EP1805109A1

EP1805109A1 - Method and apparatus for treating aqueous medium

Info

Publication number: EP1805109A1
Application number: EP05777110A
Authority: EP
Inventors: Robert Masahiro c/o Ebara Research Co. Ltd. SERIKAWA; Kenichi c/o Ebara Research Co. Ltd. SASAKI; Takashi c/o Ebara Corporation USUI
Original assignee: Ebara Corp
Current assignee: Ebara Corp
Priority date: 2004-09-01
Filing date: 2005-08-30
Publication date: 2007-07-11
Also published as: WO2006025563A1; JP2006068617A; CN101048349A

Abstract

There are provided a method and apparatus according to which an aqueous medium, in particular a poorly biodegradable aqueous medium, can be treated efficiently through electrolytic treatment using a special conductive electrode. In one embodiment of the present invention, there is provided a method for treating an aqueous medium, which comprises subjecting an aqueous medium to an electrolytic step (2) using a special conductive electrode, and then to a water treatment step (4).

Description

DESCRIPTION METHOD AND APPARATUS FOR TREATING AQUEOUS MEDIUM

TECHNICAL FIELD The present invention relates to a method for treating any of various aqueous media regardless of form such as aqueous solutions, slurries, emulsions, micelles, suspensions, concentratedsolutions and sludges; inparticulartoamethodaccordingtowhichpoorlybiodegradable aqueous media can be treated efficiently.

BACKGROUND ART

Waste water is discharged from private industrial facilities, public facilities, semipublic joint venture facilities and so on in any of various forms such as aqueous solutions, slurries, emulsions, micelles, suspensions, concentrated solutions andmixedsludges (in the present specification, these are referred to collectively as "aqueous media") . Such waste water must be made harmless by being subjected to water treatment before being discharged into public waters. The water treatment methodmost commonly carried out on waste water is biological treatment, which has been widespread from long ago due to the treatment cost being relatively low. Biological treatment can be broadly classified into aerobic treatment and anaerobic treatment. The former is mainly used in the case that the chemical oxygen demand (COD) of the aqueous medium is a few tens to a few thousands of mg/L or less, whereas the latter is mainly used in the case that the COD is at least 1000 mg/L. Moreover, with night soil treatment, sewage treatment or the like, once denitrification treatment cannot be completed with aerobic treatment oranaerobictreatmentalone, aerobicbiologicaltreatmentandanaerobic biological treatment may be used in combination so that the features of both can be utilized. In any case, biological treatment is widely usedwhentheaqueousmediumtobe treatedcontainsmainlybiodegradable matter. In the case of an aqueous medium that contains a lot of matter from natural sources such as waste water from a food, drinks or beer factory, lees, leftover food, livestock excrement, sewage, night soil, or biomass waste, biological treatment can often be used.

However, it may be difficult to use biological treatment on an aqueous medium containing a lot of poorly biodegradable matter, for example in the case that the aqueous mediumcontains chemical substance or chemically synthesized substance originating from petrochemicals, or the case that the aqueous medium contains microorganisms having hard cellwalls suchas inmethane fermentation sludge or organic sludge from a sewage treatment plant. Moreover, even being from a natural source such as lignin and humin, the biodegradability may be extremely low, in the case of macromolecules have benzene ring functional groups in the molecule. That is, biological treatment is basically difficult on aqueous media containing a lot of matter that cannot be digested by the microbesusedinthebiologicaltreatment. Moreover, evenforanaqueous medium containing biodegradable matter, some time is not possible to complete the treatment process within a practical time. Furthermore, even if the aqueous medium contains mainlybiodegradable matter, if the aqueous medium contains even a small amount of matter that is toxic to themicrobesormatterthatcausesbiologicalinhibition, thenbiological treatment may not work well. For example, it is known that ammonia, benzene and phenols cause inhibition of the activity of many microbes. Moreover, acetic acid is inherently highly biodegradable, but as can beunderstoodfrompickles actingaspreservedfood, ifanaqueousmedium containsaceticacidinahighconcentration, thenmicrobeswillnolonger be able to proliferate, and hence the aqueous medium itself will become poorlybiodegradable. Similarly, saccharose or the like is also highly biodegradable, but if such sugar is contained at a high concentration as in blackstrap molasses, then an osmotic pressure will arise due to the difference in the solute concentration between the microbe cells and the aqueous medium, and the microbes will no longer be able to proliferate, andthustheaqueousmediumwillbecomepoorlybiodegradable. The same applies to pickling syrup or the like having a high salt concentration. With such concentrated waste liquids, biological treatment may become possible upon diluting so as to eliminate the poor biodegradability. However, in this case, the amount of aqueous medium that must be treated will become high, and hence the cost, equipment space and so on required for treating the large amount of the aqueous medium will be a problem, and thus successfully establishing a water treatment process using biological treatment may be difficult.

Furthermore, if the aqueous medium contains a colorant component as in dye wastewater, biological treatment maybe extremely difficult. Colorant components are in general poorly biodegradable by microbes. When aqueous medium containing a colorant component is subjected to the biological treatment, there are some cases in which the COD components (sometimes also referredto as "COD" hereinafter) andtheBODcomponents (sometimesalsoreferredtoas"BOD"hereinafter) aresufficientlyremoved, but there is almost any color removal. Furthermore, quiteapartfromjudgingwhetherbiologicaltreatment is possible or not based only on the contained proportions of poorly biodegradable matter and readiiy biodegradable matter, there are also aqueousmediainwhichcarryingoutbiologicaltreatmentisfundamentally dangerous. In particular, waste water/liquids containing antibiotics from drug manufacturing plants, hospitals and so on are classified in such aqueous media. In biological treatment, aerobes or anaerobes are cultured so as to purify the aqueous medium. Generation change for such aerobes or anaerobes is extremely short, taking a few hours to a few days, and hence if there is a continual influx of an antibiotic into the aqueous medium treatment plant, then microbes having antibodies against the antibiotic will readily be produced, and may proliferate. It is thought that MRSA (methicillin-resistant Staphylococcus Aureus) , which is harmful to humans and for which antibiotics are ineffective, has arisen through the same principle in hospitals where antibiotics have been overused. Consequently, if suchwastewater is treatedusing biological treatment, then there will be a risk of the aqueous medium treatmentplantbecomingabreedinggroundformicrobeshavingantigens, andthesemicrobes thenbeingreleasedintothe surroundingenvironment.

Generally, andinthepresent specification, poorlybiodegradable matter refers to matter that is not readily decomposed by organisms.

As described above, even if an aqueous medium contains biodegradable matter or matter from natural sources, it is not necessarilymeans that theaqueousmediumcanbetreatedpracticablythroughbiologicaltreatment alone. In the present specification, "poorly biodegradable aqueous media" is not limited to aqueous media containing poorly biodegradable matter, toxic matter, or matter that inhibit biological treatment, but rather refers in general to all aqueous media to which it is difficult to apply biological treatment as awater treatment process as described above. Moreover, the form of a poorly biodegradable aqueous mediummay beanaqueous solution, oraslurry, anemulsion, amicelle, asuspension, a concentrated solution or a sludge containing solid matter. Examples of poorly biodegradable aqueous media under this definitionincludemixedsludgeliquids fromwatertreatmentplants such as sewage treatment plants; various sludges from methane fermentation processes and the like; waste water/liquids from petroleum refineries andpetroleumproduct plants; wastewater/liquids fromchemicalplants; wastewater/liquids fromdrugmanufacturingplants andhospitals; waste water/liquids from any of various steps in semiconductor processes (photoresist steps, washing steps, plating steps); photographic development waste liquids; any of various used cutting oil (oily or water-soluble) waste liquids from machining workshops; washing water and waste water from paint manufacturing processes; washing water and waste water from coating processes in canning plants, car body plants, and sheet metal plants; waste water/liquids from agrochemical manufacturing processes; dye waste water; dye factory waste water; ion exchange recycled waste water (condensate demineralizer waste water) frompower stations; andplatingwaste liquids andplatingwashingwater fromplatingplantscontainingorganicmatterorammonia. However, there is no limitation thereto, there beingmanyother aqueous media forwhich biological treatment is difficult. These poorly biodegradable aqueous media must be treated using another means such as physicochemical method. In the case that the COD of the aqueous medium is fewhundreds ofmg/L or less, the aqueous medium may be treated using ozone, ultraviolet radiation, hydrogen peroxide, hypochlorous acid or the like. Moreover, an advancedoxidation process (AOP) inwhich ozone or hydrogen peroxide and ultraviolet radiation are combined may be used. It is known that AOP or ultraviolet radiation is effective for treating aqueous media containing trace amounts of chlorinatedorganic compounds suchas dioxins inparticular. Treatment with ozone alone may also be effective in the case of aqueous media containing low concentrations of colorant components or dyes. Hypochlorous acid treatment is suitable in the case that sterilization treatment is required before the aqueous medium is discharged into the surrounding environment.

In the case that the COD concentration of an aqueous medium is low but the colored component concentration is high, it may be possible towellremovethecoloredcomponents throughadsorptiontreatmentusing granular activated carbon. However, if the aqueous medium contains a relatively high concentration of COD components, the adsorption performance of the activated carbon can drop in a short time. The cost of activated carbon is also not always cheap, and furthermore the used activated carbon itself must be disposed of, and consequently there is a limit of poorly biodegradable aqueous media inwhich activated carbon can be used for removing colored components.

Moreover, in the case that an aqueous medium contains a large amountofsuspendedmatter, itmaybepossibletocarryoutwatertreatment effectively by carrying out flocculation treatment. Upon adding an Al-based or Fe-based flocculant such as polyaluminum chloride (PAC) or aluminum sulfate to the aqueous medium, negatively charged suspended matter is neutralized, and flocculates together through van der Waal's forces. After floes have been formed through the flocculation, the aqueous medium can be treated by removing the floes by settling or filtration. However, in the case, for example, that the aqueous medium contains a large amount of completely dissolved matter, the treatment method such as flocculating and settling or flocculating and filtering will have almost no effect. Moreover, there is also a method of using aflocculantinwhichtheflocculantisaddedbeforedewateringthesewage sludge, methane fermentation sludge or the like, thus making it easier to squeeze filtrateout fromthe sludge. However, in the case of organic sludge, a large amount of water is contained inside of the cell walls of the microorganisms, and hence it is extremely difficult to dewater the sludge using a flocculant down to a water content of below 80%. Flocculation treatment may thus be effective only on suspended matter in the aqueous medium, and hence in most cases is used as a pretreatment rather than as a final treatment.

Regarding disposal of sludge, until little over a decade ago, ocean dumping was also used, but due to the London treaty coming into effect and so on, legal restrictions have become severe, and hence it is now becoming impossible to carry out ocean dumping. Furthermore, thereis alsothemethodoflandfilling, butduetofearsofcontamination of groundwaterresources, heightening of environmental awareness among local residents, exhaustion of disposal sites and so on, it has become difficult to carry out such disposal.

Moreover, combustion is an effective physicochemical treatment method for poorly biodegradable aqueous media. With combustion, the aqueous medium is subjected to heating and combustion treatment at a temperature of 800 to 900⁰C, whereby the water is converted into water vapor, organic matter is converted into carbon dioxide and water, and inorganic matter is converted into ash, and hence the amount of waste canbemarkedlyreduced. Combustionisusedinthetreatment of sludges, concentrated waste liquids, and various other aqueous media to which biologicaltreatmentcannotbeapplied. Inthecaseoftreatinganaqueous medium by combustion, the treatment is generally carried out in an incinerator provided with a burner of kerosene, city gas or the like, or is burned together with wastes that has high calorific value in an incinerator in which solid waste can be burned. Aqueous media such as photographic development waste liquids and agrochemical plant waste liquids are treated in an incinerator provided with a kerosene burner.

However, in the case of treating an aqueous mediumbycombustion, autothermal combustion will be impossible unless the aqueous medium contains at least approximately 40% of combustiblematter. The deficit in the calorific value must be compensated with kerosene or the like as described above, and hence if there is little combustible matter in the aqueous medium then the fuel cost will be high. Moreover, in the case of carrying out combustion by spraying the aqueous medium into a furnace in which waste with high calorific value such as waste plastic is incinerated, there is a problem that incomplete combustion is prone to occurring due to the furnace temperature dropping or the like. Furthermore, in recent years, there has been a great opposition against the burning of waste itself, with it not being possible to obtain the agreement of local residents. Operation of small-scale incinerators has been ceased due to legal restrictions in the Law concerning Special Measures against Dioxins and so on, and the establishment of new incinerationfacilitieshasnotmetwiththeagreementoflocalresidents or has been subject to legal restrictions. To maintain operation of an incinerator, statutory inspection of the dioxin concentration and so on in exhaust gas must be carried out, and the relevant authorities must be notified that this concentration is within emission standards. As a result, there are strict requirements for incineration operations to be managed thoroughly with regard to incomplete combustion, high-temperature combustion and so on, andhencemaintaining small- and medium-scale incinerators have become very difficult. There is thus a trend of disuse of small-scale incinerators, with waste being transported to distant locations, centralized and incinerated in large-scale equipment. In large-scale incineration, it is relatively easy to carry out operational management and exhaust gas control thoroughly, but the treatment cost is not necessarily cheap due to installation of large-scale exhaust gas treatment equipment, the treatment and transport cost of incineration residue and fly ash, and so on.

In recent years, an electrochemical water treatment methodusing electrolytic reactions has drawn attention as a new physicochemical treatment method for aqueous media. Electrochemical water treatment is establishing for itself a so-called "green chemical" image due to the easy operation in which the treatment starts when the current is turned on and stops when the current is turned off, chemicals not being required, it being possible to carry out the treatment in a compact apparatus, electrons serving as the substitute for chemicals, it being possible to carry out the treatment at normal temperature and normal pressure, and so on.

If the aqueous medium contains chloride ions, hypochlorous acid canbeproducedatapreciousmetalelectrodesuchasaDSA (dimensionally stable electrode) , and ammonia and colorant components contained in the aqueous medium can be decomposed by this hypochlorous acid. Ammonia in the aqueous medium can be converted into inorganic matter, i.e. into nitrogen gas, through a break-point reaction with hypochlorous acid. However, these precious metal DSA electrodes, excluding ammonia and colorantcomponents, havealmostnoeffectofdecomposingCODcomponents. Then, electrochemical water treatment using electrodes of lead dioxide or the like with high oxygen evolution overvoltage has been proposed. Withsuchelectrodes, itseemsthatdecompositionofCODcomponentsoccurs directly on the electrode surface, whereby the COD components are convertedintoinorganicmatter. However, thedecompositionefficiency of COD components for electrochemical treatment using such electrodes is not necessarily high, and moreover, because the electrodes are constituted from a heavy metal, there is a fear of the electrode-constitutingheavymetal leachingout into the treatedwater.

In recent years, an electrochemical water treatment methodusing conductive diamond has received attention. It was reported about 10 yearsagothatthereisaCODremovaleffectwhendiamondaremadeconductive andelectrochemicalreactionsarecarriedoutusingtheconductivediamond as an electrode. However, althoughthe phenomenon of COD decomposition wasconfirmedinthisreport, atthattimethemechanismwasstillunclear, andmoreover it was not possible to manufacture electrodes of practical use. However, there has been a remarkable progress of manufacturing techniques of conductive diamond electrode, due to the development of coating technologies using CVD methods in recent years. Moreover, in recent years, it has been also reported that when conductive diamond electrode are usedfor electrochemical treatment of aqueous medium, the efficiencyofCODremovalarehigherthanwhenusingleaddioxideelectrode (Ghrardini et al.: Electrochemical oxidation of 4-chlorophenol for wastewater treatment, J. Electrochemical Society, 148, D78-D82, 2001). Inthecaseofconductivediamondelectrodes, differentfromleaddioxide, even if there is some leaching out into the aqueous medium, the only component is carbon, and hence there is no problem of leaching out of a heavy metal, and moreover the efficiency of COD decomposition is extremely high due to OH radicals produced on the electrode surface. Moreover, it isalsoknownthattheOHradicalsproducedontheconductive diamond electrode have an extremely high sterilizing effect. Naturaldiamondis aninsulator, butdiamondas sameas thesilicon is an element of group IV. Diamond becomes a p-type semiconductor if dopedwith a group III element such as boron, or an n-type semiconductor if doped with a group V element such as nitrogen. Upon increasing the amount of the dopant, diamond becomes a conductor exhibiting a low electrical resistance of the order of that of a metal, e.g. lOmΩcm. If suchconductive diamond is usedas an electrode in an electrochemical reaction, then the conductive diamond exhibits a broad thermodynamic window (potentialwindow for the hydrogen evolution overvoltage and the oxygen evolution overvoltage) not seen with other electrode materials. That is, if used in electrolysis, the conductive diamond electrode is anelectrodeforwhichoxygenandhydrogenarenotpronetobeingproduced. Consequently, it is thought that when conductive diamond electrode is used as an anode, the production of OH radicals proceeds instead of the oxygenevolution, andtheseOHradicals decomposeCOD. OHradicalshave an extremelyhigh oxidizing ability, andhence can decompose almost all

COD components such as organic matter into carbon dioxide and water.

Electrochemical treatment of anaqueousmediumusingaconductive diamond electrode has merits as described above, but with prior art, therehavebeenmanyproblems withregardtomaking suchelectrochemical treatment using a conductive diamond electrode fit for practical use as a treatment process for poorly biodegradable aqueous media.

With treatment of a poorly biodegradable aqueous medium by electrolysis using a conductive diamond electrode, while the concentrationofCODcomponentscompletelydissolvedintheaqueousmedium is high, in particular in the case that the soluble COD concentration is at least a few thousands of mg/L, the COD components are converted into inorganic matter, i.e. into carbon dioxide, water, nitrogen and so on at a current efficiency of almost 100%. However, once the COD concentrationintheaqueousmediumdrops, thecurrent efficiencystarts todropmarkedly. Thepresentinventorssubjectedapoorlybiodegradable aqueousmediumdischargedfromafactorytoelectrolytictreatmentusing a conductive diamond electrode, and studied the relationship between the COD concentration in the aqueous medium and the current efficiency for the COD decomposition. The results are shown in FIG. 1.

Although dependent on the current density when operating the conductive diamondelectrode, withinan ordinarycurrent densityrange, the current efficiency for COD decomposition at the conductive diamond electrode is almost 100% if the COD in the aqueous medium is at least 500 to 2000mg/L. However, belowthis COD, thecurrent efficiencystarts to dropmarkedly. To convert the COD components into inorganicmatter, theCODcomponentsintheaqueousmediummustreachtheOHradicalsproduced on the surface of the conductive diamond electrode. OH radicals are continuallyproducedonthe surfaceoftheconductivediamondelectrode, but have a short lifetime, andhence are not released from the electrode surface into the solution so as to performingan oxidative decomposition reactionas aso-calledsolutionphasereaction. That is, thereactions in which the COD components are converted into inorganic matter only proceedontheelectrodesurface. Consequently, iftheCODconcentration in the aqueous mediumbecomes low, thenmass transferonto the electrode surface becomes rate-limiting, andhence the current efficiency for the COD decomposition drops greatly. In this case, even if the Reynolds number (the dimensionless number that indicates whether the flow is laminaror turbulent) is increasedbymechanically stirring the aqueous mediumor the like, there is a limit to the extent towhichmass transfer can be stopped from being rate-limiting, and hence it is extremely difficult tomaintain the current efficiency for the COD decomposition. IfthecurrentefficiencyfortheCODdecompositiondrops, thentheamount of electricity required for the treatment increases greatly, and hence a problem of the treatment cost increasing arises. Ifmass transferbecomes rate-limiting, thenOHradicalsproduced onthesurfaceoftheconductivediamondelectrodeareconsumedwastefully. On the surface of the conductive diamond electrode, rather than decomposition of COD, self-decomposition of the OH radicals and so on occurs,andhenceoxygengasstartstobeproduced. Theoxygengasproduced herehardlyreactswith the COD components (at least hardlyreacts below 150°C, which is the ordinary condition of use of an electrode) . In this case, theproductoftheanodicreactionisoxygengas, notcarbondioxide.

Ontheotherhand, reductionofwateroccursasthecathodicreaction, and evolution of hydrogen gas proceeds. Evolution of hydrogen gas generallyproceeds regardless ofwhetheraconductive diamondelectrode is used as the cathode, or another DSA electrode or platinum, titanium or stainless steel electrode is used. If oxygen gas and hydrogen gas are simultaneously produced inside the same electrolytic compartment, there is a fear that an explosive mixture of oxygen gas and hydrogen gas will be produced, which is a problem in terms of the safety of the process. With electrolytic compartments used in the soda industry, a separatingmembraneisgenerallyusedsothattheelectrolyticcompartment is separated into anolyte and catholyte and hence the produced gases do not mixwith one another. Fluorinated ion exchangingmembrane, which has a relatively high durability, is commonly used as such a separating membrane. However, thisfluorinatedionexchangingmembraneisrelatively expensive, and furthermore there is aproblemwithcorrosion resistance to the OH radicals produced at the conductive diamond electrode. If theseparatingmembraneintheelectrolyticcompartmentcomesintocontact with the conductive diamond electrode during operation, then therewill beahighprobabilityof the separatingmembranedeterioratingmarkedly. Furthermore, although there is no problemwhen only pure saturated salt water with thoroughly controlled water quality is introduced into the electrolytic compartment as in the case of soda industry; deterioration of the separating membrane will occur readily when treating a poorly biodegradable aqueous medium, i.e. dirty waste water or waste liquid. With apoorlybiodegradable aqueous medium, the aqueous mediumcontains various matters, and hence blockage of the separating membrane, a drop in the ion exchange capability, and so on will be prone to occurring throughattachmentofthismatterandsoon. Theuseofseparatingmembrane is generally suitable in the treatment of a "clean aqueous medium", but is difficult in the treatment of a poorly biodegradable aqueous medium as intended in the present invention.

If an electrolytic compartment using a conductive diamond electrode is operated continuously in a state such that mass transfer is rate-limitingasdescribedabove, thenmore seriousproblems relating to the stability of the conductive diamond electrode itself may arise. While the COD concentration in the aqueous medium is highandhencemass transfer of the COD components onto the electrode surface is not rate-limiting, the OH radicals produced on the electrode surface are consumed through decomposition of the COD components. Then, once mass transfer of the COD components onto the electrode surface becomes rate-limitingandhencetheCODcomponentsnolongerreachtheconductive diamond electrode, the OH radicals produced on the electrode surface ultimately become oxygen gas. However, before being converted into oxygengas, OHradicalsthatarestillactivemayreactwiththeconductive diamond electrode itself. The OH radicals produced at the conductive diamondelectrodeareabletooxidativelydecomposemost organicmatter, and hence there is not guarantee that the OH radicals will not react even with the highly stable carbon (sp³) of the diamond. The present inventors carried out tests of organic matter decomposition through electrolytic reactions using a conductive diamond electrode in a state in which mass transfer is rate-limiting as described above. Electron micrographs (SEM's) showing the state of the surface of the conductive diamond electrode before and after being used in the electrolysis are shown in FIG. 2. It can be seen that polycrystalline diamond particles aredepositeduniformlyonthesurfaceoftheconductivediamondelectrode before use (FIG. 2a), but after use for a certain period these diamond crystal particles have been etched (FIG. 2b) . It is thought that this isaresultofOHradicalsthathavenotbeenconsumedinthedecomposition of COD components oxidizing the carbon of the conductive diamond electrode.

Theconductivediamondthinfilmofaconductivediamondelectrode is manufactured by CVD using cheap organic matter such as methane as a carbon source. It can thus be expected that it will be possible to manufacture cheaper conductive diamond electrodes in the future.

However, in the present state of the art, conductive diamond electrodes are still not that cheap, and furthermore the running cost of the CVD coating process, which is carried out at a high temperature, is high. It is thus not the case that a conductive diamond electrode can merely be replaced after deteriorating every few days to every fewmonths. If it is necessary to replace a conductive diamond electrode after such a short time, then costs will arise due to the maintenance time and maintenance labor, and hence it will be difficult to make the poorly biodegradable aqueous medium treatment process viable.

In the case that the concentration of COD components dissolved inanaqueousmediumis low, theproblemdescribedabovearises; however, the problem of the current efficiency dropping also arises in the case of a slurry such as a sludge, i.e. in the case that suspended matter not dissolved in the aqueous medium is present. A cause of this is the poornessofcontactofsolid/suspendedmatterwiththeelectrodesurface.

Moreover, there are problems, such as that described below, in carrying out treatment by electrolytic treatment using a conductive diamondelectrode tomakingtheaqueousmediumcompletelyharmless; even on an aqueous medium containing a high concentration of soluble COD componentswherethemasstransferringrate-limitingissues, asdescribed above, does not occur.

Regarding the electrochemical reactions on the surface of the conductive diamond electrode, one may refer to the following electrochemical reaction formulae in redox potential tables. Production of OH radicals from water:

H₂O → OH- + H⁺ + e^' E⁰ = 2.8 V (1)

Production of oxygen gas from water: H₂O → 1/2O₂ + 2H⁺ + 2e^' E⁰ = 1.2 V (2)

The molecular weight of oxygen is 16, and hence, from Faraday's law, the electrical charge required to decompose 1 g of chemical oxygen demand, i.e. COD, in an aqueousmediumis givenbythefollowing formula.

(2 x F) / 16 = (2 x 96485 C) / 16 g = (192.970 As) / 16 g = 53.6 Ah / 16 g = 3.4 Ah/g-COD

That is, even if the current efficiency is 100%, a theoretical electrical charge of 3.4 Ah is required to decompose 1 g of COD. It is known that a conductive diamond electrode has a broad thermodynamic window and a high oxygen evolution overvoltage. If OH radicals are producedbeforeoxygengasevolution, thenthediamondelectrodepotential during the electrolysis must be at least 2.8 V of the formula (1) . The cellvoltageofconductivediamondelectrode, inthereality, forcarrying out an electrolytic reaction shouldbe at least 4 to 5 V, becausebesides this electrode overvoltage, there is an increase due to the electrical resistanceoftheelectrolytesolutionandgasproducedat theelectrode. Although dependent on the operating current density, temperature of the electrolyte, electrical conductivity of the aqueous medium and so on, a typical operating value for the cell voltage (the inter-electrode voltage for a single cell)can be mentioned as being approximately 7 V.

The amount of electricity required to decompose 1 g of COD is thus3.4Ah*7V=23.8VAh, i.e. approximately24Wh/g-CODorapproximately 24 kWh/kg-COD.

The amount of electricityrequired to treat 1 m³ of aconcentrated aqueous medium, for example an aqueous medium containing 5% (50 kg/m³) of COD components, byelectrolytic treatment using a conductive diamond electrode is thus 50 * 24 kWh = 1,200 kWh; according to this calculation, the treatment cost can by no means be said to be cheap. That is, if the COD concentration is low, then mass transfer becomes rate-limiting and hence there is the problem that electricity is wasted, whereas if the COD concentration is high, then the current efficiency for the COD decompositionisgood, buttheelectrolysiscostwillbeinanproportional amount to the absolute value of the COD, and hence there is a drawback that thecostofelectricityperunitvolumeoftheaqueousmediumtreated is high. DISCLOSURE OF THE INVENTION

As described above, there are many problems with the method in which an aqueous medium is completely treated through electrolytic treatment using a conductive diamond electrode. The present inventors carried out assiduous studies to solve these problems, and as a result have been the first to discover that the biodegradability of aqueous medium can be improvedby subjecting the aqueous medium to electrolytic treatment using conductive diamond electrode.

The present inventors first carried out various experiments focusingonBOD/CODasanindicatorforthebiodegradabilityofanaqueous medium, and as a result ascertained that the value of BOD/COD increases through electrolytic treatment using a conductive diamond electrode. Moreover, the present inventors ascertained that the reason why the biodegradability of the aqueous medium increases is due to the increase of amounts of volatile fatty acids (VFA's) and soluble sugars through the electrolytic treatment using a conductive diamond electrode. That is, because VFA's and soluble sugars have a high biodegradability, COD components are converted into BOD components through the conversion of COD components intoVFA's and soluble sugars. Furthermore, the present inventors discovered that if an aqueous medium is treated through electrolytic treatment using a conductive diamond electrode, then the viscosityandsoonoftheaqueousmediumchange, andhencetheflocculating ability, settlingability, filterabilityandsoonof the aqueousmedium improve. The present inventors then noticed that such improvement in the biodegradability of the aqueous medium and improvement in the flocculating ability, settling ability, filterability, adsorptivity, crystallizabilityandsoonoccurbeforetheCODcomponentsintheaqueous mediumhave been completelyremovedby the electrolytic treatment using a conductive diamond electrode, and thus accomplished the invention of amethodfortreatinganaqueousmediumaccordingtothepresentinvention. That is, in one embodiment of the present invention, there is providedamethodfortreatinganaqueousmedium, whichcomprisestreating an aqueous medium in an electrolytic step using a conductive diamond electrode, andtheninawatertreatmentstep, forexampleoneofbiological treatment, flocculating settling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, and crystallization treatment, or a combination of biological treatment, and one or at least two of flocculating settling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, and crystallization treatment.

As described above, according to the findings of the present inventors, thebiodegradabilityofanaqueousmediumisimprovedmarkedly throughtheelectrolytictreatmentusingaconductivediamondelectrode, andhenceitbecomeseasiertotreattheaqueousmediumthroughbiological treatment; andbycompleting the treatment of the aqueousmediumthrough biological treatment to make the aqueous medium harmless, the cost of the overall treatment process can be greatly reduced. Similarly, the flocculating ability of suspended matter in an aqueous medium is greatly improved, and hence the efficiency of flocculating settling is increased, and moreover only a small amount a flocculant need be used through the electrolytic treatment using a conductive diamond electrode. When the flocculating ability of suspended matter in an aqueous medium is poor, very often the viscous components contained in the aqueous medium are the influencing factor. Suchviscouscomponentsaredecomposedthroughtheelectrolytictreatment using a conductive diamond electrode, and hence the aqueous medium is made non-slimy. Consequently; by carrying out flocculating settling treatmentofaqueousmediumaftertheimprovementofflocculatingability and the COD components being removed as flocculated matter; the amount ofelectricityconsumedintheelectrolytictreatmentusingaconductive diamond electrode can be reduced.

Furthermore, becausetheviscosityoftheaqueousmediumisreduced throughtheelectrolytictreatmentusingaconductivediamondelectrode, any of various filtration operations such as micro-filtration (MF), ultrafiltration (UF) , reverseosmosis (RO) , sandfiltration, filterpress filtration, or belt press filtration becomes possible; moreover, by removingpollutantsintheaqueousmediumthroughsuchfiltration, savings can be made in the amount of electricity required for the electrolytic treatment using a conductive diamond electrode, and hence the cost of the overall treatment process can be reduced. Furthermore, regarding to thewater contained inside the cells of microorganisms as in the case oforganic sludge, the cellwalls aredestroyedthroughthe electrolytic treatment using a conductive diamond electrode, and hence the dewaterability of the sludge is improved.

Furthermore, if the electrolytic treatment using a conductive diamond electrode is carried out on an aqueous medium that is in the formofanemulsion, thendemulsificationandbreakdownofmicellesoccur, andhence a phase boundary appears between the oil phase and the aqueous phase, and thus oil/water separation becomes possible.

Moreover, in the present invention, a combination of biological treatment, and one or at least two of flocculating settling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, andcrystallizationtreatment canbeusedas thelatter-stage water treatment step following on from the former-stage electrolytic treatment using a conductive diamond electrode. For example, after improving the biodegradability of an aqueous medium through the electrolytic treatment using a conductive diamond electrode, it is possibletoremove suspendedmatteror solidmatterin theaqueousmedium through flocculating settling treatment, filtration treatment or the like, and then carry out biological treatment, whereby decomposition treatment can be carried out on the aqueous medium efficiently. Flocculating settling and filtration are the cheapest processes for removingsuspendedmatterorfloatingsolidmatterfromtheaqueousmedium, and suspended or solid COD components in the aqueous medium are enable toberemovedaftertheelectrolytictreatmentusingaconductivediamond electrode. Moreover, regardingCODcomponents dissolvedintheaqueous medium, which cannot be removed through the filtration or settling, biological treatment can be facilitated because the biodegradability is increased through the electrolytic treatment using the conductive diamond electrode. Moreover, in the case of treating amixed oil/water aqueous medium, oil/water separation becomes possible through the electrolytic treatment using a conductive diamond electrode, and hence it is possible to remove the oil phase and then carry out effective decomposition treatment of the aqueous phase by biological treatment. In the method of the present invention, anaerobic biological treatment or aerobic biological treatment can be used as the biological treatmentstep. PoorlybiodegradableCODcomponentsinanaqueousmedium are converted into organic acids and soluble sugars through the electrolytic treatment using a conductive diamond electrode, and then these substances can be rapidly converted into methane gas through anaerobic treatment• This method is particularlyeffective in the case that theCOD concentration of the aqueousmediumis high, andthemethane gas producedcanbeusedas anenergysource, andhencerecoveryof energy from waste becomes possible. Moreover, aerobic treatment, for which the treatment cost is cheap, can also be carried out after the biodegradability of an aqueous medium has been increased through the electrolytic treatment using a conductive diamond electrode. This methodiseffectiveinparticularwhentheCODconcentrationintheaqueous medium is not so high. Biological treatment can be used effectively onanaqueousmediumhavingalowCODconcentrationinwhichtheefficiency of the electrolytic treatment using a conductive diamond electrode is poor. Using theminimumenergy just for increasing thebiodegradability at the electrolytic treatment with the conductive diamond electrode and incorporatingbiologicaltreatment; theefficiencyofoveralltreatment process can be improved.

In the method of the present invention, the former-stage electrolytic treatment step using a conductive diamond electrode is preferably stopped in a state in which COD components in the aqueous medium have not been completely decomposed, and then the latter-stage watertreatment stepiscarriedout. IftheCODcomponents intheaqueous mediumarecompletelydecomposedthroughtheelectrolytictreatmentusing aconductivediamondelectrode, theninthecasethattheCODconcentration in the aqueous medium is high, there is a problem that the electricity consumption per unit volume is high, and in the case that the COD concentration in the aqueous medium is low, there is a problem that the current efficiency is poor and hence electricity is wasted. However, according to a preferable embodiment of the present invention, there is a provided a excellent method in terms of costs and efficiency for treating an aqueous medium; in which not all but rather only a part of the COD components in the aqueous medium is treated through the electrolytic treatment using a conductive diamond electrode; whereby thebiodegradabilityis improvedand/or the filterability, the settling abilityorthelikeisimproved, andthenwatertreatmentsuchasbiological treatmentorflocculatingsettlingtreatment iscarriedoutinthelatter stage. Furthermore, oncetheformer-stageelectrolytictreatmentusing a conductive diamond electrode is carried out only in a high range of CODconcentration, overwhichtheefficiencyoftheelectrolytictreatment using a conductive diamond electrode is good, the problem of self-deterioration of the conductive diamond electrode by the attack of OH radicals produced by the conductive diamond electrode itself can beprevented, andthenthedurabilityoftheconductivediamondelectrode can be improved.

The method of the present invention can be particularly suitably used in the treatment of a poorly biodegradable aqueous medium. In a preferable embodiment of the present invention, the biodegradability of the aqueous medium is improved such that BOD/COD of the aqueous mediumis made to be at least 0.2 through the former-stage electrolytictreatmentusingaconductivediamondelectrode. IfBOD/COD of the aqueous medium is 0.2 or less, then the biodegradability will be low, andhence latter-stage biological treatment will be difficult. In the case of carrying out aerobic biological treatment as the latter-stage biological treatment, through measures such as diluting the aqueous medium, and lengthening the biological treatment time, i.e. lengthening the residence time, itmaybepossible tohandle the aqueous medium even if BOD/COD of the aqueous medium is 0.2 or less, but the treatment efficiency will be poor. Moreover, even if BOD/COD of the aqueous medium is 0.2 or less, biological treatment may become possible by culturing the microbes for a long time, i.e. by making generation change take place so that the microbes become acclimatized. However, withthismethod, aspecialselectedmicrobephasewillbeformed. Although therewillbenoprobleminthecaseofalways supplyinganaqueousmedium with the same water quality into the biological treatment compartment, the capacity of the biological treatment maymarkedly drop if the water quality of the aqueous medium fluctuates even a little. To make the latter-stage biological treatment proceed smoothly, it is preferable to make BOD/COD of the aqueous medium be at least 0.2, more preferably at least 0.3, yetmorepreferablyat least 0.5, throughthe former-stage electrolytictreatmentusingaconductivediamondelectrode. IfBOD/COD of the aqueous medium is made to be at least 0.5, then use of methane fermentation, which is anaerobic treatment, will also become easy.

In themethod of the present invention, it is preferable to apply an electrical charge ofnotmore than 34 Ahpergramof COD in the aqueous mediumintheformer-stageelectrolytictreatmentstepusingaconductive diamond electrode. In the case that the initial COD concentration of theaqueousmediumis lowatafewhundreds ofmg/L, theelectricalcharge consumption per unit volume (e.g. per m³) of the aqueous medium will be low, and hence the treatment may be viable in terms of cost even if a electrical charge exceeding the above is applied, but in any case the electrolytic treatment using a conductive diamond electrode will come to carried out in a region in which the current efficiency is poor, and hence electrical chargewillbewasted, whichis undesirable. Inamore preferable embodiment, a electrical charge of not more than 17 Ah/g-COD is appliedin theelectrolytic treatment stepusingaconductivediamond electrode; more preferably, not more than 3.4 Ah/g-COD, which is the theoretical electrical charge, is applied, andyetmorepreferably, the electrical charge applied is kept down to not more than 95% of the theoretical electrical charge, yet more preferably not more than 90% ofthetheoreticalelectricalcharge. Thelowerlimitofthe electrical charge applied in the electrolytic treatment step using a conductive diamond electrode is preferably at least 0.034 Ah/g-COD. If the electrical charge is less than this, then it will be difficult to make the latter-stage water treatment proceed smoothly by bringing about a change in the physical properties orwater quality of the aqueous medium such as an improvement in the biodegradability of the aqueous medium oranimprovement in the settlingability, thefilterabilityorthelike. A more preferable value of the lower limit of the electrical charge is at least 0.34 Ah/g-COD, i.e. 10% or more of the theoretical electrical charge. A yet more preferable value is at least 0.68 Ah/g-COD, i.e. 20% or more of the theoretical electrical charge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the COD concentration of the aqueous medium and the instantaneous current efficiencyfor COD decompositionwhen subjectingapoorlybiodegradable aqueous medium discharged from a factory during electrolytic treatment using a conductive diamond electrode.

FIG. 2 comprises electron micrographs (SEM's) showing the state of the surface of a conductive diamond electrode before and after being used in electrolysis. FIG. 2-(a) shows the state of the surface of a conductivediamondelectrodebeforebeingusedinelectrolysis, andFIG. 2-(b) shows the state after being used in electrolysis.

FIG.3 is aflowdiagramof amethodfor treatinganaqueousmedium according to an embodiment of the present invention.

FIG.4 is a graph showing the relationship between the electrical charge applied per 1 g of COD contained in a sample liquid (horizontal axis) and the COD and BOD of the sample liquid (vertical axis) for an experiment of Example 1

FIG.5 is agraph showing the relationshipbetween the CODremoval rate and BOD/COD (in percent) for the experiment of Example 1.

FIG.6 is a graph showing the relationship between the electrical chargeinputtedandorganicacidproductionfortheexperimentofExample 1.

FIG.7 is a graph showing the relationship between the electrical chargeinputtedandthecompositionoftheproducedgasfortheexperiment of Example 1.

FIG. 8 is a graph showing the trend over time of the amount of methane produced for an experiment of Example 2.

FIG.9 is a graph showing the relationship between the electrical charge inputted and the changes in COD, BOD and color for an experiment of Example 3.

FIG.10isagraphshowingtherelationshipbetweentheCODremoval rate and the percentage of color removed for the experiment of Example 3. FIG.11 is agraphshowingtherelationshipbetweentheCODremoval rate and the concentration and ammonium nitrogen removal rate for the experiment of Example 3.

FIG.12 is agraphshowingtherelationshipbetweentheelectrical charge inputtedandCOD, MLSS andviscosity for an experiment of Example 4.

FIG. 13 is a detailed flow diagram of a method for treating an aqueous medium according to an embodiment of the present invention.

FIG. 14 is a detailed flow diagram of a method for treating an aqueous medium according to an embodiment of the present invention, and relatestomethodinwhichapoorlydecomposableaqueousmediumistreated usingaformer-stagediamondelectrodetreatment stepandalatter-stage flocculating settling step. FIG. 15 is a flow diagram relating to another embodiment of the present invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage biological treatment and flocculating settling treatment are combined.

FIG. 16 is a flow diagram relating to another embodiment of the present invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage biological treatment and flocculating settling treatment are combined.

FIG. 17 is a flow diagram relating to another embodiment of the present invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage flocculating settling treatment and filtration treatment are combined.

FIG. 18 is a flow diagram relating to another embodiment of the present invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage dephosphorization by crystallization is combined.

In FIG. 18, each symbol represents the following meanings. 1 represents poorly degradable aqueous medium (raw water) ; 2 represents electrolytic treatment step using diamond electrode; 3 represents electrolyzed water; 4 represents water treatment step; 5 represents treated water; 16 represents stirring compartment; 21 represents adjusting chemical liquid (ph adjustor, Ca²⁺, Mg²⁺, NH₄ ⁺); 22 represents crystallization-adjustedtreatedwater; 23representsdephosphorization column; 24 represents phosphorus-containing crystallized matter (HAP OT MAP ) .

DETAILED EXPLANATION OF THE INVENTION

Terms used in this specification will now be described. An"aqueousmedium"isamediumhavingwaterasaprincipalcomponent thereof, and may have any form, for example a slurry, an emulsion, or anaqueous solution, withno lJLmitationthereon. Moreover, constituent components of the aqueous mediummaybe any of organicmatter, inorganic matter, salts and so on. As described earlier, a "poorly biodegradable aqueous medium" means an aqueous medium for which treatment is difficult using a conventional water treatment method such as biological treatment. Specifically, inthecasethat theaqueousmediumcontainsalotofmatter having a low biodegradability, for example in the case of an aqueous mediumthatcontainschemicallysynthesizedmatterhavingapetrochemical as a raw material, or in the case of an aqueous medium that contains microorganisms having hard cell walls such as with organic sludge or methane fermentation sludge, the aqueous medium will be a poorly biodegradableaqueousmedium. Furthermore, anaqueousmediumcontaining matter that is toxic to, or matter that causes biological inhibition to, microbes such as ammonia, benzene or a phenol will also be a poorly biodegradable aqueous medium. Moreover, an aqueous medium containing acetic acid or sugar in a high concentration can also be said to be a poorly biodegradable aqueous medium since microbes will not being able toproliferatetherein. Furthermore, aqueousmediacontainingcolorant components and so on are also poorly biodegradable aqueous media, and moreover aqueous media for which carrying out biological treatment is problematicsuchasaqueousmediacontainingantibioticsarealsoincluded under "poorly biodegradable aqueous media" in the present invention. Inthepresentspecification, "COD" (chemicaloxygendemand) means the chemical oxygen demand of the aqueous medium determined using potassiumdichromate as anoxidizing agent (CODcr) • Aswell as theCODcr, asphysicochemicaloxygendemands, therearetheCODn_nforwhichpotassium permanganateisusedas anoxidizingagent, theTOD (totaloxygendemand) which is determined from the amount of oxygen consumedupon combustion, and the ThOD (theoretical oxygen demand) which is determined from the reaction formula for complete oxidation. The measurement method is different forthesephysicochemicaloxygendemands, andhencetherewill be differences in thevalues evenfor the same aqueousmedium. Thevalue of the CODcr used in the present specification is related to the other physicochemical oxygen demands in that ThOD ≥ TOD ≥ CODc_r ≥ COD_M11. That is, because thevalue of the chemical oxygen demandof an aqueous medium given in the present specification is CODcr_* it is often the case that a higher value would be obtained if the chemical oxygen demand were determined as ThOD or TOD, and a lower value would be obtained if the chemical oxygen demand were determined as COD_1In. "BOD" in the present specification means the biochemical oxygen demand, and is BOD₅, which is determined over 5 days. The value of BOD/COD, which is used in the present specification as an indicator of the biodegradability of an aqueous medium, is the value obtained by dividing the value of BOD₅ by the value of CODcr. and is written as a value from 0 to 1 or from 0 to 100%. Anembodiment of thepresent inventionwillnowbe describedusing FIG.3. Accordingtothisembodimentofthepresentinvention, anaqueous medium 1 is first treated in a former-stage electrolytic treatment step 2 using a conductive diamond electrode, and then the electrolyzedwater 3 produced therefrom is treated in a latter-stage water treatment step 4, from which treated water 5 is discharged.

Accordingtothepresentinvention, apoorlybiodegradableaqueous medium can be treated particularly suitably. Examples of poorly biodegradable aqueous media that can be treated using the present inventionincludesludgesfromsewagetreatmentplantsandwatertreatment plants; varioussludgesfrommethanefermentationprocessesandthelike; waste water/liquids from petroleum refineries and petroleum product plants; waste water/liquids from chemical plants; waste water/liquids from drug manufacturing plants andhospitals; waste water/liquids from anyofvarious semiconductor-relatedprocesses (photoresistprocesses, washing processes, plating processes) ; photographic development waste liquids; any of various used cutting oil (oily or water-soluble) waste liquids from machining workshops; washing water and waste water from paintmanufacturingprocesses; washingwaterandwastewaterfromcoating processes in canning plants, car body plants, and sheet metal plants; wastewater/liquidsfromagrochemicalmanufacturingprocesses; dyewaste water and dye factory waste water; ion exchange recycled waste water (condensatedemineralizerwastewater) frompowerstations; platingwaste liquids andplatingwashingwaterfromplatingplants containingorganic matter or ammonia; and membrane-filtered concentrated water from tap water treatment plants. Furthermore, the present invention is not limited to these, but rather can be applied in general to any aqueous medium for which biological treatment is difficult. There are no limitations on the form of the aqueous medium 1 to be treated. The aqueous medium 1 to be treated may, for example, be in the form of a completely dissolved aqueous solution, or slurry, an emulsion, a micelle, a suspension, a concentrated solution or a sludge containing solid matter. Moreover, the aqueous medium 1 to be treated maybe apoorly biodegradable aqueous medium that has been concentrated using any of various membrane treatments, any of various distillation treatments, flocculating settling treatment or filtration treatment. In the present invention, it is efficient for the aqueous medium to be treatedtohavebeenconcentratedtoaCODconcentrationofafewthousands mg/L, but there is no such limitation to such a concentration. In the case that the aqueous medium to be treated contains solid matter that is not in the form of a suspension but rather has a size of at least a few mm, before carrying out the former-stage electrolytic treatment usingaconductivediamondelectrode, itispreferabletopasstheaqueous medium to be treated through a strainer, a sieve or the like. In the case that the aqueous medium to be treated is not an emulsion but rather there is clearly a separated oil phase or oil film, it is preferable to remove this oil phase through liquid level separation or the like. Furthermore, in the case that the aqueous medium 1 to be treated has turbidityorsolidmatterthatissettledfromtheoutset, itispreferable to remove the settledmatter in advance or else stir so that the aqueous medium can be fed smoothly into the electrolytic treatment step using a conductive diamond electrode. In the case that the aqueous medium to be treated has a low electrical conductivity, it is preferable to addanyofvarious electrolytes suchas sodiumchlorideorsodiumsulfate beforetheformer-stageelectrolytictreatment step 2 usingaconductive diamond electrode. In the case that the electrical conductivity of the aqueous medium to be treated is 0.1 mS/cm or less, it is preferable to addanelectrolytetotheaqueousmedium. Thisisbecauseiftheelectrical conductivity of the aqueous medium is low, then the cell voltage in the electrolytic treatment step 2 using a conductive diamond electrodewill rise, and hence the cost of the electrolysis will increase. It is preferable to make the electrical conductivity of the aqueous medium to be treatedbe at least 1 mS/cm. In this case, a chemical electrolyte may be added to the aqueous medium, or in the case that another aqueous mediumhavingahighelectricalconductivityis readilyobtainable, this aqueous medium having a high electrical conductivity may be mixedwith the aqueous medium to be treated before feeding the resulting mixture into the electrolytic treatment step 2 using a conductive diamond electrode. Forexample, inthecasethat seawaterisreadilyobtainable, seawatermaybemixedwiththeaqueousmediumtobetreatedbeforefeeding the resulting mixture into the electrolytic treatment step using a conductive diamond electrode.

In the present invention, first, the aqueous medium to be treated is treatedthroughtheelectrolytictreatmentusingaconductivediamond electrode. Astheconductivediamondelectrodeusedinthiselectrolytic treatment, conductive diamondhaving any constitution known in the art in question can be used. For example, an electrode obtained by using an electrically conductive metallic material such as Ni, Ta, Ti, Mo, W or Zr as a substrate and depositing a thin film of conductive diamond on the surface of this substrate, an electrode obtained by using a semiconductormaterial suchas asiliconwaferas asubstrateandforming a thin film of conductive diamond on the surface of this substrate, or a material obtained by forming deposited electrically conductive polycrystalline diamond into a plate shape, or the like can be used as the conductive diamond electrode in the present invention. The conductive diamond thin film is made electrically conductive by doping withaprescribedamountofadopantsuchasboronornitrogenwhenforming the diamond thin film on the substrate; in general, boron is used as the dopant. In the former-stage electrolytic treatment step in the present invention, a conductive diamond electrode may be used for both the anode andthe cathode, or a conductive diamondelectrodemaybe used for one of the anode and the cathode. As the material of an electrode other than the conductive diamond electrode, an ordinary electrode material such as platinum or titanium can be used. Preferably, both the anode and the cathode may be constituted from a conductive diamond electrode in the electrolytic treatment step.

In the present invention, it is preferable for the COD components contained in the aqueous medium 1 to not be completely decomposed in theformer-stageelectrolytictreatmentstep2usingaconductivediamond electrode. This is because if the COD is completely removed in the electrolytictreatment step, theninthe case that theCOD concentration of the aqueous medium is high (at least a few tens of thousands of mg/L, i.e. atleastofpercentorder) , thenthetreatmentcostoftheelectrolytic treatment step per unit volume (L or m³) of the aqueous medium will be high, and on the other hand in the case that the COD concentration of theaqueousmediumislow, masstransferontothesurfaceoftheconductive diamond electrode will be rate-limiting, and hence electricity will be wasted in the electrolytic treatment step 2 using a conductive diamond electrode.

Inthecasethat theinitialCODoftheaqueousmediumtobetreated is of the order of at least a few percent as described above, the COD removal rate in the former-stage electrolytic treatment step 2 using a conductive diamond electrode is preferably made to be such that the COD of the electrolyzedwater 3 is at least 500mg/L, preferablyat least 1000 mg/L, more preferably at least 2000 mg/L. In the case that the initial COD of the aqueous medium is high and the COD components are soluble, the current efficiency for the COD decomposition in the electrolytic treatment using a conductive diamond electrode can be maintained at 100%. However, if the COD concentration in the aqueous medium is below such a value, then the current efficiency in the electrolytic treatment usingaconductive diamondelectrodewillbecome poor. For example, in the case that the initial COD concentration of the aqueous medium 1 is 10,000 mg/L (1%), a preferable percentage of CODremovedintheelectrolytictreatment stepusingaconductivediamond electrode is not more than 95%, preferably not more than 90%, more preferably not more than 80%; if the COD concentration becomes lower than this, thenmass transferof the COD components inthe aqueousmedium to the electrode will start to become rate-limiting, and hence the treatment efficiency will become poor. It goes without saying that if the initial COD concentration of the aqueous medium differs, then the COD removal rate at which a COD concentration at which mass transfer starts to become rate-limiting will differ. For example, taking the initialCODconcentrationtobe 50,000mg/L, the COD concentrationrange atwhichmass transferstarts tobecomerate-limitingwillnotbereached until 96% to 99% of the COD has been decomposed. However, even if mass transfer does not become rate-limiting, if a high COD removal rate is reachedforsuchahigh-concentrationaqueousmedium, thenthetreatment cost of the electrolytic treatment step 2 using a conductive diamond electrode will become high; it is thus preferable for the COD removal rate in the electrolytic treatment step using a conductive diamond electrode to be made to be not more than 95%, preferably not more than 90%, more preferably not more than 80%.

A preferable mini-mum value of the percentage of COD removed in the electrolytic treatment step 2 using a conductive diamond electrode is at least 1%, morepreferablyat least 5%, yet more preferably at least 10%, yetmore preferablyat least 15%, yet more preferablyat least 20%. Removingall of theCOD in the electrolytic treatment usingaconductive diamond electrode is not a desirable form of the present invention, but if the COD concentration were not reduced at all, then it would not be possible to changephysicalvalues andaspects of thewaterquality that affect the biodegradability, flocculating ability, settling ability, filterability and so on of the aqueous medium. To enable the water treatment in the latter-stagewater treatment step 4 to be carried out, it is thus preferable to reduce the COD concentration by at least 1% intheformer-stageelectrolytictreatmentstepusingaconductivediamond electrode. It is thus preferable to maintain the lower limit of the COD removal rate indicated above in the former-stage electrolytic treatment step using a conductive diamond electrode. By maintaining the COD removal rate within such a range in the electrolytic treatment using a conductive diamond electrode, as shown in FIG. 5 (described in detail later) , thebiodegradability (BOD/COD) of the aqueous medium can be increased through the electrolytic treatment.

In the case that the initial COD concentration of the aqueous medium 1 is one order of magnitude lower than above at a few thousands mg/L or below, the COD concentration will already be in the range at whichmasstransferoftheCODcomponentsontothesurfaceoftheconductive diamond electrode starts to become rate-limiting. Even if the COD concentration is low, there is a range over which the COD concentration in the aqueous medium drops linearly in proportion to the electrical charge inputted through the electrolytic treatment using a conductive diamond electrode. Of course, mass transfer of the COD components is rate-limiting over this range, and hence a current efficiency of 100% cannot be maintained, but when the COD drops below a few hundred mg/L, the current efficiency further worsens. As an example, the situation when an aqueous medium having an initial COD of 1300 mg/L was treated by electrolysis using a conductive diamond electrode is shown in FIG. 9. In FIG. 9, the current efficiency is approximately 56% up to the time at which an electrical charge of 2.6 Ah/g-COD has been applied, but the current efficiency drops further once the COD drops below 500 mg/L. The reason that the current efficiency is relativelyhigh during theinitial stage is that components that arereadilydecomposedthrough theelectrolysisusingaconductivediamondelectrodearepreferentially decomposed. The components that are readily decomposed by the electrolysis using a conductive diamond electrode are different to the readilybiodegradablematter inbiological treatment. As is clear from FIG.9,coloredcomponentsarereadilydecomposedthroughtheelectrolytic treatment using a conductive diamond electrode. The drop in the color proceeds at a much lower electrical charge than the drop in the COD concentration. Colored components are generally completelyremoved at anelectricalchargeof 10to 90% ofthatrequiredtocompletelydecompose the COD. Once is very common the case in which it is very difficult to remove colored components by ordinary water treatment such as biological treatment; it is preferable to remove the coloredcomponents as much as possible in the electrolytic treatment 2 using a conductive diamond electrode, even though the COD is not completely decomposed.

Even if theCOD concentrationin the aqueousmedium1 tobe treated is of the order of a few percent, in the case that the aqueous medium 1 to be treated contains solid COD components such as microbial biomass as in the case of sludge, the efficiency of the electrolytic treatment may drop before the COD reaches 500 to 2,000 mg/L, which is when mass transferoftheCODcomponentsstartstobecomerate-limitingasdescribed above. As anexample, thebehaviorinthecasethatmethanefermentation sludgewassubjectedtoelectrolytictreatmentusingaconductivediamond electrode is shown in FIG. 12. As shown in FIG. 12, once the COD concentration in the aqueous medium drops below 6,000 mg/L, the COD decomposition efficiency drops suddenly. It is thought that this is because theefficiencyof contact drops between the solidCOD components andtheelectrode surface. Ifonetries toremove 100% of theCODthrough the electrolytic treatment step using a conductive diamond electrode in such a state, then an enormous electrical power will be consumed. Therefore, carryingoutaminimumofCODdecompositionintheformer-stage electrolytic treatment step and putting the burden as much as possible on the latter-stage of water treatment will result in a cheaper cost of the whole process. A description will now be given about the mechanism by which the latter-stage water treatment step is made to proceed smoothly through the former-stage electrolytic treatment stepusingaconductive diamond electrode in the present invention. This mechanism starts to come into play before the COD has been completely decomposed through the electrolysis.

First, in an embodiment of the present invention, the biodegradability of an aqueous medium can be increased through the electrolytic treatment using a conductive diamond electrode. BOD/COD canbeusedas anindicatorofthebiodegradabilityoftheaqueousmedium, and to for increasing this value (i.e. increase the biodegradability) , it is important to convert the poorly degradable COD components into BODcomponents. Intheelectrolytictreatmentusingaconductivediamond electrode, OH radicals produced on the conductive diamond electrode do not directly convert organic matter and the like into carbon dioxide and water, but rather the organic matter first goes via intermediate productssuchasorganicacids, andthentheseorganicacidsareconverted into carbon dioxide and water. Such organic acids, of which VFA's are a representative example, have an extremely high biodegradability, and hence in the case that organic acids areproducedinahighconcentration through the former-stage electrolytic treatment using a conductive diamond electrode, it is effective to carry out methane fermentation, which is anaerobic biological treatment, in the latter stage. The biodegradabilityoftheaqueousmediumthusincreasesintheformer-stage electrolytic treatment using a conductive diamond electrode before the COD components have been completely decomposed. In the case that the aqueous medium contains macromolecular organic matter, such macromolecular organic matter generallyhas poorbiodegradability, and hence the biodegradability of the aqueous medium is poor. However, reactions thatreducethemolecularweightofthemacromolecularorganic matteroccurthroughtheelectrolytictreatmentusingaconductivediamond electrode, and hence the biodegradability of the aqueous medium can be increased. It is thought that macromolecular organic matter such as lignin, humin, and polysaccharides such as cellulose compounds are not decomposeddirectlytocarbondioxideandwater, butratherareconverted to carbon dioxide andwater via the steps in which the molecular weight is reduced.

Moreover, colored components that generally have poor biodegradabilitycanalsobereadilydecomposedthroughtheelectrolytic treatment using a conductive diamond electrode. Possible mechanisms forthedecompositionofcoloredcomponentsareasfollows. First, there is amechanism inwhich a coloredcomponent is decomposedthrough direct reactionwithOHradicals. Thisdecompositiondoesnotnecessarilyhave tobeacompletedecomposition, butratherthecolormaydisappearthrough partial molecular changes of the colored component. , In the case that the aqueous medium contains components such as a dye, the coloration maydisappear,uponjustonebondcleavagebyanOHradical, inthemolecular structurethatproduces thecoloration. Thisdestructionofthecolored molecules may occur through reaction between any of various colored components or colorants such as an azo compound and OH radicals. Furthermore, in the case that the aqueous medium contains chloride ions or sulfate ions, these ions are converted into hypochlorous acid or persulfuric acid, which are oxidizing agents, through reaction on the conductive diamond electrode, and these oxidizing agents then act to decompose the colored/colorant components.

For aqueous medium as such as organic sludge, in which the cell walls of the microorganisms contain a large amount of water and the dewaterability is extremely poor due to the elevated viscosity, decomposition of the viscous components readily occurs through the electrolytic treatment using a conductive diamond electrode. Furthermore, the cell walls of the microorganisms are destroyed by OH radicals produced on the surface of the conductive diamond electrode, and then the dewaterability of the sludge can be improved markedly.

In addition to the improvement of the dewaterability, by introducing such sludge inwhich the cellwalls have been destroyedinto anaerobicbiologicaltreatmentcompartment, i.e• anaeratingcompartment, awatertreatmentsysteminwhichsludgeisnotproducedcanbeconstructed. Thatis, bytreatingthroughtheelectrolytictreatmentusingaconductive diamond electrode all or some of surplus sludge produced in a water treatmentplantthatusesaerobicbiologicaltreatment, andthenreturning the electrolyzedwaterobtainedthroughthe electrolytic treatment into theaerobebiologicaltreatmentcompartment, it ispossibletoconstruct a water treatment system according to which sludge is not produced or else the amount of sludge produced is extremely low. That is, if the constitutionofthepresentinventionisincorporatedaspartofanaerobic biological treatment system, then the sludge solubilization and volume reduction can be achieved. This idea can be appliednot only to aerobic biological treatment, but also to anaerobicbiological treatment. That is, by treating sludge, producedfrommethane fermentation, throughthe electrolytic treatment using a conductive diamond electrode, and then returning all or some of the electrolyzed water into the methane fermentation compartment, it is possible to construct a methane fermentation system according to which sludge is not produced or else the amount of sludge produced is extremely low. As described above, organic acids are produced upon treating an aqueous medium through the electrolytic treatment using a conductive diamond electrode. These organic acids act as agood substrate formethane fermentationmicrobes, and hence the amount of methane gas produced can itself be increased. Note that there is no need to return all of the treated liquid from the electrolysis treatment step 2 using a conductive diamond electrode into the methane fermentation compartment or aerobic biological treatment compartment. It is possible to return into the methane fermentation compartmentorbiologicaltreatmentcompartmentonlyanaqueoussolution that is a filtrate obtained by subjecting the electrolyzed liquid to solid/liquid separation by filtration or the like. The cell walls of themicroorganismsaredestroyedthroughtheelectrolytictreatmentusing aconductivediamondelectrode. Althoughthedigestionofthesolidshells ofthemicroorganismsbymicrobestakestime, theaqueoussolutionportion whichcontainsorganicacids, solublesugarsandsoonthatismorereadily digested through aerobic or anaerobic biological treatment.

Asanothermechanismbywhichtheefficiencyoftreatinganaqueous medium is improved through the present invention, in the case that the aqueous medium to be treated is in the form of a suspension, an emulsion or the like, relates to the change of the electric charge or the like of the emulsion through the electrolytic treatment using a conductive diamondelectrode, andhencetheflocculatingabilityandtheseparability is improved. If the aqueousmediumtobe treatedcontainswater-soluble paint, resinor the like, then thiswill flocculateupon thepHchanging. As shown by the reaction formulae (1) and (2) for the electrochemical reactionsonthesurfaceoftheconductivediamondelectrodegivenearlier, when OH radicals are, or oxygen gas is, produced at the anode through an electrolytic reaction, protons (H⁺) are produced at the same time, and hence the electrode surface becomes strongly acidic. On the other hand, OH^"isproducedatthecathode, andhencethecathodebecomesstrongly alkaline. Particlesofwater-solubleresin, paintorthelike, suspended or emulsified in a well-balanced way in the aqueous medium; upon a repeatedly contact with the strongly acidic and strongly alkaline electrodesurfaces, instantlystartstoflocculate. Inparticular, when thepHinthevicinityofanelectrodeisacidic, thentheelectriccharges ontheparticles ofwater-solubleresinorpaint suspendedintheaqueous medium are neutralized, and hence the particles no longer repel each another and thus flocculate. Furthermore, in the electrolytic treatment of the aqueous medium using a conductive diamond electrode, the pH become acidic by the production of organic acids, whereby water-soluble paint or resin may flocculate and settle. Moreover, in the case that the aqueous medium to be treatedis ina state inwhichoilandwater #re emulsifiedtogether aswithawater-solublecuttingoil, thereisaneffectinwhichbreakdown ofmicellesanddemulsificationoccurthroughtheactionofelectrolytic reactions, and the oil particles progressively bind together to form large clumps of oil. Thus an oil phase and an aqueous phase with a clear phase boundary therebetween are formed, and hence a state in which oil/water separation can occur is produced. As described above, such effects occur before the COD is completely removed in the former-stage electrolytic treatment using a conductive diamond electrode, whereby the aqueous medium comes to have properties suitable to be in the latter-stage of water treatment.

There are no particular limitations on the constitution or form of an apparatus for the electrolytic treatment step 2 using aconductive diamond electrode in the present invention. The operation of the electrolytic compartment may be in batch mode or continuous mode. In the case of thebatchmode, an anode andacathode formedfromconductive diamond are installed in a compartment into which the aqueous medium to be treated is introduced, electrolytic treatment is carried out for a certain time, and then once a prescribed COD removal rate has been obtained, the electrolyzed water is fed into the latter-stage water treatment step. Furthermore, the electrolytic treatment step 2 using a conductive diamond electrode may be used in a circulating batchmode. Inthecirculatingbatchmode, anelectrolyticcellisprovidedinseparate to the aqueous medium tank, the aqueous medium is fed from the tank into the electrolytic cell using a pump or the like, and then the treated water in the electrolytic cell is returned into the aqueous medium tank. In this case, a forced flow of the aqueous medium due to the aqueous mediumbeing fedby the pump is formedbetween the anode and the cathode in the electrolytic cell, and hence the electrolysis efficiency can be maintained better than in the case of merely disposing the electrodes immerse inside theaqueousmedium. Also in this circulatingbatchmode, the aqueousmediumcanbe fedinto the latter-stagewater treatment step when the COD concentration of the aqueous medium in the tankhas dropped down to aprescribedvalue. Moreover, with the treatment apparatus for thebatchmodeorcirculatingbatchmodedescribedabove, theelectrolytic treatment apparatus using a conductive diamond electrode is not necessarily constituted from one tank and one electrolytic cell, but rathermayhaveamulti-stageconstitution. Thatis, anapparatushaving a constitution in which after having been treated in a first-stage electrolytic compartment, the aqueous medium is fed into the next electrolytic compartment, and electrolytic treatment using a diamond electrode is similarly carried out in this electrolytic compartment as well may be adopted. With an apparatus having such a constitution, the electrolyzed water obtained from the final-stage conductive diamond electrode electrolytic compartment can then be fed into the water treatment step 4. With an apparatus having such a constitution, there is the merit that the operating conditions for the electrolytic cell of each stage can be set in accordance with the COD concentration of the aqueous medium. More efficient operation of the electrolytic treatment using a conductive diamond electrode can be carried out by setting high current density at the electrolytic compartment in which the aqueous medium has high COD concentration range, and setting low current density to the electrolytic compartment in which the treated aqueousmediumhaslowCODconcentrationrange. ThedensityofOHradicals produced on the conductive diamond electrode surface is determined by the current density, and hence the amount of OH radicals produced can beadjustedinaccordancewiththeCODconcentrationoftheaqueousmedium to be electrolyzed. If such a constitution is adopted, then production ofexcessOHradicalscanbereduced, andhencewearingawayofaconductive diamond electrode by OH radicals can be reduced, and thus the lifetime of the electrode canbe increased. Furthermore, this little production of excess OH radicals also results in savings of the electricity used in the electrolysis.

The electrolytic treatment step 2 using a conductive diamond electrode may also use a continuous mode. In the case of carrying out the electrolytic treatment in a continuous mode, a plurality of electrolytic compartments having conductive diamond electrodes are installed connectedtogether in series, andaprescribedresidence time should be secured for each electrolytic compartment. Moreover, rather than carrying out the treatment using such electrolytic compartments, the treatment can also be carried out using a filter press-type electrolytic cellinwhichanodesandcathodesareinstalledalternately in a plurality of stages as used in the soda industry. Furthermore, as the method of passing a current through each electrolytic cell, a monopolar electrode method may be used, or a bipolar electrode method may be used. In the case that a large electrode area is required, the bipolar method has the merit that the apparatus is more compact.

In the electrolytic compartment, the temperature of the aqueous mediumthat contacts the conductive diamondelectrode is preferably set to 40 to 100°C. The electrical conductivityof an aqueous mediumvaries greatly with the temperature, with the electrical conductivity being higher at higher temperatures. Moreover, the cell voltage can be kept lowbykeeping high electrical conductivity of the aqueous medium. The temperatureoftheaqueousmediumrisesthroughtheelectrolyticreactions usingthe conductivediamondelectrode, andit is preferable tomaintain this temperature as high as possible. A more preferable operating temperatureintheelectrolytictreatmentstepusingaconductivediamond electrode is 50 to 90°C, with 60 to 85°C being yet more preferable. To maintain such a temperature, it is particularly preferable to install heat exchangers at theinlet andoutlet of theelectrolytic compartment, and make it such that heat can be exchanged therebetween, whereby heat generated in the electrolytic compartment can be reused efficiently. As the method for installing the conductive diamond electrode in the electrolytic compartment, a structure in which an effective degassing is obtained is preferable adopted. If an effective degassing cannot be carried out well, then bubbles will remain between the electrodes, causinganincreaseintheinter-electrodevoltage. Itisthuspreferable to install the electrodes vertically rather than horizontally in the electrolytic compartment. But, in the case that the electrodes are installedhorizontallyintheelectrolyticcompartment, itispreferable that at least one of the cathode or the conductive diamond electrode that acts as the anodehas a structure that can facilitate thedegassing, for example in form of a mesh, a punched plate, or an expanded metal. Depending on the composition of the aqueous mediumto be treated, fine bubbles of size not more than 1 mm may be formed by gas produced in the electrolytic reactions. For example, in the case of treating ahighlyviscous aqueous medium, while the COD removal rate is low, i.e. during the initial stage of the electrolysis step, growth of bubbles willnotoccur, andhencethetreatedwatertakenoutfromtheelectrolytic compartmentmayhave finebubblesmixedtherein. In suchacase, before theelectrolyzedwateris fedinto the latter-stagewater treatment step 4, the aqueous medium (electrolyzed water) coming out from the electrolytic compartment canbe subjected to adefoaming operationwith adegasseroramechanicaldeaeratingapparatusorinanothercompartment having a defoaming apparatus provided therein. In this defoaming operation, it is also effective to use a small amount of a chemical antifoaming agent. Note that it is at a stage when the COD removal rate is low that such production of fine bubbles occurs. Although dependent on the initialviscosityof the aqueous mediumtobe treated, theproblem of foaming generally disappears once the COD removal rate in the electrolytic treatment has reached at least 5 to 15%. In an embodiment of the present invention, biological treatment canbecarriedoutas thelatter-stagewatertreatment step. Biological treatment is one of the cheapest treatment methods among the several watertreatmentprocesses, andcanbeusedtoefficientlytreatanaqueous medium in a low COD concentration range in where the treatment by electrolytic treatment using a conductive diamond electrode would be difficult. Moreover, the biodegradability of an aqueous medium is increasedgreatlythroughthe electrolytic treatment usingaconductive diamond electrode, and hence extremely efficient biological treatment becomes possible. The biological treatment step 4 in the method of the present inventionmaybeananaerobicbiologicaltreatmentstep. Inparticular, anaerobic biological treatment is preferably used for a system inwhich the COD concentration of the aqueous medium to be treated is high, and large amounts of organic acids or soluble sugars are produced through the former-stage electrolytic treatment using a conductive diamond electrode. The anaerobic biological treatmentmaybe a standard 20-day methane fermentation, or may be high-temperature methane fermentation operated at a temperature of approximately 55°C. In the case of high-temperature methane fermentation, there is a merit that the speed of the biological treatment is fast, and hence the treatment time can be reducedto 10 to 15 days. In thepresent invention, as thebiological treatment, alternatively a high-speed UASB (up flow anaerobic sludge blanket) methane fermentation in which granules (granulated lumps of methane fermentation microbes) are charged in, or a EGSB (expanded granularsludgebed)methanefermentationwhichcanbeoperatedyetfaster andwithayethigherloadcanalsobeused. If suchanaerobic treatment is carried out as the latter-stage biological treatment in the method of thepresent invention, then there aremerits suchas it beingpossible to recover energy in the form of methane gas, and the amount of sludge producedbeing low. The gas produced fromthemethane fermentation can be used directly as fuel, or can be converted into hydrogen gas through reforming and then used as an energy source for a fuel cell or the like. Hydrogen gas is of course also produced in the electrolytic treatment step using a conductive diamond electrode, and this hydrogen gas can similarly be used as an energy source for a fuel cell.

In another embodiment of the present invention, the latter-stage water treatment step may be an aerobic biological treatment. In the casethattheelectrolyzedwaterobtainedfromtheelectrolytictreatment stepusingaconductive diamondelectrodehas alowCOD; it is preferable touseaerobicbiologicaltreatmentasthelatter-stagetreatment. There are no particular limitations on aerobic biological treatment methods that canbeusedas the latter-stage treatment in thepresent invention; for example, a flotating active sludge treatment method (in which the sludge floats in an aerating compartment) which is the standard aerobic biological treatmentmaybeused, orabiofilmfiltrationmethodinwhich microbes are fixed on a membrane may be used. Furthermore, aerobic biological treatment of a type in which aerobic microbes are fixed on a carrier such as activated carbon, anthracite (coal-based carbon) or sand may be used. Moreover, as a variation of the flotation method, amethodinwhichmicrobesarefixedongranularPEG (polyethyleneglycol) oractivatedcarbonasacarriermaybeused. Alternatively, abiological contact aeration treatment inwhichmicrobes are fixedon a string-like, web-likeorhoneycomb-likecarriermaybeused. Alternatively, rotating disk type aerobic biological treatment inwhich aeration is not carried out but rather oxygen is taken in directly from the air may be used. Rotatingdisktypeaerobicbiologicaltreatmentisanaerobicbiological treatmentmethodinwhichadiskhavingspongeorthelikeattachedthereto is disposed such that the upper half thereof: is exposed out from the aqueous medium into the air, and oxygen is taken into the aqueous medium directly from the air by rotating this disk. Moreover, in another embodiment of th.e present invention, a combination of anaerobic biological treatment and aerobic biological treatment can be used for the latter-stage biological treatment step. In this case, nitrogen and phosphorus components remaining in the electrolyzedwater canbe removed. In the case of adopting this method, thereisthusnoneedtoremoveallofthenitrogenandphosphoruscomponents in the aqueous medium in the former-stage electrolytic treatment step using a conductive diamond electrode.

Next, various embodiments of the method for treating an aqueous medium according to the present invention in which former-stage electrolytic treatment using a conductive diamond electrode and any of various latter-stage water treatments are combined will be described with reference to the drawings. In the following description, several specific examples of various embodiments of "the method for treating an aqueous medium that can be used in the present invention are given, but the present invention is not limited thereto.

FIG. 13 is a flow diagram of a specific example of a method for treating an aqueous medium according to an embodiment of the present inventioninwhicharecombinedformer-stageelectrolytictreatmentusing a conductive diamond electrode, and latter-stage biological treatment comprising a combination of anaerobic treatment and aerobic treatment. The embodiment shown in FIG. 13 can be preferably used in the case that theaqueousmediumtobetreatedisapoorlybiodegradableaqueousmedium, and contains a lot of ammonia and nitrate nitrogen but does not contain many chloride ions. For example, the embodiment shown in FIG. 13 can bepreferablyusedinthecaseof treatingapoorlybiodegradableaqueous mediumcontaining3to3000mg/Lofammoniaandnitratenitrogencomponents as total nitrogen. In the case that the nitrogen concentration in the aqueousmediumishigherthanthis, itispreferabletocarryoutnitrogen removal in the electrolytic treatment step using a conductive diamond electrode by adding chloride ions to the aqueous medium before the electrolytictreatmentusingaconductivediamondelectrode. Thepoorly biodegradableaqueousmedium1issubjectedtotheelectrolytictreatment in the former-stage electrolytic treatment step 2 using a conductive diamond electrode. In this electrolytic treatment step 2 using a conductivediamondelectrode, itispreferabletocarryout thetreatment after mixing surplus sludge 15 produced in a latter-stage aerobic biological treatment step 10 into the aqueous medium 1 to be treated. By treating the surplus sludge 15 through the electrolytic treatment using a conductive diamond electrode, it becomes possible to construct awatertreatmentprocessaccordingtowhichorganicsludgeisnotproduced or else the amount of sludge produced is extremely low. Note that, it is important to not decompose completely the COD components in the electrolytic treatment step 2 using a conductive diamond electrode; to keepdownthe costforwholetreatmentprocess. Moreover, iftheaqueous medium contains few chloride ions, then ammonia nitrogen contained in the poorly biodegradable aqueous medium 1 is converted into nitrate nitrogen in the electrolytic treatment step 2 usingaconductive diamond electrode. Next, the electrolyzed water 3 from the electrolytic treatmentstep2usingaconductivediamondelectrodeisfedintoasettling basin 6, where solid matter 8 is removed. Note that in the present embodiment, the settling basin 6 is shown as the means for carrying out solid/liquidseparation, butanyofvariousothersolid/liquidseparation methods may be used, for example a membrane filtration method, a sand filtration method, a filter press method, a belt press method, or a flocculating settling separationmethod. Solidmaterialin the aqueous medium 1 that has not been decomposed in the electrolytic treatment step 2usingaconductivediamondelectrode, cellwallsinundecomposedsludge, andsoonareremovedinthesettlingbasin 6. The solidmattercontained intheelectrolyzedwater3isnotsolidmatterhavingpoordewaterability such as with surplus organic sludge from active sludge, but rather is non-slimysolidmatter, andhencecanbereadilyremovedthroughsettling treatment, filtration treatment or the like. Next, the supernatant 7 in the settling basin is fed into an oxygen-free, i.e. anaerobic, biological treatment compartment 9. Although not shown in FIG.13, the supernatant 7 may be fed into the anaerobic biological treatment compartment9afterbeingmixedwithanotherreadilydecomposableaqueous medium. The biological treatment is in the latter stage, and hence it is undesirable to add a poorly biodegradable aqueous medium to the supernatant 7; apoorlybiodegradableaqueousmediumispreferablyfirst subjected to the electrolytic treatment using a conductive diamond electrode. Nitrate nitrogen in the aqueous medium is converted into nitrogen gas in the anaerobic biological treatment compartment 9. In theanaerobicbiologicaltreatment compartment 9, denitrifyingbacteria absorb nitrate nitrogen as a hydrogen acceptor instead of oxygen, and reduce nitrate nitrogen ornitrite nitrogen to nitrogen gas. Note that ammonianitrogenisnot decomposedhere. Onthecontrary, if theaqueous mediumcontains protein nitrogen or the like, the ammonia concentration may increase in the anaerobic biological treatment compartment 9. Moreover, the denitrifying bacteria use organic matter comprising BOD components as ahydrogendonor, andhence BOD components suchas organic acids that have been produced through the electrolytic treatment using a conductive diamond electrode are used in the conversion of nitrate nitrogen and nitrite nitrogen into nitrogen gas in the anaerobic biological treatment compartment 9. Ammonia nitrogen, which is not decomposed in the anaerobic biological treatment compartment 9, is converted into nitrate nitrogen and nitrite nitrogen in a latter-stage aerobicbiologicaltreatmentcompartment 10. Inthis stepofconverting theammonianitrogenintonitratenitrogenandnitritenitrogen, nitrite bacteria (Nitrosomonas etc.), which are aerobes, convert the ammonia nitrogen into nitritenitrogen, andthennitratebacteria (Nitrobacter) present in the aerobic biological treatment compartment 10 convert the nitrite nitrogen into nitrate nitrogen. "B" in FIG. 13 is an aerating pump that blows air into the aerobic biological treatment compartment 10. Thedigestedliquidcontainingnitratenitrogenandnitritenitrogen is fed back via a circulating line 11 into the anaerobic biological treatment compartment 9, wherethenitratenitrogenandnitritenitrogen is converted into nitrogen gas. By adjusting the proportion of the digested,liquidthat is circulatedthroughthe line 11 andtheproportion of sludge that is fed back into the anaerobic biological treatment compartment 9 through a line 14, the proportion of all nitrogen removed in the biological treatment process 4 can be controlled. Water 12 discharged from the aerobic biological treatment compartment 10 has surplus sludge removed therefrom in a settling basin 13, and is then recovered as treated water 5. The surplus sludge is fed back into the anaerobic biological treatment compartment 9 via the line 14 and into the electrolytic treatment compartment 2 using a conductive diamond electrode via a line 15. In the above, description has been given only for removal of nitrogen, but BOD components such as organic matter are alsodecomposedinthebiologicaltreatment step 4, andhencethetreated water 5 is wellpurified. Note that inFIG.13, aconstitutionwas shown inwhichoneanaerobicbiologicaltreatmentcompartment 9 andoneaerobic biologicaltreatmentcompartment 10areused, but, forexample, two-stage biologicaldenitrificationtreatmentusingafirstanaerobiccompartment, afirstaerobiccompartment, asecondanaerobiccompartment, andasecond aerobiccompartmentinthisordermayinsteadbeadoptedasthebiological treatment step 4. Furthermore, methanol may be added to the second anaerobic compartment so as to increase the denitrification rate.

Embodiments of thepresent invention inwhichadiamondelectrode treatmentstepandotherlatter-stagewatertreatmentmethodsarecombined will now be further described. As a method for a latter-stage water treatment stepcarriedout aftercarryingout former-stageelectrolytic treatment using a diamond electrode, flocculating settling treatment is useful as amethod of removing phosphorus, andmoreover SS, some COD, colorand so onat the same time. In thepresent invention, complementary effect to the former-stage electrolytic treatment using a diamond electrode, which is a partial treatment, can be expected and thus such a method is extremely effective for treating an aqueous medium.

As methods of carrying out the flocculating settling step, there areamethodinwhichflocculationtreatmentiscarriedoutusingasettling basinoraflotationseparator, amethodinwhichaflocculantis directly added into a biological treatment (active sludge method) aerating compartment, a method in which flocculating settling is carried out as advanced water treatment after the biological treatment, a method in which a flocculant is added before a filtration step also as a advanced water treatment, and so on. As the flocculant used in suchflocculation treatment, an iron salt (iron (III) chloride, iron (III) sulfate, etc.), an aluminum salt (aluminum sulfate, PAC, etc. ) , or a calcium salt (lime, etc. ) can be preferablyused. For each flocculant, there is an optimum pH for the flocculation reaction, and the pH may be adjusted by adding a necessary amount of alkali or acid solution. Moreover, polymer flocculants may be added to increase the size of the produced floes.

Besidesthephosphorusremoval, inthecasethattheaqueousmedium is waste liquid from plating or waste water from the chemical industry or the like and contains large amount of metal ions, if a flocculating settling treatment step is usedas pretreatment before the electrolytic treatment stepusingadiamondelectrode, thenthedepositionofametal, salts or the like on the electrodes can be avoided, and hence this is another useful embodd-ment of the present invention.

FIGS. 14 to 17 are flow diagrams of specific examples of methods for treating an aqueous medium according to embodiments of the present invention in which former-stage electrolytic treatment using a diamond electrodeandlatter-stageflocculatingsettlingtreatmentarecombined; such an embodiment can be preferably used in the case that the aqueous mediumtobetreatedisapoorlybiodegradableaqueousmediumandcontains a lot of phosphorus.

FIG.14isaflowdiagramshowingaspecificexampleofanembodiment of the present invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage flocculating settling treatment are combined. The poorly biodegradable aqueous medium 1 is subjected to electrolytic treatment in the former-stage electrolytic treatmentstep2usingadiamondelectrode. Next, theelectrolyzedwater 3 produced through this treatment is fed into a stirring compartment 16, and a flocculant 17 is injected in while stirring and thus mixed with the electrolyzed water 3, and then the mixture is fed into a flocculating settling basin 18, and separated into treated water 5 and solidmatter8. Dependingonthepropertiesofthefloes, theflocculating settling basin 18 may be a flotation separation compartment. FIG. 15 is a flow diagram showing a specific example of a method for treatinganaqueousmediumaccording to an embodiment of thepresent invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage biological treatment and flocculating settling treatment are combined. The poorly biodegradable aqueous medium 1 is subjected to electrolytic treatment in the former-stage electrolytic treatment step 2 using a diamond electrode. Next, the electrolyzed water 3 produced through this treatment is fed into a latter-stage aerobic biological treatment step 10, and a flocculant 17 is injectedinwhile aerating andthusmixedwiththe electrolyzedwater 3. Treated water 12 that has been subjected to the aerobic biological treatmentisthenfedintoaflocculatingsettlingbasin18, andseparated into treated water 5 and solid matter 8. Some of the solid matter 8 may bereturnedintotheaerobicbiologicaltreatmentstepasfed-backsludge 14. Moreover, an anaerobic biological treatment step may be added as a step before the latter-stage aerobic biological treatment step 10, whereby the more high level of biological treatment can be achieved.

FIG. 16 is a flow diagram showing another specific example of a method for treating an aqueous medium according to an embodiment of thepresentinventioninwhichformer-stageelectrolytictreatmentusing a diamond electrode and latter-stage biological treatment and flocculatingsettlingtreatmentarecombined. Thepoorlybiodegradable aqueousmedium1 is subjectedto theformer-stageelectrolytictreatment step2usingadiamondelectrode. Next, theelectrolyzedwater3produced through this treatment is fed into a latter-stage aerobic biological treatmentstep10. Treatedwater12thathasbeensubjectedtotheaerobic biological treatment is then fed into a stirring compartment 16, and a flocculant 17 is injected in and thus mixed with the treated water 12, and then the mixture is fed into a flocculating settling basin 18, and separated into treated water 5 and solid matter 8. Moreover, an anaerobic biological treatment step may be added as a step before the latter-stage aerobic biological treatment step 10, whereby more high levelofthebiologicaltreatmentcanbeachieved, andmoreoverasettling basin may be added as a subsequent step, with some of the sludge being fed back into the biological treatment step and/or being added into the electrolytic treatment step 2.

FIG. 17 is a flow diagram showing a specific example of a method for treating anaqueousmediumaccording to anembodiment of thepresent invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage flocculating settling treatment and filtration treatment are combined. The poorly biodegradable aqueous medium 1 is subjected to electrolytic treatment in the former-stage electrolytic treatment step 2 using a diamond electrode. Next, the electrolyzedwater3producedthroughthistreatmentisfedintoastirring compartment 16, and a flocculant 17 is injected in while stirring and thus mixed with the electrolyzed water 3, and then the mixture is fed into a flocculating settling basin 18, and separated into flocculating separation-treated water 19 and solid matter 8. The flocculating separation-treated water 19 is fed into a filtration compartment 20, fromwhich treated water 5 is obtained. A sand filtration compartment 20 is preferable as the filtration compartment, but anyofvarious types of filtration may be used as necessary.

FIG.18showstheflowofanotherembodimentofthepresentinvention in which a diamond electrode electrolytic treatment step and a latter-stagephosphorusremovalstepusingcrystallizationareprovided. Inthis flow, thephosphorusremovalstepisprovidedinthelatterstage, but in the case that the phosphorus concentration in the aqueous medium is extremely high at a few thousands mg/L, a phosphorus removal step may also be provided in a former stage, with phosphorus removal being carried out in advance to an extent that deposition of phosphorus will not occur in the piping of the diamond electrode apparatus or on the electrodes. As a method for a latter-stage water treatment step after carrying out former-stage electrolytic treatment using a diamond electrode, dephosphorizationbycrystallization is extremelyeffective in the present invention. Phosphorus can be recovered as a phosphorus resource without producing sludge once the electrolytic treatment step using a diamond electrode promotes the crystallization by reducing the amount of organic matter and other which would interfere the crystallization. As types of crystals that can be used in the dephosphorization by crystallization, there are HAP (hydroxyapatite. Ca₅(OH) (PO₄)₃) and MAP (magnesium ammonium phosphate, Mg(NH₄)PO₄). One of these types of crystals is selected in accordance with the balance of the components and ions containedin the aqueous medium. With either of these types of crystals, ionic species that are lacking are added and the pH is adjusted, and then the phosphorus removal is carried out in a dephosphorization column, which acts as the crystallization part, thusobtainingtreatedwater. FIG.18isaflowdiagramshowingaspecific example of a method for treating an aqueous medium according to an embodiment of the present invention in which former-stage electrolytic treatment using a diamond electrode and latter-stage dephosphorization treatment by crystallization are combined. This can be preferablyused inthecasethattheaqueousmediumtobetreatedisapoorlybiodegradable aqueous medium, and the electrolyzed water 3 contains at least 50 mg/L ofphosphorus (PO₃-P) . Thepoorlydegradableaqueousmedium1 is treated intheformer-stageelectrolytictreatmentstepusingadiamondelectrode, and the electrolyzed water 3 is then fed into a stirring compartment 16, where an adjusting chemical liquid 21 (a chemical liquid containing apHadjustor, andionicspeciesthatarelacking) isadded, thusadjusting to conditions required for the crystallization. The resulting crystallization-adjusted treated water 22 is then fed into the dephosphorization column 23, and phosphorus is removed through growth of the crystals (HAP or MAP) in the dephosphorization column 23. Phosphorus-containing crystallized matter 24 is taken out from the dephosphorization column 23, whereby treatedwater 5 is obtained. The phosphorus-containingcrystallizedmatter24isvaluableasaphosphorus resource, and can be used as a fertilizer or mixed into a fertilizer. Various embodiments of the present invention are as follows. 1. A method for treating an aqueous medium, which comprises subjecting an aqueous medium to an electrolytic step using a conductive diamond electrode, and then to a water treatment step.

2. The method for treating an aqueous medium according to above item 1, wherein the latter-stage water treatment step comprises one of biological treatment, flocculating settling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, and crystallization treatment.

3. The method for treating an aqueous medium according to above item 1, wherein the latter-stage water treatment step comprises a combination of biological treatment, and one or at least two of flocculating settling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, and crystallization treatment.

4. The method for treating an aqueous medium according to any ofaboveitems 1 through3, whereinthebiologicaltreatment is anaerobic biological treatment or aerobic biological treatment.

5. The method for treating an aqueous medium according to any of above items 1 through 4, wherein the former-stage electrolytic step is stopped in a state in which COD components have not been completely decomposed, and then the latter-stage water treatment step is carried out.

6. The method for treating an aqueous medium according to any of above items 1 through 5, wherein the aqueous medium to be treated is a poorly biodegradable aqueous medium. 7. The method for treating an aqueous medium according to above item 6, wherein through the former-stage electrolytic step, BOD/COD of the aqueous medium is made to be at least 0.2.

8. The method for treating an aqueous medium according to any of above items 1 through 7, wherein an electrical charge of not more than 34 Ah is applied in the former-stage electrolytic step per gram of COD in the aqueous medium.

9. An apparatus for treating an aqueous medium, which comprises an electrolytic compartment using a conductive diamond electrode into which an aqueous medium to be treated is introduced so as to carry out electrolytic treatment thereon, and at least one water treatment compartment into which electrolyzed water that has been treated in the electrolyticcompartment is introducedsoas tocarryoutwatertreatment thereon.

10. The apparatus for treating an aqueous medium according to above item 9, wherein the at least one latter-stage water treatment compartment comprises one of a biological treatment compartment, a flocculating settling treatment compartment, a filtration treatment compartment, an oil/water separation treatment compartment, an adsorption treatment compartment, and a crystallization treatment compartment.

11. The apparatus for treating an aqueous medium according to above item 9, wherein the at least one latter-stage water treatment compartmentcomprisesacombinationofabiologicaltreatmentcompartment, andoneorat least twoofaflocculatingsettlingtreatmentcompartment, a filtration treatment compartment, an oil/water separation treatment compartment, anadsorptiontreatmentcompartment, andacrystallization treatment compartment. 12. The apparatus for treating an aqueous medium according to any of above items 9 through 11, wherein the biological treatment compartment is constituted from an anaerobic biological treatment compartment or an aerobic biological treatment compartment or a combination thereof. [Examples] Example 1

Inthepresentexample, anaqueousmediumdischargedfromafactory A having a water quality as shown in below Table 1 was treated through electrolytic treatment using a conductive diamond electrode.

Table 1: Water quality of aqueous medium treated in Example 1

The aqueous medium to be treated had a high COD of 10,600 mg/L, andBOD/CODof 15.3% andthus lowbiodegradability, andhencewasapoorly biodegradable aqueous medium that would be difficult to handle using ordinary biological treatment. Taking 3 L of this aqueous medium as asample liquid, anelectrolytictreatment experimentusingaconductive diamond electrode was carried out under electrolytic conditions of a current density of 140 mA/cm² and an average cell voltage of 7.5 V. The temperatureoftheaqueousmediumduringtheelectrolytictreatmentrose to 46⁰C due to heat generated through the electrode reactions. In the electrolytic cell, the anode was a conductive diamond electrode, a titanium plate was installed as the cathode, the electrode gap was 2 mm, and a separating membrane was not disposed between the electrodes. The diamond electrodewas one obtainedby depositing conductive diamond on a 6-inch silicon wafer substrate using hot filament CVD. The electrolytic treatment was carried out using circulating batch mode; the sample liquid was fed into the electrolytic cell by a pump from a sealed reservoir of total volume 5 L, and the liquid discharged from the electrolytic cell was returned into the reservoir tank. Air in the head space of the reservoir tank was replaced with inert argon gas in advance, and gas in the reservoir tankproducedduring the electrolysis was periodically released, and subjected to quantitative analysis by gas chromatography. The results are shown in FIGS.4, 5, 6 and 7. FIG. 4isagraphshowingtherelationshipbetweentheelectricalchargeapplied per 1 g of COD contained in the sample liquid (horizontal axis) and the COD and BOD of the sample liquid (vertical axis). While the COD of the sample liquid is still above approximately 2000 mg/L, the COD decomposition proceeds at a current efficiency close to 100%, as shown in FIG. 4. The dashed line in FIG. 4 is a straight line showing the reduction in the COD for the case that the current efficiency is 100%. However, once the COD becomes 2000 mg/L or less, mass transfer of the COD components in the aqueous medium starts to become rate-limiting, and hence the COD concentration reduction curve deviates away from the dashed line in FIG. 4. Moreover, whereas the BOD of the sample liquid was 1630 mg/L initially, the BOD became 2.8 times higher at 4600 mg/L when an electrical charge of 1 Ah/g-COD had been applied. That is, the BOD concentration in the aqueous medium was highest when approximately 29% of the theoretical electrical charge had been applied. At an electricalchargeabovethis, theBODconcentrationdroppedagain. FIG. 5 shows the relationship between the COD removal rate and BOD/COD for the present example. BOD/COD is the value obtained by dividing BOD₅ by CODc_r and expressed in percent. The biodegradability of the COD components contained in the aqueous medium is higher, as higher as is this value. Compared to the BOD/COD of the raw water that was 15.4%, BOD/COD reached 50% when the COD removal rate was 15%, and BOD/COD was above 60% for a COD removal rate from 30% to 75%. Once the COD removal rate exceeded 95% in the electrolytic treatment using the conductive diamond electrode, BOD/COD conversely became lower than the value of therawwater. Inthecaseofcarryingoutanaerobicbiologicaltreatment asthelatter-stagewatertreatment, itispreferabletoletCODcomponents remaining to increase the amount of methane gas recovery, and hence in the former-stage electrolytic treatment using the conductive diamond electrode; it is preferable to stop at a COD removal rate between 10 and 60%. Moreover, in the case of carrying out aerobic biological treatment as the latter-stage water treatment, it is preferable for the COD to be low, and hence the COD removal rate in the former-stage electrolytic treatment using the conductive diamond electrode is preferably made to be higher, for example 30 to 95%. FIG. 6 shows the relationship between the electrical charge inputted and organic acid production for the present example. The most produced organic acid in the electrolytic treatment using the conductive diamond electrode was formicacid, followedbylacticacid, aceticacid, andthenasmallamount of propionic acid. The total concentration of these organic acids is shown in FIG. 6 as the VFA (volatile fatty acid) concentration. The increase of biodegradability of the aqueous medium through the electrolytic treatment using the conductive diamond electrode is due to this VFA production. Note that in the present example, the VFA concentrationwashighestwhenanelectricalchargeof 1.8Ah/g-COD (i.e. approximately53%ofthetheoreticalelectricalcharge) hadbeenapplied. At anelectricalchargeabove this, theVFAis furtherdecomposedthrough the electrolytic treatment using the conductive diamond electrode, and hence the concentration thereof drops. VFA is produced in the electrolytic treatment using the conductive diamond electrode due to partial oxidation of organic matter. If further electrical charge continues to be applied in the electrolytic treatment until the COD is completelydecomposed, thentheVFAwillbeconvertedintocarbondioxide and water. FIG. 7 shows the concentration composition of the produced gas in the present example. It can be seen that the principal component of the gas produced through the electrolytic reactions using the conductivediamondelectrodeishydrogengas, followedbycarbondioxide. Moreover, when an electrical charge exceeding 3.2 Ah/g-COD had been applied, i.e. whenanelectricalchargeexceeding 94% of thetheoretical electrical charge had been applied, oxygen gas started to be produced. This was because the COD of the aqueous medium became low andhence mass transfer of the COD components onto the electrode surface started to become the rate-limiting, and thus oxidation of water which produces oxygenstartedtooccurinsteadofthedecompositionoftheCODcomponents. In this way, hydrogen gas and carbon dioxide are producedwhile the COD removal rate is low and there is no problem. But once the COD removal ratebecomeshigh, hydrogengasandoxygengasareproducedsimultaneously, and in this case there is a danger for hydrogen explosion.

Electrolyzed water having a COD removal rate between 15% and 95% obtained through the present example can be sufficiently handled by ordinary water treatment such as biological treatment. Example 2

To show that electrolyzedwaterhaving a COD removal rate between 15% and 95% obtained in Example 1 had been converted into water that could be sufficiently handled by ordinary water treatment such as biological treatment, electrolyzed water to which had been applied a electrical charge of 1.8 Ah/g-COD in Example 1 (i.e. the electrolyzed water when the VFA had reached a maximum, having a COD removal rate of 55%) was used as a sample, and anaerobic biological treatment (methane fermentation) was carried out. Moreover, as a comparative experiment, a methane fermentation test was also carried out using, as a sample, the raw waste water from factory in which the water quality is shown in Table 1. Adjustment was carriedout such the totalvolume of methane fermentation seeding sludge and the respective sample was 40 mL, and this was sealed in a 100 mL vial together with an inert gas. Moreover, ablank specimen obtainedby filling only the seeding sludge into avial was also prepared. Each vial Was put into a vibrating constant-temperature tank at 55°C, and a high-temperature methane fermentation test was carried out. The results are shown in FIG. 8. The vertical axis in FIG. 8 shows the amount of methane gas produced per 1 g of COD contained in the sample (with the amount of methane gas producedfortheseedingsludge(blank)havingbeensubtractedtherefrom) . In the case that the untreated raw water was subjected to the methane fermentation, evenuponcontinuingthemethanefermentationfor 14 days, only a very small amount of methane was produced, although the amount of methane produced was seen to increase gradually. Note that in this methane fermentation test on the untreated rawwater, for the first two days, a phenomenon was observed in which upon subtracting the amount ofmethane producedfor the blank, anegative amount ofmethane produced wasobtained. Itisthoughtthatthisisbecausesomecomponentcontained in theuntreatedwaterinhibitedthemethane fermentation. Onthe other hand, in the case of carrying out the methane fermentation treatment ontheelectrolyzedwaterobtainedfromtheelectrolytictreatmentusing the conductive diamond electrode, the amount of methane gas produced was not seen to increase from the seventh day onwards, but the amount of methane gas produced was higher than in the comparative experiment. That is, this shows that in the case of the electrolyzedwater obtained throughtheelectrolytictreatmentusingtheconductivediamondelectrode, because VFA's account for the majority of the contained COD components, themethanefermentationproceededatahighrate, andhencewascompleted within approximately 7 days. Example 3

In the present example, the used water sample was prepared by biologically treating night soil and concentrating, and that samplewas treated through electrolytic treatment using a conductive diamond electrode. The water to be treated was a poorly biodegradable aqueous medium having a water quality as shown in Table 2 below.

Table 2: Water quality of aqueous medium treated in Example 3

The COD concentration of the water sample was 1,300 mg/L and not so high, but the BOD was low at 80 mg/L, and hence the biodegradability wasextremelylow(BOD/COD: 6%) . Furthermore, thewatersamplecontained a large amount of a colored component thought to originate from bile (color: 6,200) , and this colored component could hardly be removed at all by biological treatment. The water to be treated also contained approximately 300 mg/L of ammonia nitrogen. Taking 3 L of this poorly biodegradable aqueous medium as a sample, an electrolytic treatment experiment using a conductive diamond electrode was carried out under conditions of a current density of 40 mA/cm² and an average cell voltage of 4.8V. The temperature of the aqueous mediumduring the electrolytic treatment rose to 40⁰C due to heat generated through electrolytic reactions. The constitution of the electrolytic cell andthe treatment operationusedwere the same as in Example 1. The results of the diamond electrolytic treatment are shown in FIGS. 9, 10 and 11. As can be seen from FIG. 9, the initial COD of the water sample was 1,300 mg/L, and because that is in the range in which mass transfer is rate-limiting, the initial current efficiencywas approximately 56% only. The current efficiency dropped as the COD of the aqueous medium dropped, and the current efficiency was seen to further drop when an electrical charge exceedingthetheoreticalelectricalchargehadbeenapplied. Regarding the BOD, the highest value was exhibited when an electrical charge of 1.3Ah/g-COD (38% of the theoreticalelectricalcharge) hadbeenapplied (COD removal rate 22%). BOD/COD at this time was 39%, which was 6.5 times higher than BOD/COD of the raw water. Regarding the color, the colordroppedmarkedlyas theelectricalchargeinputtedincreased, with virtually 100% having been removed when an electrical charge of 2.6 Ah/g-COD (76% of the theoretical electrical charge) had been inputted. The percentage of COD removed at this timewas 56%, andhence it is clear that to remove the color, it is not necessary to make 100% COD removal rate. TherelationshipbetweentheCODremovalrateandthecolorremoval rate was calculated from the data of FIG. 9, and is plotted in FIG. 10. FromFIG.10, itisagainclearthattoremovethecolor, itisnotnecessary tomake 100% COD removal rate. Moreover, FIG.11 shows the relationship between the concentration of ammonia nitrogen and the COD removal rate for the present example. Virtually 100% of the ammonia nitrogen had been decomposed when the COD removal rate was 80%. It is thought that this is because the raw water contained chloride ions, and hence some of the chloride ions were converted into hypochlorous acid through an electrolyticreactionattheconductivediamondelectrode, andtheammonia nitrogen was decomposed by hypochlorous acid through the break-point reaction.

When the COD removal rate had reached 22%, the electrolyzedwater was diluted with water by a factor of ten, and then aerobic biological treatmentwas carriedout. Fortheaerobicbiologicaltreatment, atest was carried out using an activated sludge method at a load of 0.2 g-BOD/g-VSS-sludge^•day. ThefinaltreatedwaterhadaCODconcentration of 10 mg/L or less, and a color of 2. Example 4

In the present example, sludge from amethane fermentation plant that had been obtained by treating night soil and kitchen garbage was taken as sample, and that was subjected to electrolytic treatment using a conductive diamond electrode. The liquid to be treated was a poorly degradable aqueous medium having a water quality as shown in the below Table 3. Table 3: Water quality of aqueous medium treated in Example 4

Themethanefermentationsludgewasaresiduethathadbeenalready subjected to anaerobic biological treatment, and hence had a low biodegradability (BOD/COD: 8.8%), and was constituted from microbes containing a lot of water inside the microorganisms cell, and moreover had a relatively high viscosity of 195 mPa*s, and hence was an aqueous medium having poor dewaterability. The T-COD (COD including solid matter) concentration was relatively high at 11,100 mg/L, and approximately90% ofthis CODwas not soluble, butrathercomprisedsolid COD components originatingmainly fromthemicrobe biomass. 7,290 mg/L of this solid matter was contained as MLSS (moisture liquor suspended solids) . Taking 3 L of this aqueous medium as a sample, an electrolytic treatment experjunent using a conductive diamond electrode was carried out under conditions of a current density of 120 mA/cm² and an average cell voltage of 13.5 V. The temperature of the aqueous medium during the electrolytic treatment rose to approximately 55°C due to heat generated through electrolytic reactions. The constitution of the electrolytic cell and the treatment operation used were the same as in Example 1. The electrolytic treatment results of the methane fermentation sludge are shown in FIG. 12. Looking at the trend of the drop intheT-COD, it canbe seen that decompositionof theCODproceeded at acurrentefficiencycloseto 100%upto thetimeatwhichanelectrical charge of 1.85 Ah/g-COD (54% of the theoretical electrical charge) had been applied. The COD removal rate at this time was 48%. It can be seen, however, that the efficiency of the COD decomposition dropped markedly when the electrical charge inputted was above this value. Although not shown in FIG. 12, it was necessary to input an electrical charge of at least 276 Ah/g-COD (81 times the theoretical electrical charge) to obtain a COD removal rate of 96% or more. From this value, it can be calculated that the amount of electricity required to treat 1 m³ of the methane fermentation sludge was at least 2,060 kWh. It is clear that to completely decompose the COD through the electrolytic treatment using the conductive diamond electrode, a large quantity of electricity must be consumed, and hence good cost performance is not obtained. It is thought that the reason why the current efficiency for theCODdecompositionishighduringtheinitialstageoftheelectrolytic treatmentisduetothatthefactthatthereisapreferentialdecomposition of components that increase the viscosity or soluble COD components readily decomposed by the electrolytic treatment using the conductive diamondelectrode. Moreover, whenanelectricalchargeof 1.85Ah/g-COD had been applied, the MLSS was 3,760 mg/L (SS removal rate 52%), and the viscosity was 43 mPa'S (viscosity reduction rate 77%) , and the COD removal rate at this time was 48%. The amount of electricity used in the electrolysis up to this time was 138 kWh/m³-sludge, which was 1/15 of the amount of electricity required to obtain a COD removal rate of 96%. Moreover, because the MLSS dropped, it is thought that through this electrolytic treatment using the conductive diamond electrode, OH radicals attacked the cell walls of the microorganisms, breaking the cellmembranes, andbringingaboutareductioninthevolumeoftheorganic sludge. Ifthecellwallsarebrokenandthecontentsofthemicroorganisms dissolve out into the aqueous medium, then these contents of the microorganisms will also in turn be decomposed by the electrolytic treatment using the conductive diamond electrode. Note that the efficiency of decomposition by the electrolytic treatment using the conductive diamond electrode is poor for the remaining hard cell walls• Specifically, if one attempts to oxidize this solid cell wall component on the electrode surface, then problems with the efficiency of contact and so on will arise. It is thought that this is why the drop in the MLSS is sluggish when an electrical charge of more than 1.85 Ah/g-COD is applied.

Electrolyzedwater when the COD removal rate had reached 48% was filtered with a 0.45 μm membrane filter. The raw sludge was similarly subjected to such a filtration test. With the raw sludge, clogging of the filter soon occurred and hence filtration could hardly be carried out at all, whereas the electrolyzed water having a COD removal rate of 48% couldbe easily filtered. Moreover, the solids in the raw sludge were very slimy, whereas the residual solid matter after filtering the electrolyzed water having a COD removal rate of 48% was relatively non-slimy.

INDUSTRIAL APPLICABILITY

Amethodfortreatinganaqueousmediumis providedin thepresent invention. In the present invention, electrolytic treatment using a conductive diamond electrode is carried out in a former stage, and then awater treatment step comprising at least one of biological treatment, flocculating settling treatment, filtration treatment, oil/water separationtreatment, adsorptiontreatment, crystallizationtreatment, and so on is carriedout in alatter stage, wherebythe overall treatment processbecomesefficientandcheap. Accordingtothepresentinvention, in particular poorly biodegradable aqueous media can be treated efficiently. In the process of the present invention, the electrolytic treatment step using a conductive diamond electrode is responsible for onlyapart of the treatment process inwhere the electrolytic treatment is good, and the latter-stage water treatment step is responsible for a part of the treatment process in where the biological treatment for example is good; the overall treatment of the aqueous medium thus goes smoothly. Furthermore, according to the present invention, the durability of the conductive diamondelectrode can be greatly improved.

Claims

1. A method for treating an aqueous medium, which comprises subjecting an aqueous medium to an electrolytic step using a conductive diamond electrode, and then to a water treatment step.

2. Themethodfortreatinganaqueousmediumaccordingtoclaim 1, wherein the latter-stage water treatment step comprises one of biological treatment, flocculating settling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, and crystallization treatment.

3. Themethodfortreatinganaqueousmediumaccordingtoclaim 1, whereinthelatter-stagewatertreatment stepcomprises acombination ofbiologicaltreatment, andoneorat leasttwoofflocculatingsettling treatment, filtration treatment, oil/water separation treatment, adsorption treatment, and crystallization treatment.

4. Themethodfor treating an aqueous mediumaccording to any of clams 1 through 3, wherein the biological treatment is anaerobic biological treatment or aerobic biological treatment.

5. Themethodfor treating an aqueous medium according to any ofclams1through4,whereintheformer-stageelectrolyticstepisstopped in a state in which COD components have not been completely decomposed, and then the latter-stage water treatment step is carried out.

6. Themethodfor treating an aqueous mediumaccording to any of clams 1 through5, wherein the aqueous mediumtobe treatedis apoorly biodegradable aqueous medium.

7. Themethodfortreatinganaqueousmediumaccordingtoclaim 6, wherein through the former-stage electrolytic step, BOD/COD of the aqueous medium is made to be at least 0.2.

8. Themethod for treating an aqueous mediumaccording to any of clams 1 through 7, wherein an electrical charge of not more than 34 Ah is applied in the former-stage electrolytic step per gram of COD in the aqueous medium.

9. Anapparatusfortreatinganaqueousmedium,whichcomprises an electrolytic compartment using a conductive diamond electrode into which an aqueous medium to be treated is introduced so as to carry out electrolytic treatment thereon, and at least one water treatment compartment into which electrolyzed water that has been treated in the electrolyticcompartmentisintroducedsoastocarryoutwatertreatment thereon.

10. The apparatus for treating an aqueous medium according to claim9, whereinthe latter-stagewatertreatment compartment comprises one of a biological treatment compartment, a flocculating settling treatmentcompartment, afiltrationtreatmentcompartment, anoil/water separation treatment compartment, anadsorptiontreatment compartment, and a crystallization treatment compartment.

11. The apparatus for treating an aqueous medium according to claim 9, whereinthe latter-stagewater treatment compartment comprises a combination of abiological treatment compartment, andone or at least two of a flocculating settling treatment compartment, a filtration treatment compartment, an oil/water separation treatment compartment, an adsorption treatment compartment, and a crystallization treatment compartment.

12. The apparatus for treating an aqueous medium according to anyof claims 9 through 11, wherein thebiological treatment compartment is constituted from an anaerobic biological treatment compartment or an aerobic biological treatment compartment or a combination thereof.