EP0652851A1

EP0652851A1 - Electrochemical treatment process and device for calcium- and/or magnesium-containing water or waste water

Info

Publication number: EP0652851A1
Application number: EP94917668A
Authority: EP
Inventors: Friedrich Schwaiger
Original assignee: PHOSTRIP-ABWASSER-TECHNIK GmbH
Current assignee: PHOSTRIP-ABWASSER-TECHNIK GmbH
Priority date: 1993-06-01
Filing date: 1994-05-26
Publication date: 1995-05-17
Also published as: DE9408658U1; WO1994027915A1; AU6929894A

Abstract

A process and device are disclosed for electrochemically treating calcium- and/or magnesium-containing water or waste water, in particular communal and/or industrial waste water which is chemically and/or biologically treated by means of micro-organisms in order to eliminate phosphorus and/or nitrogen. At least two electrodes are introduced in the phosphorus- and/or nitrogen-containing water to be purified and are connected to a direct current souce. An electrochemical treatment is then applied in the presence of an oxidising agent while current or voltage are kept constant.

Description

Method and device for the electrochemical treatment of water containing calcium and / or magnesium or waste water

The invention relates to a method and a device for the electrochemical treatment of calcium- and / or magnesium-containing water or wastewater, in particular municipal and / or commercial wastewater, which is chemically and / or biologically using microorganisms for the purpose of Phosphorus and / or nitrogen elimination is treated; It relates in particular to a method and a device for the electrochemical and / or microbiological elimination of phosphorus and / or nitrogen from water, in particular municipal or commercial waste water, which in a pretreatment system or in a sewage treatment plant, a cleaning process (preferably a biological one) Cleaning).

The phosphorus and nitrogen content of the water, in particular the municipal and commercial waste water, forms the nutrient basis for an often uncontrollable algae wax Turn, so that great importance must be attached to the extensive elimination of the phosphorus and / or nitrogen contents in each wastewater treatment in order to relieve the water.

In the course of water protection, legislators in Europe are constantly setting higher standards for wastewater treatment with regard to phosphorus and nitrogen elimination. This nutrient elimination can be achieved on the one hand by biological processes (nitrification, denitrification, biological phosphorus storage in microorganisms) and on the other hand by chemical processes (magnesium ammonium phosphate precipitation, phosphate precipitation).

In the case of phosphate precipitation, the phosphate dissolved in the water or waste water is converted into sparingly soluble iron or aluminum or calcium phosphates by adding precipitants, preferably based on iron or aluminum salts (in exceptional cases also by adding calcium hydrate) - phate converted. The dissolved iron or aluminum salts used as precipitants contain chloride and / or sulfate anions. These water-soluble anions remain in the water after the phosphate precipitation and bring about its salting-up, for example according to the equation:

FeClS0 ₄ + P0 ₄ ^{3 "} → FeP0 ₄ 1 + Cl ^" + S0 ₄ ²

The precipitants used can also cause undesirable side reactions, for example undesirable changes in the acid capacity, the pH value and impairments in the biological wastewater treatment. For the phosphate elimination with the help of chemical precipitants, correspondingly complex storage and dosing devices have to be built and product quantities have to be kept in stock. This is associated with additional investment and operating costs as well as for metering devices including measuring and control devices and for the disposal of the additional precipitation sludge.

In Germany, around 500,000 tonnes of chemical precipitants are currently used to treat waste water each year. This is not only unsatisfactory from the point of view of water protection (e.g. because of an undesired salting up), but also a logistical and traffic-technical problem (undesirable increase in road traffic, currently approx. 100 trucks per day). In addition, the storage and handling of chemical precipitants is problematic with regard to environmental hazards and worker protection.

In order to minimize these disadvantages, the field of wastewater treatment is constantly on the lookout for alternative methods for eliminating phosphorus and / or nitrogen, preferably based on biological processes.

The object of the invention was to further develop the state of the art in the field of phosphorus and / or nitrogen elimination from water, in particular waste water, in order to achieve the undesirable phosphorus and / or nitrogen contents in an even more effective and substantially more economical and environmentally friendly manner to be further minimized in order to meet the increasing requirements of the legislature and to ensure the best possible environmental protection.

It has now been found that this object can be achieved according to the invention in that the water or wastewater containing phosphorus and / or nitrogen to be treated is in a line or in a basin, preferably in a ventilated activated sludge basin, using electrodes Made of the same or different electrically conductive materials connected to a DC power source are subjected to an electrochemical and / or biological treatment at constant voltage or constant current flow.

The invention therefore relates to a method of the type mentioned at the outset, which is characterized in that at least two electrodes made of the same or, preferably, different materials are introduced into the water to be purified and contain phosphorus and / or nitrogen and are connected to a direct current source be connected, with a constant current flow or a constant voltage in the presence of an (optionally dissolved) oxidizing agent, optionally with the addition of alkalis, an electrochemical treatment is carried out to reduce the phosphorus and / or nitrogen dissolved in the water leads, on the one hand the phosphorus-containing precipitation products formed are separated off and on the other hand the phosphate-enriched biomass, which arises as a result of increased biological activity of the microorganisms contained in the treated water, is drawn off as excess sludge.

The invention further relates to a device for carrying out the method described above, which is characterized in that it comprises a (preferably) closed control cabinet (S) in which a direct current source, devices for regulating the current flow and / or the voltage of the direct current source and Line connections are arranged to the direct current source,

Electrodes made of the same or, preferably, different materials, which are attached to a support frame (T) which is inserted into the water to be treated, and

Connection lines (L) between the direct current source and the electrodes (E). The device for electrochemical / biological P and / or N elimination from water and waste water used to carry out the method according to the invention, as can be seen from the attached FIG. 2, consists of a control cabinet S and one or more electrode packages connected to it that are immersed in the water to be treated.

The control cabinet is fed by a 220/380 volt power connection and its design / insulation corresponds to the regulations of the applicable standards. It is preferably set up outdoors in the vicinity of the connected electrodes E.

So that a direct current can be produced in the range from 5 to 100 volts and 2 to 40 amperes, preferably a smoothed direct current in the range from 10 to 40 volts and 10 to 20 amperes, a rectifier device provided inside the control cabinet is used. In addition, a device is provided which ensures an adjustable constant current output by automatically adapting the voltage to the continuously changing conductivity of the water to be treated. However, it is also possible to keep the voltage constant with variable current output. The control and transformation in the control cabinet is carried out in such a way that under no circumstances, even in the event of a fault, can the primary voltage break through to the output part and the voltage of a maximum of 48 volts is never exceeded there.

The control device has one or more DC outputs for connecting the electrodes. The control unit can be programmed in such a way that an automatic polarity reversal of the current flow takes place after adjustable time intervals of 2 minutes to 1 month, preferably 2 times a day for 5 minutes to 3 hours, in order to remove deposits the cathode (s). If several electrode packs or sets of electrodes are used, the polarity reversal is preferably carried out sequentially.

Each of the electrode sets used consists of at least one cathode and at least one anode, preferably one anode and two cathodes. With regard to the electrode material, electrode shape, electrode size and electrode arrangement, reference is made to the explanations below.

In the practical implementation of the method according to the invention, it has been shown that a simultaneous elimination of the undesirable phosphorus and nitrogen contents in the treated wastewater can be achieved with extremely low operating costs, so that the overall method works extremely economically and almost none Maintenance required. In addition, the addition of chemicals can generally be completely dispensed with by the process according to the invention and, if appropriate, the phosphorus precipitated from the waste water in the form of a phosphate can easily be recovered and marketed.

The electrical power consumption is very low, for example, per 1000 m ^{3 of} wastewater and day with approx. 5 kWh. The voltage is less than 40 volts. A medium-hard water with a total hardness of> 5 ° dH is favorable for the function of the method (l ° dH falls 3.68 g P to 19.9 g Ca ₅ (P0 ₄ ) ₃ OH). The higher the hardness and the pH of the water or wastewater, the better the effectiveness of the invention (ELKOPHOS), ie P-elimination with run-off values below 1 mg / 1 P _total ^without precipitant.

When using the method according to the invention and the device according to the invention, an electric field builds up in the treated wastewater, which as a result of Electrode reactions and ion migration bring about chemical changes which lead to a surprisingly strong precipitation of phosphorus, the phosphorus accumulating in the activated sludge and then being drawn off from the cleaning cycle together with the excess sludge. At the same time, there is an increased biological activity with regard to phosphorus and nitrogen elimination, as a result of which the phosphorus and nitrogen content in the aerated aeration tank or in the denitrification stage can be reduced further, so that environmentally friendly

In this way, an extremely strong reduction in the phosphorus and / or nitrogen content can be ensured without the addition of chemicals, far beyond the level required by law.

For example, in a sewage treatment plant for approx. 10,000 residents in the aerated aeration tank, two plate-shaped electrodes made of rust-proof V4A steel (cathode) or iron (anode) connected to a 24 V DC voltage source were used an edge length of about 180 cm, which dipped into the wastewater at a mutual distance of about 50 cm, carried out an electrochemical treatment which, without the addition of chemical precipitants, eliminated about 90 to 95% of the phosphorus content within a few hours to well below the legally prescribed limit. Another effect: was the improvement of the biological nitrogen degradation, which was improved by more than 30% in the activation zone.

Preferred embodiments of the method according to the invention and the device according to the invention result from the subclaims.

The application of the method according to the invention in the context of a wastewater treatment is exemplified below. ter explained with reference to the accompanying drawing (Fig. 1).

In a biological wastewater treatment plant, the wastewater is subjected to a cleaning cycle, it being passed through a rake and sand trap 1, a pre-clarification tank 2, an unventilated aeration tank 3, an aerated aeration tank 4 and a post-treatment tank 5. A return sludge screw pump 6 conveys the sludge from the secondary clarifier 5 back into the aerated activation tank 4. Part of the sludge is withdrawn from the circuit as excess sludge 9 and reaches the sludge treatment plant 10.

The treated water flows from the secondary clarifier 5 into a receiving water (not shown). In the ventilated living pool 4, electrodes 7, which are connected to a direct current source 8, dip into the waste water and enable an electric field to be built up. This leads to phosphate precipitation, which is associated with the excess

Sludge can be removed from the system. At the same time, the phosphorus is further broken down by microorganisms together with other organic substances and nitrogen compounds in the aerated activation tank 4, special ventilation devices (not shown) ensuring the corresponding supply of oxygen.

In the electrodes that can be used to carry out the method according to the invention, the electrode material of the anode is (unalloyed) iron or aluminum, the electrode material of the cathode should be made of a high-quality alloy, for example V2A steel, V4A steel or another electrical conductive material, for example made of an electrically conductive plastic or an electrically conductive plastic or graphite or titanium. The direct voltage to be applied to the electrodes and the resulting direct current flow depend on the physical data of the electrodes, the conductivity of the aqueous medium located between the electrodes and the desired electrochemical effect. When the method according to the invention is carried out in practice, either the voltage or the current flow should be kept constant, it is particularly preferred to keep the current flow constant by adjusting the voltage accordingly.

The voltage of the DC voltage source is preferably between 5 and 100 volts, in particular between 10 and 40 volts.

The current flow of the direct current source, which is preferably kept constant, is preferably within a range from 1 to 100 amperes, in particular from 10 to 20 amperes.

The electrodes used according to the invention are preferably of the same size and preferably have an area of 1 to 20 m ² , in particular 2 to 10 m ² . The electrodes (both the sacrificial anode and the permanent cathode), which are preferably plate-shaped, can each consist of one or more parts, preferably plate-shaped parts, of different or the same size.

They are preferably introduced into a mixed reaction tank, in particular into an aerated aeration tank or into a denitrification stage.

The mode of operation of the method according to the invention is, as is currently assumed, based on the following mechanisms, without the invention being restricted thereto, since these mechanisms have not yet been clearly clarified. If the anode is made of iron, for example, the electrolytic effect of the direct current releases bivalent iron ions (Fe ²⁺ ). As a subsequent reaction, the water is decomposed electrolytically at the cathode, ie split into H ⁺ and θH ~ ions. The hydrogen ions (H ⁺ ) are discharged and escape from the system as hydrogen gas. The hydroxyl ions (OH ~) increase the pH, especially in the area of the cathode, far into the alkaline area. Under these conditions, the calcium ions (Ca ²⁺ ) and magnesium ions (Mg ⁺ ) contained in the water or waste water react with the phosphate ions dissolved therein to form insoluble calcium or magnesium phosphate. At the same time, the divalent iron ions (Fe) formed are oxidized to trivalent iron (Fe ³⁺ ) by the oxygen dissolved in the water, which in turn reacts with the phosphate ions to form insoluble iron phosphate FeP0 ₄ , which precipitates.

The divalent iron ions (Fe ²⁺ ) formed on the anode are not only oxidized by the oxygen dissolved in the water, but also by nitrates (NO .--, ^{* "} ) dissolved in the treated water or waste water are reduced to elemental nitrogen so that denitrification takes place at the same time.

In addition, the biological activity of the microorganisms contained therein with respect to P and / or N elimination is greatly increased in the aeration basin or in the denaturation basin in a manner not previously clarified. Surprisingly, this effect occurs particularly impressively when the anode is made of metallic, unalloyed iron or aluminum and the cathode is made of a high-alloy steel (e.g. V2A or V4A steel) or another conductive material, e.g. made of an electrically conductive plastic or graphite.

This is demonstrated impressively by the following experiment 1. Trial 1

An anode and a cathode were each installed in a 10 1 plastic container with a small agitator and connected to a direct current source. The DC power source worked at 20 V. The distance between the electrodes was 5 cm.

The test vessel was filled with wastewater from the activation of a biological sewage treatment plant, and the stirrer and direct current source were switched on. Samples were taken at intervals of 10 min each, filtered and the dissolved phosphate content was determined analytically. Anodes and cathodes made of different materials were tested. The following Table I shows the results.

Table I

ANODE → iron V ₂ A iron V ₂ A aluminum CATHODE → iron V ₂ A v _2A - ^• iron V ₂ A mεΛP mc lP ε lP ms lP mε lP

Start 10 10.0 10.0 10.0 10.0 after 10 minutes 10 9.8 p.2 10.0 9, 1 after 15 minutes 10 9.7 6.3 9.7 8.3 after 20 minutes 9 10.0 5, 1 9.4 7 , 6 after 30 minutes 9 9.7 3.0 9, 1 6.0 after 45 minutes Q 8.8 0.5 8.7 4.3 after 60 minutes S 9.1 0.3 9.0 2.6

It can be concluded from the results obtained that the biomass from a wastewater treatment plant with activated sludge is able to accelerate and accelerate the degradation of phosphorus with the help of electrical fields.

Trial 2

In a second test series, solid-free process water was used in the same device as in test 1 an excessive phosphorus concentration is used. From the outset, only one iron anode and one cathode made of alloyed V4A steel was used in this test series. It has been shown that phosphorus elimination was also found in this installation (ie without biomass), albeit to a lesser extent than with biomass (experiment 1).

In a further test series with phosphate-containing waste water, it was found that the calcium and magnesium concentrations also changed in parallel with the phosphate content. The results are summarized in Table II.

Table II

PO4-P Ca Mg mg / 1 mg / 1 mg / 1

Start 1 1.3 1 17 21 after 10 minutes 9.1 109 19 7.2 101 after 15 minutes after 20 minutes IS 6.4 93 ^'17 4.3 after 30 minutes after 45 minutes S2 17 1.8 79- 16 after 60 Minutes 0.9 75 16

With the electrochemical treatment according to the invention, the water became cloudy, a precipitate formed, which was filtered off and analyzed. The precipitate consisted of calcium carbonate and calcium phosphate, as well as small amounts of iron phosphate or iron hydroxide.

In search of an explanation for the surprising effect of phosphate precipitation with the aid of suitable anodes and cathodes by applying a defined direct current, the pH was measured at various points in the electric field. It was done with a pipette Wastewater withdrawn because pH measuring devices cannot measure in the electrical field. The measurements showed that the pH of the water was significantly higher in the area of the cathode surface than in the rest of the area. PH values of over 12 were measured.

The surprising effect of P-elimination in the sense of the invention can therefore be explained in such a way that the lime-carbonic acid equilibrium of the water is shifted by partial increase in the pH value on the cathode surface and thus the calcium dissolved in water as a cation for the Precipitation with phosphate is available. According to the invention, P elimination in wastewater and industrial water is therefore also possible without the use of special chemical precipitants. This is of enormous ecological and economic importance because not only can considerable amounts of chemical precipitant be saved (including the necessary storage, dosing and control systems), but also no additional and disruptive components get into the wastewater ¬ zung and no acidification occurs.

Trial 3

An operational test was carried out in a sewage treatment plant with wastewater treatment using the activated sludge process. As in the laboratory test, electrodes (anode made of iron, cathode made of V4A steel) were connected to a 24 V DC source. The electrodes were installed for a period of one month in the aeration tank and in the pre-clarification tank. In the pre-clarification basin, the electric field only had an effect on mechanically cleaned waste water, in the aeration tank on biologically cleaned waste water in connection with activated sludge. The purpose of the experimental setup was to find out to what extent an interrelation in the effect of phosphorus elimination exists between purely electrochemical processes and biochemical ^¬ rule operations.

The phosphate concentration was measured regularly (daily) in the wastewater treatment plant. It was found that the P-elimination with the aid of the invention (ELKO) in the Bele ^¬ environment is clearly more effective than in the pre-treatment. From this it can be deduced that the overall wastewater treatment can be improved if, in the sense of the invention, electrical fields are built up with the aid of suitable electrodes which are connected to a direct current source. This follows from Table III below.

Table III

Electrochemical P elimination

Trial operation

Trial time in days, daily sample

Trial 4

According to the invention, the electrochemical P elimination was carried out in a test facility. Iron was used as the anode and graphite was used as the cathode, each in the form of 10 cm × 10 cm plates. The electrodes were in one A distance of 10 cm from one another is introduced into waste water containing phosphorus and nitrate from the activation tank of a biological purification stage of a municipal sewage treatment plant and connected to a direct current source. The current was set to 18 amperes at a voltage of 30 volts. The test plant was continuously charged with waste water at a volume of 200 l at 200 l / h, so that the mean residence time was 1 h.

The phosphate and nitrogen content of the waste water was analyzed at 30 minute intervals in the inlet and outlet of the test facility. The following values were found:

Table IV

mg / 1 P / N start 30 60 90 120 150 180 210 min min min mm min mm mm

mσ / 1 P0 ₄ -P

Inflow 5.5 5.3 5.2 5.3 5.5 5.1 5.2 5.3

Process 5.5 4.7 3.4 2.5 1.6 1.2 1.1 1.1

Elimination rate in% 0 11.3 34.6 53.8 70.9 76.5 78.8 79.2

mσ / 1 NQ ₃ -N inflow 18.2 18.4 17.9 18.8 18.6 19.1 18.7 18.4

Expiration 18.2 17.5 15.2 14.4 12.9 12.3 12.2 11.9 Elimination rate in% 0 4.9 15.1 23.4 30.6 35.6 34.8 35.3

The test results reproduced above show in an impressive manner that, at the same time as the P elimination, down to a value far below the legally prescribed limit of 2 mg P0 ₄ ~ P / 1, there is a significant increase in the biological N elimination among the legally prescribed limit value of 18 mg N0 ₃ - N / 1 has taken place, an effect that was not expected even for the specialist in this field.

Claims

Patent claims

1. Process for the electrochemical treatment of calcium- and / or magnesium-containing water or waste water, in particular municipal and / or commercial waste water, which is chemically and / or biologically using microorganisms for the purpose of phosphorus and / or nitrogen - Elimination is treated, characterized in that

at least two electrodes made of the same or, preferably, different materials are introduced into the water to be purified, containing phosphorus and / or nitrogen, and are connected to a direct current source,

with a constant current flow or a constant voltage in the presence of an (optionally dissolved) oxidizing agent, optionally with the addition of alkalis, an electrochemical treatment is carried out which leads to a reduction in the phosphorus and / or nitrogen dissolved in the water,

on the one hand the phosphorus-containing precipitation products formed are separated off and on the other hand the phosphate-enriched biomass, which arises as a result of increased biological activity of the microorganisms contained in the treated water, is drawn off as excess sludge.

2. The method according to claim 1, characterized in that electrodes made of different materials are used.

3. The method according to claim 2, characterized in. that the electrode connected as the anode (sacrificial electrode) consists of iron or aluminum and that as the cathode switched electrode made of an electrically conductive material, preferably of V2A or V4A stainless steel, electrically conductive or made plastic or graphite.

4. The method according to any one of claims 1 to 3, characterized in that the electrodes are in the form of plates.

5. The method according to any one of claims 1 to 4, characterized in that the individual electrodes are the same size or different sizes and each have an area of 0.3 to 50 m ² , preferably from 2 to 10 m ² .

6. The method according to any one of claims 1 to 5, characterized in that the electrode surfaces are arranged parallel to one another at intervals of 5 to 200 cm, preferably from 20 to 80 cm.

7. The method according to any one of claims 1 to 6, characterized in that both the anode and the cathode consist of several plates of different or the same size.

8. The method according to any one of claims 1 to 3, characterized in that the electrodes have the shape of grids, cylinders, rods and / or wire.

9. The method according to any one of claims 1 to 8, characterized in that the voltage of the, preferably smoothed, direct voltage source at a value between 2 and 100 volts, preferably between 10 and 40 volts, is kept constant.

10. The method according to any one of claims 1 to 8, characterized in that the current flow of the, preferably ge smoothed, DC power source is kept constant within half a range of 1 to 100 amps, preferably 10 to 20 amps.

11. The method according to any one of claims 1 to 10, characterized in that the voltage or the current flow of the

DC source is kept constant manually or automatically during the electrochemical treatment.

12. The method according to any one of claims 1 to 11, characterized in that the cathode (s) is (at) freed from deposited deposits at predetermined time intervals by manual or automatic reversal of the polarity of the current flow.

13. The method according to claim 12, characterized in that the polarity reversal is carried out after an operating time of 1 min to 1 month, preferably 1 h to 1 day.

14. The method according to claim 12 or 13, characterized gekenn¬ characterized in that when using several electrode sets (anode + cathode (s)) individual electrode sets are reversed in time.

15. The method according to any one of claims 1 to 14, characterized in that the electrodes in a line or basin through which the water to be treated flows, preferably in a mixed, aerated activation basin or in a mixed, non-aerated denitrification basin, be introduced.

16. The method according to any one of claims 1 to 15, characterized in that oxygen or nitrate is used as the oxidizing agent.

17. The method according to any one of claims 1 to 16, characterized in that the alkali is sodium hydroxide solution, sodium aluminum nat, calcium hydroxide and / or magnesium hydroxide is used.

18. Device for performing the method according to one of claims 1 to 17, characterized in that it comprises

a (preferably) closed control cabinet (S) in which a direct current source, devices for regulating the current flow and / or the voltage of the direct current source and line connections to the direct current source are arranged,

Electrodes (E) made of the same or, preferably, different materials, which are attached to a support frame (T) which is inserted into the water to be treated, and

Connection lines (L) between the direct current source and the electrodes (E).

19. The apparatus according to claim 18, characterized gekennzeich¬ net that the electrodes (E) consist of several sets of electrodes (anode + cathode (s)).

20. The apparatus of claim 17 or 18, characterized ge indicates that one or more sets of electrodes are used, each of which consists of a cathode and an anode, preferably two cathodes and an anode.