DE102015006706A1 - Continuous process for removal of micro-contaminants from biologically clarified municipal wastewater - Google Patents

Abstract

The invention relates to a continuous, physical process - based on electrolysis processes - for the removal of micropollutants from the water, which runs automatically. This happens under optimized conditions where an electrolytic coagulation, chemical oxidation and AOP-methods have been integrated as an advanced oxidation process. Here, micropollutants are mineralized by the generation of OH * radicals and removed from the water.

Description

The invention relates to a continuous, physical process using electrolysis technology to remove organic compounds from water. It is in reactors connected in series, worked with different electrodes and suggestions, the water flows constantly through pumps with appropriate flow, through the process plant.

A danger that has only been recognized and serious in the last decade is based on the accumulation of chemical compounds in the groundwater. Through the daily use of drugs, pesticides, corrosion inhibitors, fracking fluids, biocides, X-ray contrast agents, antibiotics, cosmetics, and the like. These substances accumulate in the groundwater. These substances are now found in all waters and have effects on all biological systems that can be classified as toxic, but have so far hardly been researched in detail in long-term effects. The micro-pollutants are mainly in concentrations between 0.1 and 100 micrograms per liter of water or in other words formulated in nano-g to micro-g and can not be removed by biological wastewater treatment.

This topic has been intensively developed only in the last 2-3 years and edited and published in various research projects. The solution to this problem is the use of PAC (activated carbon) and / or ozone. The best achievable yield is on average at 80% to 90% removal of micropollutants.

• • Ozonierung – Addition von Ozon (O₃) nach einer klassischen, biologischen Abwasserbehandlung in einem Kontakt-Turm mit einer HRT von etwa 20 Minuten gefolgt von einer biologischen Filtrationsstufe oder Bett-Reaktor um Oxidations-Beiprodukte abzubauen bzw. zu entfernen
• • Aktivkohle Adsorption – PAC wird in Tanks zugesetzt und mit einer HRT von 30–60 Minuten reagieren gelassen. Ein Filtrationsschritt ist notwendig um von Aktivkohle wieder abzutrennen. Es kann auch über Aktivkohle direkt abfiltriert werden mit entsprechender HRT.

Two technical processes were tested and recommended and used to remove micro-contaminants (mp):

• • ozonation - addition of ozone (O ₃ ) after a classical biological wastewater treatment in a contact tower with an HRT of about 20 minutes followed by a biological filtration stage or bed reactor to remove or remove oxidation by-products
• • Activated Carbon Adsorption - PAC is added to tanks and allowed to react with HRT for 30-60 minutes. A filtration step is necessary to separate again from activated carbon. It can also be filtered off directly via activated charcoal with appropriate HRT.

The "European Commission" - COM (2013) 574 final - has the "Urban Waste Water Treatment Directive" 91/271 / EEC supplemented with an indication of the use of new techniques (tertiary treatment technologies). Here are listed ozonation, chlorination, UV, membrane technology and sand filter with respect to the micro-pollutants at entry into the aquatic environment and given as promising solutions above technologies.

In the prior art, the following techniques are seen at this time: a) activated carbon activated carbon / PAC (powder activated carbon); b) direct use of ozone; c) AOP (Advanced Oxidation Process) methods. Laboratory tests and pilot plants / pilot plants showed that the following organic compounds were used and evaluated as an indicator of the effectiveness for removing micropollutants from a process. Table 1 Substances for measuring the cleaning effect by ozone treatment comment material acquisition substance group condition hydrochlorothiazide drug ingredient carbamazepine drug ingredient ozone treatment clarithromycin drug ingredient Very well diclofenac drug ingredient eliminable substances metoprolol drug ingredient Venlafaxine drug ingredient Citalopram drug ingredient aliskiren drug ingredient benzotriazole corrosion protection Well eliminable mecoprop Biocide, PSM matter carbendazim Biocide ... Candersartan drug ingredient

With the development and application of the antibiotics, bacterial infectious diseases could initially be successfully combated. With effluents from urban areas and hospitals, large amounts of antibiotic-resistant bacteria are released into the environment, as conventional treatment plants serve as "transfer stations" for resistance. This applies to all biological types of treatment plants also for the membrane bioreactor. Typical of wastewater from the above areas are levels of hormones, antibiotics, cytostatics, psychotropic drugs, X-ray contrast agents, anti-inflammatory drugs / analgesics such as ibuprofen, dichlophenac, etc. and disinfectants and other drugs or their metabolites, from conventional (biological) wastewater treatment plants ) can not be removed, but remain in the water. In sewage treatment plants, antibiotic levels have already been measured, which in some cases are well above the active concentration. These substances accumulate over time in the groundwater and thus drinking water and can be regarded as a latent danger for biological cell systems.

Based on the experience of individual plants in Switzerland and Germany, one can assume an annual cost of approx. 0.1 Euro / m ³ for a 250,000 PE plant and 0.25 Euro / m ³ for a 25,000 PE plant. For a wastewater of 150-200 l / hp / d, this gives about 5-7 Euro / pe for the large plant and 15-20 Euro / pe for the smaller one. Since the population equivalents (EW) also include industrial and industrial wastewater, annual costs per inhabitant are around 10-15 euros for the 250,000-euro plant and 20-40 euros for the 25,000-euro plant. Especially for smaller systems vary the previously available cost information very.

It is clear, however, in any case that the advanced wastewater treatment is associated with significant costs. The investment costs for downstream ozone treatment are up to around 25,000 PE to 500,000 CHF / € for small plants and 250,000 PE to 1.1 million CHF / € for large plants. In Switzerland, a law will come into force in 2016 according to which the federal government (according to FOEN information) has budgeted 1.2 billion CHF for about 100 installations. In other EU countries, different legal provisions have been or are being prepared.

A previously not mentioned possibility of cleaning drinking water for micropollutants is the reverse osmosis - unfortunately with the disadvantage that really everything is removed, even vital salts. However, osmosis also has the disadvantage that about one third of the wastewater used would be lost and that a prefiltration would have to take place via at least a 5 micron filter. However, this is hardly possible and uneconomical for large amounts of waste water. Methods have also been described that represent a combination of 3 technologies - namely: coagulation, activated carbon adsorption and electrochemical oxidation ( Dept. Enviro. Closely. TU Crete, Greece - Feb. 2008, Michael Mavros et al. ). Furthermore, one could evaporate biologically treated wastewater under vacuum, but with even higher costs.

• • Nebenprodukte entstehen, die wieder entfernt werden müssen;
• • dass Sauerstoff in Tanks oder Flaschen zur Erzeugung von Ozon eingesetzt werden;
• • dass der erzeugte Überschuss von Ozon wieder zerstört werden muss;
• • dass die Infrastruktur in verfahrenstechnische Anlagen für die Ozonierung aufwendig und teuer ist, um entsprechende Verweilzeiten (HRT von 20 Minuten) zu garantieren;
• • dass die klassische Erzeugung von Ozon relativ teuer ist.
• • dass die Rate der Entfernung von organischen Verbindungen nur 80 bis 90% beträgt.

The criticism of the state of the art is that in ozonation

• • By-products are formed, which must be removed again;
• • that oxygen is used in tanks or bottles to produce ozone;
• • that the surplus of ozone generated must be destroyed again;
• • That the infrastructure in process plants for the ozonation is complex and expensive, in order to guarantee appropriate residence times (HRT of 20 minutes);
• • that the classical generation of ozone is relatively expensive.
• • that the rate of removal of organic compounds is only 80 to 90%.

The criticism of the prior art can be continued to the effect that the efficiency of a maximum of 80-90% of all three abovementioned technologies of removal of undesirable substances is simply too low - and the procedure in batch mode in the PAC method or is semi-continuous with ozone treatment and ozone can cause too many by-products to be removed. Ozone in direct treatment of wastewater containing nitrobenzene is not very suitable and shows only about 80% removal.

Ozone is a toxic gas known as a climate killer. Since up to 0.6 g of O ₃ per liter or more are used depending on the temperature, the solubility limit is reached very quickly under normal conditions. It fights for every ppm ozone in the application at room temperature. An excess of ozone means but also unwanted by-products that need to be removed again. Ozone is unstable in water. The half-life ranges from seconds to hours depending on pH, NOM (normal organic matter).

Another disadvantage is above all not a fully continuous procedure and the efficiency that can remove a maximum of about 80-90% of the micropollutants in the activated carbon adsorption and is about the same with the ozonation. Another disadvantage is the high expenditure on equipment of various basins, tanks and other apparatus, which require a larger base area and thus land area for large volumes.

In addition, absorption of substances is limited by the use of activated carbon, which is not only proportional to the active surface. Since the polar surface adsorbs everything, there must always be a certain surplus, which is also reflected in other parameters ( Water Science & Technology Vol 66 No 10 pp 2115-2121, 2012 and Drink. Water Eng. Science 2, 57-62, 2009 , In general, a difference is also described and recognizable: "Treatment options for wastewater effluents from pharmaceutical companies" in Int. J. Environ. Sci. Tech. 8 (3), 649-666 - 2011 between ozonation and activated carbon. A particular disadvantage of the PAC method is the negative CO ₂ balance in the production of the activated carbon.

The use of activated carbon (AK) has not gained acceptance, because the AK requires large basins, agitators and possibly also a temperature control. Also, the reprocessing / regeneration of AK with appropriate transport too expensive and expensive. The results did not reach the values of ozone treatment on average or only just. After weighing the advantages and disadvantages of ozone treatment, the costs for an investment and the operating costs are considerably high. Thus, a technology that achieves better or even better results at a lower cost is of great benefit.

The invention is based on the object to provide a continuous process that does not have previous disadvantages. Thus, the object of the proposed invention is to carry out a reduction or elimination of micropollutants, with the proviso of achieving a higher percentage of removal at a reduced cost compared to the previously described ozone treatment. This is in principle possible for biologically clarified wastewater, which generally has a reduced COD level with a reduced total nitrogen content, as well as for groundwaters.

As a technical solution to the problem is proposed by the invention to carry out an electrolytic treatment, which performs an electrocoagulation in the first stage.

• • Reduktion des Härtegrades mit Beseitigung von Karbonat-Ionen und Phosphat
• • Bindung von organischen Verbindungen im erzeugten Metallhydroxid und deren Entfernung durch Filtration
• • Erzeugung von Radikalen wie Hydroxyl-Radikal – Im ersten Schritt entsteht das Superoxide Anion Radikal: ·O₂ ^– (Reaktion erster Ordnung) gefolgt von der Formation vom Hydroxyl-Radikal (Reaktion 2.-ter Ordnung).

In this first stage several issues are solved in parallel:

• • Reduction of hardness with elimination of carbonate ions and phosphate
• • Binding of organic compounds in the produced metal hydroxide and their removal by filtration
• • Generation of radicals such as hydroxyl radical - In the first step, the superoxide anion radical is formed: · O ₂ ^- (first-order reaction) followed by the formation of the hydroxyl radical (second-order reaction).

In the following step, an air oxidation is carried out by blowing in air, which leads to an improvement in the coagulation and readily oxidizable compounds such as polar functional groups such. As sugars and alcohols oxidized. In addition, a reduction of the ammonium content in the alkaline pH range is possible, which is generally known as gas stripping, wherein free ammonia is expelled. Also, ferrous iron is oxidized to ferric III. This process is automatically controlled, continuous and thus provides a solution for the removal of micropollutants in near real time.

According to cost calculations, which are based solely on mass flow per unit time for the technology, the investment costs are calculated with about 30 to 50% less, compared to ozone plants. The operating costs experience a reduction of the same order of magnitude. If the costs are reduced to about 1/3, the costs per inhabitant per year are reduced to values of 3.3 to 5 CHF / € and for small investments to about 10 CHF / €.

The procedural design of a plant looks like that in the first step, a reactor with iron electrodes that are low in phosphates but may contain a manganese content. After this reaction stage, which shifts the pH to alkaline, air is blown. The addition of air oxidizes organic compounds, which due to their structure allow this and promotes coagulation, while also ammonia traces are removed. Further, a reduction of carbonate occurs, which is troublesome in the reaction with occurring radicals, since carbonate is known as radical scavenger. The occurring iron hydroxide, which occurs in large, stable flakes, is filtered off and thus removed from the circulation. Here it binds shares of organic compounds, which are removed by filtration.

In the second stage, a reactor with aluminum electrodes is used. The pH remains in the alkaline level. Again followed by an air injection, the previously described effect is repeated. In the third stage, a reactor is equipped with titanium electrodes. Here, the pH changes and goes into weak acid. In the overall process, the salt load and thus the conductivity is reduced in succession in the three stages, carbonate removed to perform in the last and fourth stages of an advanced oxidation process (AOP). There are three types of electrodes that have been found suitable so far: diamond electrodes (bbd), lead dioxide electrodes on titanium as support (Ti / PbO ₂ ) and stainless steel grid electrodes. While the first electrodes are under DC conditions, the stainless steel grid electrode is driven under AC condition. Thus, in the first to third stage already a significant reduction of "micropoluttants" achieved.

This fourth and final step generates only radicals, such as hydroxyl radicals, where the superoxide anion Radical: · O ₂ ^- (first-order reaction) is followed by the formation of the hydroxyl radical (second-order reaction). The redox potential is much higher compared to the ozone and, since the reaction is carried out in the flow, there are no oxidation by-products.

AOP methods are very different. To "crack" micro-pollutants (micropollutants) oxidatively requires radicals - z. For example, OH * radicals with very high redox potential to oxidize a terminal oxidation of organic molecules to water and carbon dioxide, which proceeds via intermediates. Thus, UV-based applications - direct photolysis such as UV / H ₂ O ₂ , UV / O ₃ and UV / TiO ₂ - UV / Fentons reagent as photocatalysis are a known solution. Important to this technology is the general mechanism of how contaminants are removed, factors that affect performance, and practical application and execution considerations such as energy use and time.

Electrocoagulation alone by known and described methods is not able to completely remove or mineralize various micropollutants. However, some substances or molecules in the wastewater can hardly be removed in this way. These substances include sulfides / sulfites, mercaptans and ammonium compounds, as well as volatile fatty acid-acetic acid to propionic acid (C ₂ to C ₅ ) and few, other compounds, such as polar compounds such as sugars and alcohols. In order to remove these substances it is necessary to use special electrodes, which were mentioned before.

There are publications with the topic of electro-coagulation known from almost all areas of industrial wastewater treatment. As a disadvantage, however, it should be noted that the reaction preferably proceeds well from the acidic to the basic pH range. When this pH is reached, it often makes little sense to continue the process. When breaking down the known sum parameters then hardly anything happens. Often, a step of homogenization must be made in advance to standardize particle content, particle distribution, pH, which can be degassed by ultrasound or heating or freed by steam introduction of excessive amounts of methane gas or ammonia and volatile fatty acids. Also, a process step which is known as gas stripping is preceded, the volatile compounds z. B. in alkaline medium NH ₃ expels by introducing air. Also, a degassing with ultrasound is possible whereby light volatile substances are expelled in the same way depending on the pH and temperature increase of the water. Here also steam-volatile components can be expelled.

This means in principle that after a specific electro-coagulation for example wastewater, an electrochemical oxidation is carried out with the use of OH radicals. As an alternative, third step, which generates high amounts of oxygen by using a DSA electrodes type titanium-based metal oxides (IrO ₂ / Ta ₂ O ₃ ) or Ti / SnO ₂ or Pb / PbO ₂ , an oxidation of molecules is achieved, the one Attack of hydroxyl radicals are not accessible. The lead oxide anode is at a low ion concentration (about 0.2 g / l NaCl) at about 25 mA / cm ² current density, in the alkaline pH range, at room temperature (20 ° C) one of the best and cheapest electrodes. OH radicals are nonselective, nonspecific, with few molecules of certain structures being insufficiently oxidized and better able to cope and be more successful in-situ treatment with oxygen species integrated into the process. Electrocoagulation alone or air oxidation alone will never be able to eliminate micropollutants.

In the process itself, this is superficially not visible from the outside, since reactors only with other electrodes and gegf. Power supplies are equipped. Important for the solution of the problem is a timely treatment of water containing micropollutants, the proposed method to remove these substances, so in this form has not been described. Another advantage over the timely, faster treatment beyond, is a higher efficiency of up to 99% of all substances removed and can be classified as environmentally friendly, since the energy balance is positive, since electrodes as well as metal hydroxide can be fully recycled and resulting hydrogen to Energy can be used.

By adding acid, at the start of electrocoagulation, to maintain the pH and thus prolong the reaction time is not absolutely necessary. Depending on the set polarity switching of 10 to 60 seconds, more material turnover can be achieved and thus more OH radicals are generated, which lead to a higher degree of mineralization. But are reactive compounds present, which react spontaneously with iron ions, preferably, as in sulfite / sulfide or often by organic compounds such. B. Mercapto compounds that act as complexing agents, it makes little sense to extend the pH acidic reaction phase. These substances are removed as described above.

It is better to remove these compounds which precipitate out of the matrix and further treat the recovered liquid in a second reactor. Essential for the success of the process is a mechanical precursor to separate solids, as only liquids for electrolysis can be used. However, a small proportion of solid particles can be tolerated. In the precipitation of the iron hydroxide, stable flakes with a size of about 10 to 30 microns form, which binds the smallest particles and removed by a filtration step.

Since the process produces hydrogen and oxygen, it can be collected and used for energy production. The intermediates such as iron hydroxide are present here in purified form and can after dehydration where iron oxide is then reduced by mixing with coal in the blast furnace. The yield of pure iron is more than 10 times higher compared to the use of iron ore. This will allow a recycling cycle to be closed, which will characterize the proposed process as truly environmentally friendly.

The theoretical foundations for electrocoagulation are known and in particular at the cathode: 2H ₂ O (1) + 2e → H ₂ (g) + 2OH- (aq) (1) and 8H ⁺ (aq) + 8e → 4H ₂ (g) (2) at the anode: 2H ₂ O (1) → O ₂ (g) + 4H (aq) + 4e (3) Fe (s) → Fe ^{2 +} (aq) + 2e (4)

As the OH ion concentration at or near the cathode increases, the pH of the medium increases. As the anode decreases and releases iron ions into the solution - illustrates the equation below Fe ²⁺ (aq) + 2OH ^- (aq) → Fe (OH) ₂ (s) (5) 2Fe ²⁺ (aq) + 1 / 2O ₂ (g) + 5H ₂ O (1) → 2Fe (OH) ₃ (s) + 4H ⁺ (aq) (6) S = fixed; aq = in solution; g = gaseous; Dye = dye / pesticide

For further model consideration, the elimination of dyes / pesticides and much more by the combination of the processes of flocculation / coagulation with iron ions (Fe ^{3 +} ), their Precipitation, reduction to small molecules at the cathode, formation of a competitive reaction at the Anode as: 4Fe ²⁺ + 6H ₂ O + O ₂ ↔ 4 (OH) OFe + 8H ⁺ followed by
Dye-H + (OH) OFe → Dye-OFe + H ₂ O.

The precipitate as precipitate is then removed by sedimentation and filtration in the process continuously. It should be noted here that hydroxyl radicals are formed in the first stages of electrocoagulation.

Due to the simplified representation of anodic oxidation and cathodic reduction: 2H ₂ O = 4H + + 4e + O ₂ , 4H ₂ O + 4e = 2H ₂ + 4OH ^-

The representation of atomic hydrogen (H *) is one of the intermediate states in the reaction of molecular hydrogen of cathodic reduction. Here, the atomic hydrogen initiates the following sequence of the reaction: H * + O ₂ = HO ₂ , HO ₂ + H * = H ₂ O ₂ .

In the case where the electrodes are made of metal (M), hydrogen peroxide appears and becomes part of the reaction to generate hydroxyl radicals: H ₂ O ₂ + M ²⁺ = M ³⁺ + OH • + OH ^-.

The overall process step shown runs dynamically, continuously, so that always a liquid without excessive particle content is transferred to the next reactor. Depending on the concentration of micropollutants, a correspondingly large electrode surface area, contact time (HRT) and, depending thereon, the flow rate with an adequate current density in mA / cm ^{2 are} required. Other parameters are the starting pH - preferably slightly acidic - and the electrolyte concentration, which is reflected as conductivity, at low temperature of, for example, room temperature under atmospheric pressure.

The process of the first stage of an electric coagulation is terminated when characteristic values are achieved: For some dyes can be reached at a pH range of 7-9, the highest efficiency for the removal from the reaction of iron with hydroxide ions (OH ^- ). At a pH lower than 6, precipitation is not expected since Fe (OH) _{3 is present} in soluble form as Fe ³⁺ . If, on the other hand, the pH is higher than 9, Fe (OH) _{3 can} again be found in soluble form as {Fe (OH) ₄ ^- }. Since the formation of Fe (OH) _{3 is} essential in the removal of z. As dyes, the pH should not be much higher than 8-9.

• • Erreichen eines basischen pH-Wertes
• • Signifikante Änderung des Redox-Wertes – mit Stoffumsatz
• • Erniedrigung der Leitfähigkeit
• • Absinken der Stromstärke/Stromdichte bei konstanten Verhältnissen
• • Erniedrigung der Trübung
• • Keine signifikante Ausfällung von Eisen-Hydroxid mehr sichtbar
• • Reduktion der Farbstärke – bei Messung im UV-Abnahme bzw. der UV-Absorption u. a.

The Pourbaix diagram provides information about this situation. These characteristic values are:

• • reaching a basic pH
• • Significant change in redox value - with material turnover
• • reduction of conductivity
• • Drop in current / current density at constant conditions
• • reduction of turbidity
• • No significant precipitation of iron hydroxide more visible
• • Reduction of the color intensity - when measuring in UV reduction or UV absorption, among others

For the reduction of the COD level, it is important that conditions prevail that allow the generation of · OH * radicals. This • OH * radicals are unstable at alkaline pH: CO _x (adsorbed HO *) → CO _x + ½O2 + H ⁺ + e; Carbonate is a radical scavenger. These measurements of the sum parameters can partly be done online or determined in preliminary tests. Care should be taken to achieve complete iron hydroxide precipitation.

The proposed method is based on removal of the carbonate in the water and then in the second step, a reaction by generated radicals, such as hydroxyl radicals, reach the mineralization. Since a slightly acidic pH equilibrium of Ca (HCO ₃ ) ₂ and dissolved CO ₂ in the water, the expulsion of CO _2, the pH would rise and drift into the slightly alkaline and thus reached a critical supersaturation with respect to the CaCO ₃ and calcite deposited. Furthermore, the presence of ferric ions and the prevailing potential in mV according to the Pourbaix diagram has an influence on calcite deposition, another configuration / structure of CaCO ₃ . The calcite is deposited at Fe ³⁺ - Concentrations, see Poubaix diagram, below 0.1 mmol / l and accelerate the seed crystal formation and thus the resulting CaCO ₃ crystal growth.

With the exception of the alkali carbonates are only slightly soluble in water, so that most metals are precipitated upon reaction with alkali metal carbonates. The carbonate ions react with water to form bicarbonate and hydroxide ions as an acid-base reaction. The proposed method is thus based on the removal or minimization of carbonate ions, since otherwise no reaction of organic compounds with hydroxyl radicals can take place. Thus, in the course of the reaction with iron and aluminum electrodes, the pH is basic, but is shifted slightly acid when used in the third stage of titanium electrodes. Carbon dioxide can be driven out of the water as an acid gas at an acidic pH. Acid medium / acid pH: -redox potential E ° [V] +2.85 · OH + H ⁺ + e ^- ⇆ H ₂ O +1.49 HO · ₂ + H ⁺ + e ^- ⇆H ₂ O ₂

Resistant micropollutants are better degraded by hydroxyl radicals and, in comparison, are better seen as ferrate or ozone. The presence of hydroxyl radicals and concentration is detected as a "spin trap" method with N, N-dimethyl-p-nitrosoaniline (RNO) and the half-life is about 10e-9 seconds. The reaction runs in the temperature range of 20 to 37 ° C and shows concentrations of about 10e6 to 10e9 hydroxyl radicals per ml of water.

After reaching the desired values in a continuous "first-stage treatment", the following reactors contain a classic electrolysis unit designed for higher mass flows. The selection of the electrodes takes place after previous measurements or investigations. The electrodes can here be dimensionally stable anodes or other forms also as "mixed-electrodes", which primarily depend on the properties of the water. Mixed - electrodes are in this case a mixture of iron electrodes with DSA electrodes.

Mixed electrodes have not yet been described in connection with the removal of micropollutants. Mixed electrodes have been described only in electrocoagulation - for example, aluminum with iron. Industrial wastewater is different from urban water. The pH must be adjusted before the start of the treatment as DSA electrodes do not allow alkaline conditions.

The choice of electrodes depends primarily on the requirements of a solid oxidation. The activity of catalytic oxides correlates with the possibility of electron-valence state in the redox process and the potential in the region of the oxygen process. The most common catalytic oxides are RuO ₂ and IrO ₂ . This DSA electrode works in chloride-containing media and does not significantly lower the COD value, as the resulting compounds are weaker oxidants - so it is often less suitable. Various DSA electrodes for the generation of oxygen are known and described. These are metal oxides of z. As Ir, Ru, In, Co to Ta, inter alia, act on a titanium substrate applied mixture of metal oxides as electrodes or Ti / SnO ₂ and Pb / PbO ₂ .

In addition to the bdd electrode (boron doped diamond), which works in a wide pH range and generates OH * radicals in chloride-free medium, other electrodes are also possible. A publication that describes this is z. B. - Oxidation of organic pollutants on BDD anodes using modulated current electrolysis - M. Panizza, A. Kaplaka and Ch. Comninellis - UNI Genoa, EPFL - Lausanne. In this publication, an efficiency of nearly 100% - described in the oxidation of 3,4,5-trihydroxy-benzoic acid. Diamond electrodes generate in a z. B. chloride-free medium OH radicals, but in the presence of chloride and chlorine / oxygen radicals and simple chlorine, which reacts to HOCl pH dependent.

However, it is precisely this content of radicals over time that is crucial for the function of generating radicals which occur very diminished even in the case of DSA electrodes, whereby an excessive excess of hydroxyl radicals dimerizes and forms H ₂ O ₂ , but in the presence of divalent Metals reacts again to hydroxyl radicals. The success with which organic compounds / molecules can be oxidized and with which conditions is the decisive question for a reduction of the sum parameters COD and DOC. Only a special DSA electrode in the mix with steel or iron electrodes will do justice to this situation with a corresponding overvoltage of a higher current intensity.

So you do not always need an expensive diamond electrode to generate OH radicals. Urban wastewater is usually never free of chloride. However, electrocoagulation is able to reduce the chloride content in addition to phosphate and other species. The laws of Faraday generally apply to electrolytic processes, which certify increased substance conversion at a higher current density. The excitation - pulsation of a DC voltage, polarity switching in short intervals, use of alternating current at different frequencies z. B. 50 Hz, etc. - how electrodes are arranged, monopolar or bipolar, are already a factor in deciding how the electrode works and how radicals are better produced. For the "high-current electrolysis" a bipolar configuration is possible, but is often not used here.

Investigations of a chrono-amperometric method have revealed, when taking a characteristic of different iron alloys, that a choice is possible between electrodes which release a great deal of iron ions over time under the same conditions and few iron ions. You can determine this photometrically. However, this means at the same time at a current density of about 60 mA / cm ² electrode surface a different release or formation of radicals in sacrificial anodes. This is different from the consideration of the working potential in volts of an electrode in relation to the normal hydrogen electrode, where an OH radical formation is possible only from about 2.7V.

Four materials were tested for the first step of electro coagulation: GG (gray cast iron), St 37 (black plate), Standard steel 1.1730 and stainless steel (V2A). The production of iron ions decreased from GG to V2A as well as the formation of Fe (OH) ₃ as precipitate. The reduction, if differences in COD value were noticeable, increase in opposite order. The impression was that when DC was used, which was subjected to a voltage surge every 10 to 30 seconds, more radicals were generated. This coincides with some publications.

This is all focused on the second, subsequent step of electrochemical oxidation of micropollutants to remove the remaining "residue". This specifically addresses the possibility of producing as many radicals as possible, preferably hydroxyl radicals, in a suitable process. In general, it became clear that OH radical does not react as a single-electron acceptor, but either abstracts an H (proton) or attaches itself to multiple bonds. Compounds in which both reactions are not possible or energetically disadvantaged are therefore difficult to degrade by the attack of a hydroxyl radical.

• • Elektrokoagulation mit 3 verschiedenen Elektroden-Materialien (Fe, Al, Ti),
• • Nach jeder einzelnen Koagulationsstufe ein gas-stripping mit verbesserter Koagulation durchzuführen um auch leichtflüchtige Stoffe zu entfernen,
• • Luft oder Sauerstoff einzuleiten, um eine Oxidation von organischen Stoffen zu ermöglichen,
• • in letzter Stufe eine elektrochemische Oxidation durchzuführen mit unterschiedlichen Elektroden, die vorhergehend beschrieben wurden.

Therefore, it is crucial in the proposed method of removing micropollutants to integrate and optimize three elementary process steps.

• Electrocoagulation with 3 different electrode materials (Fe, Al, Ti),
• • After each coagulation stage, perform gas-stripping with improved coagulation to remove even volatile substances,
• • introduce air or oxygen to allow oxidation of organic matter,
• • perform last-stage electrochemical oxidation with different electrodes previously described.

Only this method in the described arrangement uses very different oxidation methods, which guarantee that the highest possible proportion of organic substances can be oxidized.

The use of bdd electrodes (boron doped diamond) has been described in numerous publications and is an alternative to the selection of electrodes, but brings little or no advantages in practice with chloride-containing waters. The favored choice, however, is an iron alloy that comes close to stainless steel. Pure stainless steel with a high chromium content should only be used under alternating current conditions, since no ions are emitted to the matrix here. In the case of electrolysis under DC conditions, chromate may form, which is undesirable or not permitted in drinking water. However, electro coagulation is able to remove almost all trivalent chromium from the water. Bdd electrodes are simply too expensive to invest in, and thus uneconomic, for industrial use with very large electrode areas.

• • Einsatz von Eisen-Legierungen unter den Bedingungen einer Elektrolyse mit
• a. Gleichstrom unter kurzer Polaritätsumschaltung zur Erzeugung eines Spannungs-Strom-Stosses z. B. alle 10 bis 30 bzw. 60 Sekunden b. Einsatz von Wechselstrom bei z. B. 50 Hz oder c. Pulsung von Gleichstrom
• • Einsatz von Eisenlegierungen bei elektrolytischen Bedingungen einer elektrochemischen Oxidation unter Addition von Ultraschall bei z. B. hohen Frequenzen (500 kHz), wo primär Hydroxylradikale in Wasser gebildet werden.

To generate hydroxyl radicals, the following possibilities are realistic:

• • Use of iron alloys under the conditions of electrolysis with
• a. DC under short polarity switching to generate a voltage-current surge z. B. every 10 to 30 or 60 seconds b. Use of alternating current at z. B. 50 Hz or c. Pulsation of direct current
• • Use of iron alloys in electrolytic conditions of an electrochemical oxidation with addition of ultrasound at z. B. high frequencies (500 kHz), where primary hydroxyl radicals are formed in water.

As can be deduced from various experiments, a current density of 60 mA / cm ^{2 is} a good and still economical choice. Due to the sufficiently high flow rate of water, a temperature rise at the cell constant used with a 3 mm electrode gap is small. Cold water of about 20 ° C should be used at the start. With an increase in the current density, the reaction and the yield are accelerated as a metabolic rate. By successive and parallel reactors, the discontinuous operation is canceled and a higher mass flow possible. Here, classical stages of process engineering such as gas-stripping, coagulation, flocculation, sedimentation, filtration are integrated.

The alkaline pH from the first two stages of the proposed process is shifted to acid in the third stage after treatment with titanium electrodes. In the laboratory experiment, an AOP method was carried out, but by adding H ₂ O ₂ a reaction according to Fenton, but under electrolytic conditions with iron electrodes. Fenton's reaction is disturbed and stopped by phosphate, sulfate, fluoride, bromide, chloride ions. The inhibition can precipitate iron compounds which trap HO · radicals or result in a soluble form of Fe (III) compounds which form less reactive complexes ( Pignatello et al., 2006 ). Hydroxyl radicals form electrolytically preferably better in the acidic pH range. The addition of H ₂ O ₂ gave no advantages or better results. Therefore, the proposed method dispenses with the addition of any chemicals.

The inventive method further has the advantage that a. In the laminar flow pattern, only water that is in contact with the surface of the electrode allows an equivalent conversion of material over time - ie no surpluses are generated and b. in continuous flow there is a limited mass transfer as oxidation which produces fewer by-products; c. all by-products that arise as iron hydroxide can be fully recycled. All starting materials used, such as iron electrodes, are available at low cost and can be recycled after use.

Whether ultrasound should be added during electrocoagulation or electrochemical oxidation ultimately depends on the question of timely treatment of large quantities of water. An ultrasonic insert also drastically reduces the normally required electrode area for a water volume per unit time in the flow and is also particularly suitable for frac backflow / deposit water. However, for large municipal effluent volumes, the reactors equipped with ultrasound are too expensive and less suitable for the dimension. The addition of ultrasound significantly increases the efficiency - thus addition also becomes an economic factor. Further extensive investigations are necessary in order to finally obtain statements under exactly which conditions an optimum lies for a matrix system or application.

• 1. Homogenisation (Vorfiltration – etwa 100 micron, pH-Einstellung, Temperierung)
• 2. Reaktor-Einleitung für die elektrolytische Koagulation mit den benannten Elektroden (Fe, Al, Ti) im Durchfluss
• 3. Überleitung zur Lufteinleitung als integrierte Bogensieblösung mit Abtrennung von Schaum, Feststoff und Wasserphase – deren Metallhydroxidschaum über eine Pressfilteranlage verdichtet wird und nach Trocknung als Eisenoxid recycelt werden kann.
• 4. Einleitung in Reaktor 4 für die elektrochemische Oxidation mit unterschiedlichen Elektroden – vorzugsweise Edelstahlgitter-Elektrode
• 5. Filtration über ein Sand- bzw. Karbonfilter mit Membrane
• 6. UV-Messung im Durchfluss zur Kontrolle – mit Rückleitungsmöglichkeit
• 7. Mögliche anschliessende Ultrafiltration für Trinkwassergewinnung

The procedural procedure is as follows:

• 1. Homogenization (prefiltration - about 100 micron, pH adjustment, temperature control)
• 2. Reactor initiation for electrolytic coagulation with the designated electrodes (Fe, Al, Ti) in the flow
• 3. Transfer to the air inlet as integrated Bogensieblösung with separation of foam, solid and water phase - whose Metallhydroxidschaum is compressed via a press filter unit and can be recycled after drying as iron oxide.
• 4. Introduction in reactor 4 for the electrochemical oxidation with different electrodes - preferably stainless steel mesh electrode
• 5. Filtration through a sand or carbon filter with membrane
• 6. UV measurement in the flow for control - with return possibility
• 7. Possible subsequent ultrafiltration for drinking water production

The proposed method of integrated electrocoagulation with electrochemical oxidation as a dynamic process, which can proceed continuously, is an ecological and economical process.

example 1

As an example, a treatment of press water was carried out in the instant coffee production. Roasted, ground coffee is used under high pressure at elevated temperature for extraction. This water consists of 2 fractions press water and hydrolyzate. It contains several hundred organic compounds such as alkaloids (caffeine, theophylin etc.), 2-fufurylthiol, chlorogenic acid, polysaccharides, trigonelline, lignin, purines and flavorings of different structure such as aldehydes, ketones and carboxylic acids. Therefore, this example is particularly suitable because of the variety of compounds present in higher concentrations than micropollutants in the biologically treated wastewater. The resulting in the process hydrolyzate is a classic precipitation with precipitants such as polyacrylamide, iron chloride or milk of lime u. a. inaccessible. It was previously checked how high the content of carbonate anions in the original water was. In contrast to micropollutants, which are present in the μg range, the concentration is much higher here and is present in the higher mg range.

Experiments were carried out in the laboratory up to 1 to 2 liters of liquid and in the small technical scale of a maximum of 140 liters. The press water was allowed to stand overnight first, with sedimentation giving about 1 to 2% solids. Thereafter subjected to filtration through a 10-20 micron filter and introduced into the apparatus or the reactor. The dark brown process water was electrocoagulated into three reactors with iron, aluminum and titanium electrodes. After each step, an obtained precipitate was filtered off and blown air into the solution for 10 minutes with a porous aquarium stone. The average reaction time with the electrodes in an electrocoagulation step was 20 minutes, which was considered HRT.

At about 60 mN / cm ² current density with change in polarity as a surge, iron electrodes were used with an active area of 2200 cm ² and on a laboratory scale of 200 cm ² . With a flow rate of 300 to 500 l / h, about 1.8 hours were treated on a semi-pilot plant scale in a reactor with iron electrodes and an extremely consistent foam was obtained on the surface. This foam was so stable that it could be skimmed off. Three fractions were obtained: 1. very stable foam, 2. yellowish, turbid water and 3. little black sediment. The water was subjected to further filtration to obtain a pale yellowish clear filtrate, which was then sequentially treated in reactors with aluminum and titanium electrodes. The retention time was always about 20 minutes per electrocoagulation reactor.

In the subsequent laboratory experiment with 1-2 liters of this filtrate was subjected to electrochemical oxidation using stainless steel grid electrodes at 50 Hz, treated 20-40 volts and subjected to another sample in the last step with DSA electrodes for oxygen generation. After filtration through a 0.22 micron filter, the resulting liquid was clear and colorless. The scan in the UV spectrum was driven from 190 nm to 720 nm. There was no UV absorption - just a straight baseline. The resulting filtrate was further concentrated in a rotary evaporator to half volume and subjected to classical COD analysis. No readable value was received anymore. A measurement of the filtrate in an IC-MS system is in progress - the result is still pending.

Example 2

2 liters of the effluent after aerobic treatment of a sewage treatment plant was subjected to the same process. After filtration through a 100 micron filter, water was added in 3 coagulation stages each with HRT of 20 minutes, filtration of the precipitate and 10 minutes of aeration - sequentially with Fe, Al and Ti electrodes. The current density was also about 60 mA / cm ^{2 here} . The final step of hydroxyl radical electrochemical oxidation was cycled with stainless steel mesh electrodes at 50 Hz and 20-40 volts at constant flow at 15 liters per hour. After 30 minutes, filtration was continued through a 0.2 micron filter. A clear, colorless water was obtained, which gave only a baseline without absorption value in the UV spectrum. An amount of about 100 ml was concentrated in a rotary evaporator and about 25 ml and the residue subjected to a COD measurement. No value could be read.

In a secondary experiment, ultrasound at 20 kHz was added after treatment with stainless steel electrodes to generate hydroxyl radicals. The voltage supply electrodes were driven by a pulsating electric field at 50 Hz with one at a voltage of 20 to 40 volts with a current density of about 60 mA / cm ² . The conditions such as electrode area, temperature, flow rate were kept the same. Here, the contact time was halved to about 11 minutes, for the same result, after filtration through a 0.22 micron filter, which was UV adsorption and COD was measured. The resulting liquid was clear and colorless at the end of the treatment. The scan in the UV spectrum was driven from 190 nm to 720 nm. The resulting filtrate was concentrated in a rotary evaporator to half the volume and subjected to a classical COD analysis. No readable value was received anymore. A measurement of the filtrate in an IC-MS system is still pending.

LIST OF REFERENCE NUMBERS

1

Water reservoir

2

Reactor 1 with Fe electrodes

3

Curved screen with integrated air injection

4

Reactor 2 with Al electrodes

5

Curved screen with integrated air injection

6

Reactor 3 with Ti electrodes

7

Curved screen with integrated air injection

8th

Electrochemical reaction stage

9

pump

10

Band filter or filter press

11

sand filter

12

Carbon filter with membrane

13

UV detector

14

Air injection

15

Drain - purified water

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• "European Commission" - COM (2013) 574 final - has the "Urban Waste Water Treatment Directive" 91/271 / EEC [0005]
• Dept. Enviro. Closely. TU Crete, Greece - Feb. 2008, Michael Mavros et al. [0010]
• Water Science & Technology Vol 66 No 10 pp 2115-2121, 2012 and Drink. Water Eng. Science 2, 57-62, 2009 [0015]
• "Treatment options for wastewater effluents from pharmaceutical companies" in Int. J. Environ. Sci. Tech. 8 (3), 649-666 - 2011 [0015]
• Standard steel 1.1730 [0054]
• Pignatello et al., 2006 [0061]

Claims (10)

Continuous process of electrolytic, electrochemical treatment of water for the removal of micro-impurities from the water, characterized in that water is passed in the form of a solution or dispersion with a defined flow through a plurality of electrolysis reactors having different types of electrodes and different in the process Reactions as well as oxidations occur to mineralize organic compounds. A method according to claim 1, characterized in that in the process in the primary stage and successive steps, carbonate / bicarbonate is removed to allow a reaction by hydroxyl radicals and further organic compounds such as humic acids are depleted by the step of electrocoagulation. A method according to claim 1, 2, characterized in that three different metals - iron, aluminum and titanium - are used as electrodes in this order in reactors in the primary treatment as electrocoagulation, which form in this process step hydroxyl radicals, wherein an increased proportion of manganese in Iron electrodes prove to be advantageous. A method according to claim 1, 2 or 3, characterized in that by gas stripping, in the intermediate process step, volatile compounds are removed from the water, such as NH ₃ from an alkaline medium or fatty acids as well as steam-volatile compounds. A method according to claim 1 to 4, characterized in that for an electrochemical reaction stage for the production of hydroxyl radicals, diamond electrodes known as boron doped diamond or stainless steel electrodes are used and for the formation of oxygen for oxidation, DSA electrodes preferably type metal oxides based on titanium as IrO ₂ / Ta ₂ O ₃ or Ti / SnO ₂ or Ti / PbO ₂ generate high amounts of oxygen. Method according to one of claims 1 to 5, characterized in that the addition of ultrasound takes place in the range of, for example 20 kHz and 500 kHz, wherein a positive, synergistic effect occurs, which leads to an increased release of hydroxyl radicals or turnover, wherein Preferably steel alloys are used as electrodes, which cause in this catalytic, electrochemical step, a reaction rate increase, which applies to the electrocoagulation and for the electrochemical oxidation. Method according to one of claims 1 to 6, characterized in that at the end of the process, a filtration step takes place, which uses a sand filter, a carbon filter with integrated membrane and finally defines a quality level of the effluent water by a UV measurement. Method according to one of claims 1 to 7, characterized in that the method depleted in the first step of electrocoagulation also heavy metals, isotopes, phosphate, chloride, sulfate and other ions in the content. - except alkali metals. Method according to one of claims 1 to 8, characterized in that in the overall process atmospheric oxygen by blowing, oxygen by electrochemical reaction and hydroxyl radicals, and their derived by-products such as H ₂ O ₂ and other transition compounds as well as radicals are used, wherein also an addition of UV Radiation in the range of 254 nm is allowed to increase the effectiveness of the process. Method according to one of claims 1 to 9, characterized in that in all process stages different forms of oxidizing agents are used, such as atmospheric oxygen, oxygen-anion radicals, hydroxyl radicals, intermediary products such as hydrogen peroxide under the action of UV radiation and in the presence of Metal ions cause an advanced oxidation process and to increase the material conversion ultrasonic countercurrent preferably in the range of 20 kHz and 500 kHz. can be added.

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