Application of electrochemical technologies for the reclamation of treated wastewaters

Transcription

1 Application of electrochemical technologies for the reclamation of treated wastewaters MANUEL A. RODRIGO*, JAVIER LLANOS, CRISTINA SÁEZ, PABLO CAÑIZARES Department of Chemical Engineering, Faculty of Chemical Sciences and Technologies University of Castilla-La Mancha Edificio Enrique Costa Novella, Avda. Camilo José Cela 2, 37 Ciudad Real SPAIN Abstract: - This work focuses on the application of several electrochemical technologies in the reclamation of treated wastewaters. Conductive-Diamond Electrolysis (CDE) is successfully applied to eliminate the faecal coliforms of an actual effluent from wastewater treatment facilities with energy consumptions below. kwh m -3. Electrocoagulation (EC) with aluminum electrodes is applied to simultaneously decrease the concentration of COD and turbidity of actual effluents and it also attains good disinfection efficiencies also for energy consumptions below. kwh m -3. Integration of electrochemical processes is presented as a new challenge for the improvement in the efficiency of regeneration processes and very interesting results are obtained when electrolysis is combined with electrodialysis or electrocoagulation. Finally, it is described a pilot plant for the regeneration of treated wastewaters using electrochemical technology. This demonstration plant, which has been built in the Department of Chemical Engineering of the University of Castilla-la Mancha (Spain), is sized for an influent flow rate of l h - and it is based only on the use of electrochemical technologies. Key-Words: - Conductive-diamond, electrolysis, electrocoagulation, electrodialysis, wastewater reclamation, integrated processes, pilot plant, COD removal, turbidity removal, process intensification. Introduction Water demand is increasing fast and consequently the availability of high-quality sources of water is being lowered. For this reason, the regeneration and reuse of treated urban wastewaters is becoming a hot research topic for addressing water shortage [- 3]. Wastewater treatment facilities (WWTFs) reduce the COD, nitrogen and phosphorous content of urban wastewaters but the effluent quality attained is not enough for reuse. Consequently, further treatments should be applied in order to meet the requirements for different uses (industry, irrigation in agriculture, gardens, etc.). Before selecting the technique to be used in the regeneration process, it is necessary to define the quality standards that the final effluent should fulfill according to its subsequent use. This definition is one of the most controversial aspects of the regeneration system as every country and international organization use different criterion to establish which should be the quality requirements of the regenerated water. In the case of Spain, the RD 62/27 regulation establishes the legal framework for the quality standard that regenerated water should fulfill depending on its use. This regulation differentiates between urban, agricultural, recreational and environmental uses and it fixes the maximum threshold of microorganisms (nematodes and Escherichia coli), total solids and turbidity that are allowable for regenerated water. In the framework of European Union, the Directive 98/83/EC establishes the maximum allowable ions concentration for a regenerated effluent. Among the different technologies available to face these challenges, electrochemical techniques have attracted high interest due to some very promising results obtained in previous works done during the recent years [4-6]. One of the electrochemical technologies is electrocoagulation. This technique consists of the addition of coagulant species by means of the electrodissolution of a sacrificial electrode, which generally consist of aluminium or iron plates. This species helps to remove pollutants such as colloids, emulsions or anions [7-8] by a later sedimentation / filtration process. The electrocoagulation is a process currently used for the treatment of many industrial wastewaters and also in the production of drinking water from surface water reservoirs. Other technique which can be applied in the regeneration of treated is electrolysis for disinfection [9]. It consists of the direct electrolysis of the treated ISBN:

2 wastewater to be reused in an electrochemical cell, without dosing any reagents except for electricity. Initially, this technology produces chlorine and hypochlorite from chlorides contained in the effluent of the municipal WWTFs and consequently [], it can be considered as an especial type of chlorination technology. Simultaneously, the electrochemical reduction of some of the nitrates to ammonium may also be expected []. Hypochlorite reacts preferentially with ammonium, producing chloramines (breakpoint chlorination) and they become the main disinfection reagents in this technology. Special attention should be paid to the production of large amounts of chlorine in order to prevent oxidation of organic matter and subsequent formation of organo-chlorinated by-products. In addition, this overproduction of chlorine could also yield large amounts of chlorates (the production of which is related to the ageing of the hypochlorite) and even in some cases, perchlorates [2]. Both species are also harmful, and their production should be prevented. One of the advantages of electrochemical processes is that it is possible to perform multipurpose operations, taking advantage of the reactions that take place on the electrodes. Another advantage is that they can be easily integrated with other electrochemical or non-electrochemical technologies. With this integration of processes, it is possible to decrease both the investment and the operating costs of the treatment and attain very high efficient processes. 2 Objectives This work aims at presenting some relevant and recent results about the application of electrochemical technologies to the reclamation of treated wastewaters. These electrochemical technologies are Conductive-diamond Electrolysis (CDE), electrocoagulation (EC) and combinations of CDE with other electrochemical processes. Moreover, this work also presents a demonstration facility constructed in the Department of Chemical Engineering of the University of Castilla-la Mancha consisting of a pilot plant for the regeneration of wastewaters by electrochemical technologies. 3 Results and discussion 3. Conductive-Diamond Electrolysis Figure shows the variation in E.Coli concentration during electrolyses with conductive-diamond anodes of two actual effluents taken from the outlet of the secondary clarifier of two WWTFs at three different current densities (covering a one fold range from. to 5. A m -2 ). Ecoli / E.Coli Q /Ah dm -3 Figure. Variation of E.Coli during the electrolysis with diamond electrodes of WWTF :.3 A m -2, 6.5A m -2, 3 A m -2 ; and WWTF 2 :.3 A m -2, 6.5A m -2, 3 A m -2. The most remarkable result is that the electrolysis with conductive-diamond electrodes is able to attain the complete removal of E.Coli from treated wastewaters regardless of the characteristics of the effluent treated, and therefore, it behaves as a robust disinfection technology in the complete range of current densities studied. However, the disinfection rate and efficiency clearly depend on the current density used and on the characteristic of the influent water. In general, the higher the current density the faster the disinfection, although the results obtained clearly depend on the characteristics of the effluent treated. The disinfection process is carried out mainly because of the formation of hypochlorite, from the oxidation of chlorides, and of chloramines, due to the reaction of hypochlorite with ammonium ions. For the most efficient test, the electric charge required to completely eliminate the E. coli of the initial effluent is around.2 kah m -3. Considering this electric charge and an optimized cell potential of 5 V, energy consumption for the disinfection process would represent only. kwh m Electrocoagulation Fig. 2 shows the changes in the concentration of COD and turbidity during the electrocoagulation (using aluminium electrodes) of actual effluents ISBN:

3 obtained from two different WWTFs. In addition, the removal of E. Coli and nematode by electrocoagulation was also assessed. Fig. 3 shows the results obtained. COD /COD Q / kah m -3 Figure 2. Influence of the current applied on the removal of COD and turbidity (after 4 µm filtration) WWTF turbidity COD; WWTP 2 turbidity COD. Operation conditions: T: 25ºC; flowrate: 5 dm 3 h - ; Current density: A m -2. As it can be observed, both, turbidity and COD, decrease with the applied electric charge and good removal efficiencies are obtained for very low applied current charges. This means that this is a good technology to refine the quality of the effluent of WWTFs. Nem. eggs/nem. eggs Q / kah m -3 Figure 3. Variation of the concentration of nematodes eggs ( ) and E. Coli ( ) during the electro-coagulation process of two secondarilytreated wastewaters aluminium electrodes. Operation conditions: T: 25ºC; flowrate: 5 dm 3 h - ; Current density: A m -2. However, the more interesting results are shown in Fig. 3. As it can be observed, the concentration of nematode eggs and faecal coliforms decrease during the electrochemical process. In the first case the E. Coli/E. coli Turbidity/Turbidity decrease is very rapid and these changes can be explained in terms of filtration and not of the electrochemical process. At this point, it is worth mentioning that the samples were filtered (4 µm cut-off) after the electrocoagulation process. For this reason, an electrocoagulation process would require a subsequent filtration stage to be effective in the removal of nematode eggs. With regard to the concentration of faecal coliforms, it is possible to attain its complete removal for applied currents lower than. kah m -3. The efficiency of the disinfection process depends on the type of treated wastewater. During the process it was observed the production of small concentrations of oxidants (mainly hypochlorite), being these species responsible of the disinfection process. 3.3 Combined electrochemical processes Process integration emerges as a good alternative to minimize the costs related to energy consumption as well as the investment in electrochemical process. This process integration represents the union, in one single stage, of basic operations that are traditionally carried out separately. This process integration allows working towards the intensification of water treatment processes. Here, two examples of the integration of electrochemical processes are presented Electrodialysis-electrodisinfection The first approach for the integration of electrochemical processes is done with the combination of electrodialysis and electrolysis processes. In this case, the main aim is taking advantage of the electrode-rinsing solution in order to synthesize oxidants, which can be later used for the disinfection of the target effluent, simultaneously with the desalination of reclaimed wastewater. According to this proposal, an actual effluent previously treated in a WWTF is conducted through an electrodialysis / electrolysis integrated cell. In this cell, a portion of the initial effluent is concentrated (concentrate), whereas other fraction is desalted (diluate). Moreover, in the third compartment of the electrochemical cell (electrode rinsing) a solution is electrolyzed in order to produce oxidant agents with disinfectant capacity. This solution can be either a synthetic solution or a third portion of the initial target effluent, if the salts concentration of this effluent is high enough to produce a sufficient concentration of oxidants. Next, this disinfectant solution can be dosed to the diluate ISBN:

4 in order to produce a valued-added stream (desalted+disinfected) which can be reused for many different uses. With this configuration, the only limitation is the conductivity of the concentrated stream, which should be limited to the maximum allowable value for its discharge to the environment. Preliminary tests have been carried out using a third portion of the initial effluent as the electrode rinsing solution. In these tests (Fig. 4), it was observed that it is possible to eliminate completely the E. Coli present in the electrode rinsing solution due to the formation of oxidants, mainly hypochlorite, from the chloride initially present in the target effluent. Moreover, it was observed that it is possible to decrease the conductivity and the Sodium Adsorption Ratio of the effluent to values that allows the reclamation of the wastewater for irrigation purposes. Conductivity /Conductivity Q / Ah dm -3 Fig. 4. Evolution of the conductivity in the diluate solution ( ) and the concentration of E. coli in the electrode-rinsing solution ( ) during an electrodialysis - electrodisinfection process. Working conditions: 6 cell pairs; Working voltage: 5 V Electrocoagulation-electrodisinfection Other attempt to integrate electrochemical processes was done with electrocoagulation (EC) and electrolysis. In this case, a novel design of the electrochemical cell is presented. It consists of a single-compartment filter-press electrochemical cell, where a perforated aluminium plate is placed between anode and cathode. This Al foil acts as a bipolar electrode, in which the face that confronts the anode works as cathode and vice versa. Thus, the side of the Al foil which confronts the cathode is dissolved, allowing the generation of Al species for the electrocoagulation process E. Coli/E. Coli Fig. 5 shows the changes of E. Coli and turbidity of an actual effluent from a WWTF inside the EC- ED cell, using BDD as anode, stainless steel as cathode and a perforated aluminum bipolar electrode. As it can be observed, it is possible to decrease simultaneously both E. Coli and turbidity, in this integrated electrochemical cell. The main responsible of the disinfection process is the hypochlorite electrogenerated from the initial chloride and also chloramines, which are formed due to the reaction of hypochlorite and ammonium. E. Coli / E. Coli Q / kah m -3 Figure 5. Evolution of E. Coli ( ) and turbudity ( ) with the applied electric charge during the ED- EC process of urban wastewater. Anode: BBD; j: 6.65 A m -2 ; E. Coli : 24 CFU/ ml. Turbidity : 8 NTU. For the case of turbidity, this parameter is removed due to the dissolution of the face of the Al bipolar electrode that confronts the cathode (aluminium behaves as a bipolar electrode). In this case, the required electric charge to remove turbidity is considerably higher than that necessary to eliminate completely the E. Coli concentration. Nevertheless, these tests are still preliminary and the future work will be focused on the optimization of the electrochemical cell for the simultaneous removal of both parameters. 3.5 Pilot plant for waste water reclamation Taking advantage of the promising results observed when these electrochemical techniques are used for the reclamation of treated wastewaters, a pilot plant sized for a treatment flow-rate of l h - was projected and constructed in the Electrochemical and Environmental Engineering Laboratory of the Department of Chemical Engineering of the University of Castilla La Mancha for demonstration purposes. The first treatment process included in this plant is an electrocoagulation-electroflotation unit to.9.2. Turbidity/Turbidity ISBN:

5 remove turbidity, organic matter and suspended solids (Fig 6.). In this first stage, the formed solids can be separated either by flotation or by filtration of the bulk solution. Fig.6. Electrocoagulation Plant Next, a CDE unit (Fig. 7) is included in order to remove organic matter as well as to disinfect the water effluent. Fig. 8. Electrodialysis plant This plant was designed with a great versatility because of its demonstrative purposes. Thus, it is possible to work just with one or two of these operations, depending on the initial effluent to be treated and on the final use of the reclaimed water. Moreover, the plant is equipped with photovoltaic panels and with a transportation unit, with which it is possible to transport any single unit. These last two characteristics of the plant make it possible to perform the treatment process at remote locations. Fig 7.CDE plant Finally, a desalination plant can be applied to decrease the concentration of dissolved solids. This plant consists of a reverse osmosis and an electrodialysis plant (Fig. 8). Depending on the characteristics of the inlet effluent, it is possible to select one of these two desalination processes. 4 Conclusion The most relevant conclusion of this work is that electrochemical technologies emerge as a promising alternative to face the reclamation of wastewaters. It has been found that it is possible to disinfect actual effluents from a municipal WWTF and also to decrease considerably its concentration of COD and turbidity, in a very robust way and with reduced energy consumption. References: [] K. Zhang, K. Farahbakhsh, Removal of native coliphages and coliform bacteria from municipal wastewater by various wastewater treatment processes: Implications to water reuse. Water Research 4 (2), 27, pp [2] A. Joss, C. Baenninger, P. Foa, S. Koepke, M. Krauss, C. S. McArdell, K. Rottermann, Y. ISBN:

6 Wei, A. Zapata, H. Siegrist, Water reuse: >9% water yield in MBR/RO through concentrate recycling and CO2 addition as scaling control. Water Research 45 (8), 2, pp [3] M. Molinos-Senante, F. Hernández-Sancho, R. Sala-Garrido, R., Cost-benefit analysis of water-reuse projects for environmental purposes: A case study for spanish wastewater treatment plants. Journal of Environmental Management 92 (2), 2, pp [4] M. A. Rodrigo, P. Cañizares, C. Buitrón, C. Sáez. Electrochemical technologies for the regeneration of urban wastewaters. Electrochimica Acta 55 (27), 2, pp [5] H. Bergmann, A. T. Koparal, A. S. Koparal, F. Ehrig. The influence of products and byproducts obtained by drinking water electrolysis on microorganisms. Microchemical Journal 89(2), 28, pp [6] Yavuz, A. S. Koparal, U. B. Öǧütveren, Treatment of petroleum refinery wastewater by electrochemical methods. Desalination 258 (- 3), 2, pp [7] J. L. Trompette, H. Vergnes, C. Coufort, Enhanced electrocoagulation efficiency of lyophobic colloids in the presence of ammonium electrolytes. Colloids and Surfaces A: Physicochemical and Engineering Aspects 35 (-3), 28, pp [8] S. Zodi, O. Potier, F. Lapicque, J.P. Leclerc. Treatment of the industrial wastewaters by electrocoagulation: Optimization of coupled electrochemical and sedimentation processes, Desalination 26(-2), 2, pp [9] Z. Frontistis, C. Brebou, D. Venieri, D. Mantzavinos, A. Katsaounis, BDD anodic oxidation as tertiary wastewater treatment for the removal of emerging micro-pollutants, pathogens and organic matter, J. Chem. Technol. Biotechnol. 86, 2, pp [] A. M. Polcaro, A. Vacca, M. Mascia, S. Palmas, R. Pompej, S. Laconi, Characterization of a stirred tank electrochemical cell for water disinfection processes, Electrochim. Acta. 52, 27, pp [] E. Lacasa, J. Llanos, P. Cañizares, M.A. Rodrigo, Electrochemical denitrificacion with chlorides using DSA and BDD anodes, Chem. Eng. J. 84, 22, pp [2] G. Pérez, J. Saiz, R. Ibañez, A. M. Urtiaga, I. Ortiz. Assessment of the formation of inorganic oxidation by-products during the electrocatalytic treatment of ammonium from landfill leachates. Water Res. 46(8), 22, pp ISBN: