ORIGINAL ARTICLE Advanced treatment of swine wastewater by electrodialysis with a tubular ion exchange membrane

Transcription

1 Blackwell Science, LtdOxford, UKASJAnimal Science Journal Blackwell Publishing Asia Pty LtdOctober Original ArticleElectrodialysis of swine wastewatery. FUKUMOTO and K. HAGA ORIGINAL ARTICLE Advanced treatment of swine wastewater by electrodialysis with a tubular ion exchange membrane Yasuyuki FUKUMOTO and Kiyonori HAGA National Institute of Livestock and Grassland Science, Tsukuba-shi, Japan ABSTRACT To remove the excess nitrogen and phosphorus in swine wastewater, an electrodialysis technique was applied to an advanced treatment method. The laboratory-scale swine wastewater treatment system constructed for the present study consisted of an activated sludge process, as the main treatment unit, and electrodialysis, as an advanced treatment unit. This system was operated for 200 days and the processing performance was evaluated. By electrodialysis, approximately 99% of NO 3 and PO 3 4 in the activated sludge-treated water (AT solution) was removed during operation. Furthermore, electrodialysis decreased the color density of the AT solution at a rate of 58%. The advanced treatment of swine wastewater by electrodialysis proved to be an efficient technique to remove excess nitrogen and phosphorus, and decrease color density. KEYWORDS: color density, electrodialysis, nitrogen, phosphorus, swine wastewater. INTRODUCTION Swine feces, urine and the washing water from piggeries are separated into solid and liquid fractions by mechanical solid-liquid separation. The solid fraction is composted and used as an organic fertilizer. The liquid fraction (swine wastewater) is generally treated by biological wastewater treatment methods, for example, activated sludge process and anaerobic treatment (Haga 1998). These biological treatment methods can decrease biochemical oxygen demand (BOD), suspended solids, nitrogen and phosphorus in wastewater. However, at the high levels of regulation standard, especially nitrogen and phosphorus, it is difficult to sufficiently reduce them to levels that comply with the effluent standards of the water pollution control law in Japan using biological methods alone (Osada et al. 1991). Some parts of nitrogen and phosphorus compounds exist in wastewater as ions. After biodegradation processing, the ratio of ionic nitrogen and phosphorus increases. Therefore, if this ionic nitrogen and phosphorus can be removed from wastewater, it is possible to reduce the risk of water and environmental pollution by the discharge of wastewater into public water bodies. An electrodialysis method with ion exchange membranes has been applied to a variety of processes, including the desalination of seawater (Yawataya 1982), the food industry (Yawataya 1982), fermentation technology (Hongo et al. 1986) and the recovery of nickel (Markovac & Heller 1981, 1982). Electrodialysis is an electro-membrane process, in which ions are transported through ion exchange membranes from one solution to another under the influence of a potential gradient. The electrical charges on the ions allow them to be driven through the membranes. Charging a voltage between two end electrodes generates the potential field required. As the ion exchange membrane has the ability to selectively transport ions Correspondence: Yasuyuki Fukumoto, Department of Livestock Industry Environment, National Institute of Livestock and Grassland Science, Tsukuba-shi, , Japan. ( yasuyuki@affrc.go.jp) Received 21 January 2004; accepted for publication 20 April Animal Science Journal (2004) 75,

2 Y. FUKUMOTO and K. HAGA with a positive or negative charge and to reject ions of the opposite charge, the useful concentration, removal or separation of electrolytes can be achieved by electrodialysis. Therefore, it is possible that the excess nitrogen and phosphorus, which are formed as ions in swine wastewater, can be removed by this technique (Indusekhar et al. 1991). In the present study, the electrodialysis method was applied into an advanced processing method in the treatment of swine waste. Therefore, the swine wastewater was firstly treated by a biological degradation process (activated sludge process), and then the treated water was introduced into the electrodialysis unit to remove the ionic nitrogen and phosphorus. The swine wastewater processing system combined with the electrodialysis unit was constructed on a laboratory scale. A thin film type of ion exchange membrane seemed inadequate to apply to the advanced treatment of swine wastewater because of its physical weakness. The tubular ion exchange membrane (EDcore, Tokuyama, Tokyo, Japan) has a thick, strong membrane (membrane thickness, 3 mm). In the present study, the tubular anion exchange membrane was used to construct the electrodialysis unit. This system was operated for 200 days and the processing performance was evaluated for nitrogen, phosphorus and other water quality factors. MATERIALS AND METHODS Swine wastewater Fresh swine feces and urine were collected from a pigpen at the National Institute of Livestock and Grassland Science and were completely mixed at a weight ratio of one (feces) to two (urine). The mixture was screened by a sieve (0.5-mm mesh) and stored at 4 C as a stock wastewater before the experiment. When the mixture was used for the experiment, it was diluted with water to adjust the BOD to approximately 750 mg/l (BOD loading rate was 0.25 kg/m 3 ). This diluted stock wastewater was used as influent in this study. Swine wastewater processing system with electrodialysis The system consisted of the activated sludge processing unit, and the electrodialysis treatment unit (Fig. 1). In the activated sludge processing, a fermentor (3 L operational volume, MBF-500 M; Eyela Tokyo Rikakikai, Tokyo, Japan) was used as a reactor. Intermittent aeration was conducted at 1-h intervals. The unit was set for a 24-h cycle. At the start of the operation cycle, 1 L of swine wastewater was charged in the reactor, and then a 1-h non-aeration period was set to stimulate the denitrification process. After the end of the aeration periods, the sludge in the mixed liquor was allowed to settle for 1.5 h and 1 L of the supernatant (activated sludge treated water: AT solution) was obtained. This operation program was controlled by a process controller (EPC-1000, Eyela Tokyo Rikakikai). A tubular anion exchange membrane (diameter, 6.1 cm; length, 45 cm) was set in a plastic cylinder (volume, 3 L), with an injection port on the lower side of the wall. An iron pipe was inserted in the tubular ion exchange membrane as the anode, and an iron net was set surrounding the tubular membrane as the cathode. The electrodialysis unit consisted of two areas. The first was the space between the tubular Fig. 1 Schematic diagram of the activated sludge processing system combined with electrodialysis unit (tubular anion exchange membrane) for swine wastewater treatment. AT solution, activated sludge treated water; ED solution, treated water from electrodialysis unit; CC solution, circulated concentration water. Animal Science Journal (2004) 75,

3 Electrodialysis of swine wastewater membrane and inner wall of the plastic cylinder (outside cell, 2 L), the second area was the space inside the tubular membrane (inside cell, 1 L). The AT solution was introduced from the lower entry port of the plastic cylinder by a circulation pump at a rate of 1 L/day, and passed through the outside cell. The duration of the AT solution in the outside cell was 2 days, and one litre of effluent from the electrodialysis unit (treated water from electrodialysis unit: ED solution) was obtained every day. The circulated concentration solution (CC solution: water, 2 L), which was utilized for concentrating ions from the AT solution, was circulated between the inside cell and the stock bottle of the CC solution by circulation pump at a rate of 8.4 L/day. One half the volume of the CC solution (1 L) was replaced with fresh water at approximately 1-month intervals. Direct current was applied by a DC power supply (EX-375L2, Takasago, Kawasaki, Japan) at a constant current of 0.1 A (current density, 0.14 ma/ cm 2 ) during operation. The swine wastewater processing system with the electrodialysis unit was operated continuously in a room with a constant temperature of 20 C for 200 days. Analysis The water quality of the AT, ED and the CC solutions was analyzed once a week for ph, NO 3, PO 4 3, Cl, SO 4 2, Na +, NH 4 +, Mg 2+, K +, Ca 2+ and color density. The ph was determined by a ph electrode (Horiba, Kyoto, Japan). The concentrations of anions and cations were determined by ion-chromatography using a Dionex DX-120 analyzer attached to IonPac CS12A and AS12A columns. The color density was determined by a Drainage Analyzer NDR-2000 (Nippon Denshoku, Tokyo, Japan). RESULTS AND DISCUSSION The average concentration of BOD in the AT solution was 11 mg/l, and the decreasing rate of BOD was 98.5%. After the activated sludge processing, the ph of the AT solution ranged between 5.6 and 8.0 (average ph was 6.7). The changes in the anion (NO 3, PO 4 3, Cl and SO 4 2 ) of the AT, ED and CC solutions during experimental operation were shown in Fig. 2. The NO 3 -N concentration in the AT solution ranged between 1.2 and 59 mg/l (average 28 mg/l). By electrodialysis treatment, nitrate ions were removed from the AT solution and collected into the CC solution. The NO 3 -N concentration in the ED solution ranged between 0.03 and 0.55 mg/l; thus the average removal rate was calculated as 98.6% (Table 1). Fig. 2 Change in the concentration of anion (NO 3, PO 4 3, Cl, and SO 4 2 ) in the AT, ED and CC solution during the operation. AT solution, activated sludge treated water; ED solution, treated water from electrodialysis unit; CC solution, circulated concentration water. Animal Science Journal (2004) 75,

4 Y. FUKUMOTO and K. HAGA Table 1 Water quality of the AT and ED solution in long-term continuous operation about anion components NO 3 -N PO 4 -P Cl SO 4 -S AT ED AT ED AT ED AT ED Average (mg/l) Range (mg/l) Decreasing rate (%) AT solution, activated sludge treated water; ED solution, treated water from electrodialysis unit. Decreasing rate = (AT - ED) AT 100. The PO 4 3 -P concentration in the AT solution ranged between 19 and 35 mg/l (average, 30 mg/l). After the electrodialysis treatment, the concentration of PO 4 3 -P in the ED solution ranged between 0 and 0.29 mg/l (average, 0.21 mg/l). The average removal rate was 99.3% (Table 1). From these results, it was shown that electrodialysis treatment was effective in removing ionic nitrogen and phosphorus in swine wastewater. The effectiveness of electrodialysis depends on the electric current. Therefore, it is possible to vary the electric current to correspond to the treatment of higher concentrations of nitrogen and phosphorus than the concentrations in the present study. In this system, only an anion exchange membrane was used to construct the electrodialysis unit. Therefore, cations (e.g. NH 4 + ) in the AT solution could not be removed by this treatment system. The nitrogen compounds in the AT solution mostly took the form of nitrates because nitrification occurred during the AT processing. However, there is an actual possibility that some nitrogen compounds were discharged from the AT process in the form of cations because of inadequate nitrification activity. In that case, a new cation exchange membrane would be necessary to remove excess nitrogen in the AT solution. If oxidation in the AT process is adequate and the target ion compounds can form in anion, the advanced treatment can be conducted using only an anion exchange membrane as shown in this study. The removal rate for other anion compounds (i.e. Cl and SO 4 2 ) in electrodialysis treatment was also high (97.3% and 96.6%, respectively; Table 1 and Fig. 2). It is this movement of anion compounds that leads to the change in the ph of the ED and CC solution (Fig. 3). By electrodialysis treatment, the ph of the ED solution increased (average, 11.6), and that of the CC solution decreased (average, 2.2). When we discharge wastewater to public water bodies, the ph of the wastewater must be in the range between 5.8 and 8.6. Therefore, a high ph solution such as the ED solution can not be discharged. For this reason, we attempted to neutralize the ph of the ED solution by Fig. 3 Change in the ph in the AT, ED and CC solution during the operation. AT solution, activated sludge treated water; ED solution:, treated water from electrodialysis unit; CC solution, circulated concentration water. Fig. 4 Change in the ph of the treated water from electrodialysis unit (ED solution) by aeration at different aeration rates. aeration with air or pure CO 2 gas. By aeration with pure CO 2, the ph of the ED solution was neutralized within 1 min. It took longer to attain the allowed discharge ph level by aeration with air, but after 10 h of aeration the ED solution was neutralized using a low airflow rate (0.5 L/min; Fig. 4). When the aeration airflow rate was higher (1.5 L/min), the ph of the ED solution was neutralized in 4 h. The changes in cation (K +, Na +, Ca 2+ and Mg 2+ ) in the AT, ED and CC solution during operation were shown in Fig. 5. In this electrodialysis unit, only an anion Animal Science Journal (2004) 75,

5 Electrodialysis of swine wastewater Fig. 5 Change in the concentration of cation (Na +, K +, Ca 2+, and Mg 2+ ) in the AT, ED and CC solution during the operation. AT solution, activated sludge treated water; ED solution:, treated water from electrodialysis unit; CC solution, circulated concentration water. Fig. 6 Change in the color density in the AT and ED solution during the operation. AT solution, activated sludge treated water; ED solution:, treated water from electrodialysis unit. exchange membrane was used, so theoretically cations can not be removed from the AT solution by this treatment unit. K +, Na + and NH 4 + (NH 4 + was not confirmed in the AT solution during the experimental operation; data not shown) conformed to theoretical behavior. That is, the concentrations of these cations in the AT solution did not change during electrodialysis. However, the concentrations of Ca 2+ and Mg 2+ in the AT solution decreased during electrodialysis, contrary to the theory. Moreover, they were not concentrated in the CC solution in this study (Fig. 2). During the operation of the electrodialysis unit, crystal materials were confirmed in the outside cell, and the composition of the crystal material was analyzed. The crystal material consisted of 13% phosphorus, 24% calcium and 8% magnesium based on the weight ratio. Therefore, these ions formed crystals followed by precipitation in the outside cell. This accounts for the disappearance of Ca 2+, Mg 2+ and PO 4 3 during electrodialysis. The high ph (approximately 10 11) in the outside cell solution would also promote crystallization (Suzuki et al. 2002). This result suggested the possibility that the nutrient salt in the livestock wastewater could be collected as a solid material by electrodialysis. Another area of speculation is the disappearance of PO 4 3 in the electrodialysis unit. During operation, iron ions were melted from the anode that accumulated in the CC solution because the iron ions can not go to the cathode through the anion exchange membrane. A quantity of PO 4 3 in the AT solution might be transported into the CC solution, but there it could combine with the iron ions and be deposited in the inside cell (Donnert & Salecker 1999). During the operation of the electrodialysis unit, the color density of the AT solution decreased (Fig. 6). The average color density of the AT solution was 269 (range, ), and that of the ED solution was 122 (range, ). Therefore, the average decrease of the color density was calculated as 58%. The cause of the color in the AT solution is regarded to be humic substances (Matsubara & Urano 1994). Animal Science Journal (2004) 75,

6 Y. FUKUMOTO and K. HAGA Humic substances are electronegative because of their carboxyl content. Therefore, there is a possibility that they were also removed from the AT solution by electrodialysis similar to NO 3 and PO 3 4. However, the question remains as to whether humic substances penetrated the membrane and concentrated in the CC solution. We attempted to check the accumulation of humic substances in the CC solution by investigating the concentration of total organic carbon (TOC) and color density. However, because iron ions had melted from the anode and accumulated, the CC solution was dark reddish-brown in color, making it impossible to check the color density. The iron ions also made it impossible to check the accumulation of humic substances by checking the change in TOC because iron ions can combine with humic substances. From these results, we could not confirm the movement of humic substances in the CC solution in the present study. The surface of the ion exchange membrane was a dark brown color after operation. This pigmentation could be caused by the adsorption of color substances in the AT solution during electrodialysis. After the experimental operation, the ion exchange membrane was cut and the cross section of the membrane was observed. Although the surface of the membrane was colored dark brown, the coloring was not observed inside the membrane. From this observation, most of the humic substances accumulated on the surface of the membrane. If color removal is mainly owing to adsorption on an ion exchange membrane, the colored matter adsorbed will gradually be saturated, and the color removal ability could decline. During 200 days of operation, no remarkable decline in color removal ability occurred. Although operation continued in an abbreviated manner half a year after the exam, there was no large decline in colored-matter removal ability (data not shown). Therefore, it was suggested that the capacity for color removal is quite high, mainly by adsorption on the membrane surface. CONCLUSIONS A laboratory-scale activated sludge processing system combined with electrodialysis unit was constructed, and the processing performance was evaluated during 200 days of operation. Advanced treatment of swine wastewater by electrodialysis was effective in removing NO 3, PO 4 3 and other ions from the solution. The removal rates were high (approximately 99%), and the treated water after electrodialysis met the effluent standards of the water pollution control law regarding the concentrations of nitrogen and phosphorus. Although the ph of the ED solution increased to approximately 11 after electrodialysis treatment because of the ion movement, the ph of the ED solution was easily neutralized by aeration. In the outside cell of the electrodialysis unit, crystal materials consisting of phosphorus, calcium and magnesium were confirmed. This crystal material could be used for fertilizer. It was shown that nutrient salt in the livestock wastewater was recovered as a solid material by electrodialysis. The color density of the AT solution was decreased by electrodialysis treatment. The average rate of decrease in the color density was calculated as 58%. The tubular ion exchange membrane is fit for the advanced treatment of swine wastewater because this membrane is strong against a physical shock, and can be equal to prolonged use. ACKNOWLEDGMENTS The research was conducted as a part of Joint Regional Intensive Research Project of Kumamoto Prefecture financially supported by Ministry of Education, Culture, Sports, Science and Technology. The authors express sincere thanks to Dr Yasuo Tanaka and Mrs Keiko Sumiya for their valuable help in ion-chromatography analysis. We wish to thank Dr Kazuyoshi Suzuki for his technical advice on determination of elements of the crystal material. REFERENCES Donnert D, Salecker M Elimination of phosphorous from municipal and industrial waste water. Water Science and Technology 40, Haga K Animal waste problems and their solution from the technological point of view in Japan. Japan Agricultural Research Quarterly 32, Hongo M, Nomura Y, Iwahara M Novel method of lactic acid production by electrodialysis fermentation. Applied and Environmental Microbiology 52, Indusekhar VK, Trivedi GS, Shah BG Removal of nitrate by electrodialysis. Desalination 84, Markovac V, Heller HC Principles of electrodialysis for nickel-plating rinsewater. Plating and Surface Finishing 68, Markovac V, Heller HC Engineering aspects of electrodialysis for nickel plating rinsewater. Plating and Surface Finishing 69, Matsubara H, Urano K Analytical method for determining aromatic constituents of humic substances. Journal of Japan Society on Water Environment 17, (in Japanese) Animal Science Journal (2004) 75,

7 Electrodialysis of swine wastewater Osada T, Haga K, Harada Y Removal of nitrogen and phosphorus from swine wastewater by the activated sludge units with the intermittent aeration process. Water Research 25, Suzuki K, Tanaka Y, Osada T, Waki M Removal of phosphate, magnesium and calcium from swine wastewater through crystallization enhanced by aeration. Water Research 36, Yawataya T Ion Exchange Membranes, pp. 27, 120. Kyoritsu Shuppan, Tokyo. Animal Science Journal (2004) 75,