| Struvite(MgNH4PO4·6H2O) is a kind of white crystal poorly dissolved in water, it contains two kinds of nutrition elements nitrogen and phosphorus, which is a good slow-release fertilizer. Struvite can be formed in sewage、animal waste water and sludge dehydration liquor which is rich in nitrogen and phosphorus elements. Formation of struvite is an effective way to remove nitrogen and phosphorus pollutants from sewage and turn "waste" into "wealth" at the same time. Although nitrogen and phosphorous can be removed by means of struvite, which can be formed by adding chemicals or stripping carbon oxide in the wastewater, few studies focused on using electrochemical precipitation method to recover struvite.In order to explore the feasibility of recycling struvite by electrochemical precipitation, two series experiments, series I and series II, were carried out at first for determining the optimal operation conditions. In experiments, pH values were adjusted by the method of electrolysis. In series I, the cathode was stainless steel, and the anode was carbon rod. In series II, the cathode was stainless steel, and the anode was magnesium rod. Then, the treatment of domestic wastewater with the A2/O process and the combination of electrochemical precipitation and A2/O process were studied respectively. Conclusions are elicited as follows:(1) When anodes were the carbon rod and cathodes were stainless steel network, optimal electrolysis voltages were3V and best electrolytic time was2h. When anodes were the magnesium rod and cathodes were stainless steel network, optimal electrolysis voltages were3.6V and best electrolytic time was also2h. The struvite was enriched in the stainless steel nets and it was convenient for collecting(2) No stirring or low-velocity stirring was beneficial for the formation of the struvite in electrochemical precipitation and temperature did not have significant effects on the formation of the struvite. Low concentration Ca2+hardly affected the velocity of the electrolysis reaction, and high concentration Ca2+affected not only the reaction velocity, but also the struvite purity. Increasing of N:P ratio will promote the removal rate of PO43-, the best nitrogen and phosphorus removal ratio of N:P=2:1.(3) Treating the sludge dewatering filtrate by carbon rod or magnesium rod as anodes did not have significant differences. High purity struvite could be obtained under both conditions.(4) when started the A2/O process alone with low DO, the system could obtained92%-97%COD removal rate, could got74%～83%of the total phosphorus removal rate,62%-74%of TN and86%～90%of NH4+-N could be removed. In the combined technology of adding the electrochemical deposition mechanism to the system, the total phosphorus removal rate increased to96%-98%, the average removal rate was97.2%. The removal rate of COD、TN and NH4+-N was93%-96%,69%-76%and96%-69%, respectively. The outlet water concentration of COD, TN, and NH4+-N could be stable reached the standard.(5) There was good and stable removal efficiency for COD by the combined technology and A2/O process. Compared with the traditional A2/O process, the combination process had a great improvement of phosphate remove, the phosphorus concentration of effluent reduced about0.38-0.69mg/L, the removal rate was increased about18.4%and outlet water quality was relatively stable. After adding electrochemical deposition device, the outlet water of the total nitrogen decreased in some degree and output water could keep stable quality, compared with traditional A2/O process, the total nitrogen of effluent reduced about0.17～0.31mg/L.(6) Carrying out a cost comparison of struvite production with electrochemical precipitation when used stainless steel net as cathode, carbon rod and magnesium rod as the anode, respectively, and the method of adding dosage of Mg2+. When used traditional method of adding reagents for treating It sludge dewatering filtrate, it cost14.19yuan, while used carbon rod and magnesium rod as the anode in electrolytic precipitation, respectively, it cost10.15yuan and10.79yuan. When used carbon rod and magnesium rod as the anode to treat1t sludge dewatering filtrate could saved4.04yuan and3.4yuan that compared with traditional method of adding reagents.(7) Recycling struvite with electrochemical precipitation can not only lower operating costs of the sewage treatment plant and reduce the difficulty of subsequent process, but also produce the high purity of struvite which can be used as agricultural fertilizer. Furthermore, using magnesium electrolysis as anode greatly reduce the operating costs and most of the precipitation are enriched in the stainless steel net, which is helpful for workers to collect struvite precipitation. Electrochemical precipitation method combined with other processes will also solve the cost problems of reform process that sewage treatment plants face when the quality of processed water increased... |
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