Study On Swine Wastewater Treatment And Resource Recovery Based On Electrocoagulation And Electrochemical Precipitation

With the large-scale and intensive development of livestock and poultry breeding in China,the conventional anaerobic-aerobic biological treatment process for livestock wastewater treatment widely used in most farms has suffered from the problems of poor treatment efficiency and operation instability.However,the existing processes of the treatments only aim to degrade or remove the nitrogen and phosphorus without their recycling.To mitigate the pollution load and biological toxicity caused by high concentration organics,suspended solids and ammonia nitrogen in raw livestock wastewater to the subsequent biological treatment,electrocoagulation process was investigated as a pretreatment for improving the comprehensive efficiency and quality of the overall treatment process.To improve the poor removal of carbon and nitrogen by subsequent aerobic biological treatment process caused by insufficient carbon source and high nitrogen content of biogas digestion slurry,the recovery of nitrogen and phosphorus in the form of struvite was investigated to improve the carbon nitrogen ratio and its impact on the efficiency of subsequent aerobic biological treatment.The impact of methanogenic efficiency between raw swine wastewater and different pretreated effluents and organic matter on the recovery of nitrogen and phosphorus via struvite electrochemical precipitation were also investigated.The treatment and resource recovery process of swine wastewater with different pollution characteristics based on electrochemical method for“electrocoagulation/struvite electrochemical precipitation-anaerobic digestion-struvite electrochemical precipitation-aerobic SBR”was preliminarily constructed.The COD,ammonia nitrogen and total phosphorus of final effluent meet the Chinese wastewater discharge standard of pollutants for livestock and poultry breeding(GB 18596-2001).The main outcomes were achieved as follows:1.Electrocoagulation batch tests under different structural parameters(anode material,inter-electrode distance)and operational parameters(current density,stirring speed,initial p H and reaction time)were systematically conducted for raw high-strength swine wastewater pretreatment.The optimal conditions in this study were the Fe anode(better than Al anode),inter-electrode distance of 2.0 cm(better than 1.0 and 3.0 cm),current density of 30 m A/cm~2(better than 20 and 40 m A/cm~2),no stirring(better than 200 rpm),initial p H of 6.3 without control(better than 2.9 and 11.1)and reaction time of 35 min(low removal increase beyond it),where the good solid-liquid separation with supernatant recovery of 60%was achieved,the removal of SS,TP,COD and BOD was 98.5%,99.5%,50.6%and 10%,respectively,while the BOD/COD increased from 0.36 to 0.66.For comparison,conventional chemical coagulation using the same iron dosage with electrocoagulation achieved the poor solid-liquid separation with supernatant recovery of 10%,the SS,TP,COD and BOD removals of 95.7%,99.5%,26.8%and 20%respectively,while the BOD/COD increased from 0.36 to 0.40.Thus,electrocoagulation had significantly better performance than chemical coagulation.The results that higher COD removal,lower BOD removal and higher BOD/COD were achieved by electrocoagulation than chemical coagulation implied the formation of new biodegradable substances by electrooxidation occurred during electrocoagulation process,which was partially demonstrated by the newly formed fluorescent substances(tryptophan-like substances with Ex/Em of 220-240/370-400 nm)from fluorescent spectroscopy.2.Lab-scale batch experiments were systematically conducted to investigate the effects of initial p H,current density,organic substances(BSA,SA and HA)upon nutrients removal,and precipitates quality(crystal structure,morphology and purity)during the struvite electrochemical precipitation process.Under the optimal conditions of the initial p H of 7.0and current density of 4 m A/cm~2 in the absence of organic substances,the orthophosphate phosphorus and ammonium nitrogen removal efficiencies and purity of precipitates produced were 96.2%,11.5%and 94.2%,respectively.The nutrients removal and precipitate quality were significantly affected by different kinds,concentrations,and combinations of organic substances.XRD analysis revealed that the crystal form of the struvite precipitates produced was not changed in the presence of organic substances.SEM analysis revealed that compared with BSA,SA,and HA,the morphology of struvite crystals significantly changed.Purity analysis revealed that there was a positive correlation with nutrients removal efficiencies.Low concentration combination of bovine albumin,sodium alginate,and humic acid showed antagonistic inhibition effects,whereas a high concentration combination showed the accelerating inhibition effects.Consequently,more adverse effects were revealed upon nutrients removal,morphology,and purity of precipitates by HA than BSA and SA.Initial p H adjustment from 7 to 8 could effectively mitigate the adverse effects on struvite electrochemical precipitation under high concentration combined with organic substances(500 mg/L bovine albumin,500 mg/L sodium alginate,and 1500 mg/L humic acid).3.“Electrocoagulation/struvite electrochemical precipitation-anaerobic digestion-struvite electrochemical precipitation-aerobic SBR”process can realize the recovery of biogas energy and nitrogen and phosphorus resources of swine wastewater with different pollution characteristics and remove pollutants.For low concentration swine wastewater,the pretreatment of struvite electrochemical precipitation method showed a greater promotion of methanogenic performance than electrocoagulation method and the methane yield reached368 m L/g.Compared with the raw swine wastewater,the anaerobic biological treatment effluent significantly improves the efficiency of struvite electrochemical precipitation,and coupled membrane filtration helps to produce purer struvite with a purity of 96%.Compared with chemical precipitation,electrochemical precipitation pretreatment significantly improved the aerobic biological treatment efficiency of biogas digestion slurry.And the COD,ammonia nitrogen and total phosphorus of final effluent easily meet the Chinese wastewater discharge standards.