Constructed Wetland Coupled With Isolated Chlorella Sorokiniana ZM-5: Wastewater Treatment Performance And Microbial Response Mechanism

In recent years,with the rapid development of social economy and the trend towards scale,the environmental pollution and resource waste caused by livestock and poultry breeding wastewater(such as swine wastewater)begin to receive special attention.Swine wastewater,as a typical type of livestock and poultry wastewater,is discharged in large quantities,causing high concentrations of pollutants to be directly released into the natural environment and posing a serious threat to ecological balance and human health.Therefore,how to treat swine wastewater has become an urgent issue.At present,anaerobic digestion technology is a widely used swine wastewater treatment technology.However,after anaerobic digestion treatment of swine wastewater,a large number of nitrogen components(especially ammonia nitrogen)remain in the wastewater,which needs further treatment before reaching the discharge standard.Constructed wetlands(CWs)are considered an economically effective system for treating swine tailwater.However,due to the lack of carbon source in the denitrification stage,the nitrogen removal effect of CWs is limited.In recent years,studies have shown that the CW coupled with microalgae can solve the problem of lacking carbon source in the denitrification stage of the CW,and improve the pollutant treatment capacity.This study used a single cell sorting technique based on laser induced forward transfer(LIFT)to successfully separate Chlorella sorokiniana ZM-5 from an CW near a swine farm in the vicinity of Harbin using the PRECI SCS single cell sorting instrument.The enrichment and expansion of single cell Chlorella sorokiniana ZM-5 were achieved.Through single factor experiments,it showed that Chlorella sorokiniana ZM-5 could produce energy substances(biolipids)from swine tailwater(COD 400 mg/L、TN 266 mg/L and TP 9 mg/L).Based on this characteristic,reasonable treatment and resource utilization of swine tailwater could be achieved.Through response surface methodology(RSM)optimization results,it was found that the optimal reaction conditions for treating swine tailwater with Chlorella sorokiniana ZM-5 were illumination time of 24 hours,C/N 4.3:1,p H 7.2,and temperature of 30℃.Under optimized conditions,the degradation rates of COD,TN,and TP reached 84.38%,72.77%,and 64.06%,respectively,and the accumulation of biological lipids reached 352.61 mg/g.The results of life cycle assessment(LCA)and life cycle cost assessment(LCC)showed that the optimized conditions reduced the net environmental negative impact and the total economic cost of swine tailwater treatment by about 48% and 10.24%.At the same time,the optimized conditions increased the environmental benefits and economic benefits of biological lipid production by about 49% and 50.28%.Based on the above analysis,the treatment of swine tailwater and the production of biolipids by Chlorella sorokiniana ZM-5 can lay the foundation for the development of microalgae resource utilization of swine tailwater.This study also explored the feasibility of improving the nitrogen removal capacity of the Chlorella sorokiniana ZM-5 coupled with CW.Compared with a single constructed wetland(CK),the coupled system of microalgae-CW(EG)increased the removal rates of COD and TN by about 20%.At the same time,Chlorella sorokiniana ZM-5 could interact with the denitrification bacteria in the CW microbial community to make up for the deficiencies in the denitrification stage.HT q PCR chip analysis also showed that the abundance of denitrification genes(nos Z,nir S and nir K)in EG increased significantly(p<0.05)by 28.48%,35.89% and 20.55%,respectively.Moreover,the absolute abundance of pho D and PPK in EG increased by about 68% and 48% compared to CK,demonstrating stronger phosphorus mineralization ability.The above results indicated that the addition of Chlorella sorokiniana ZM-5 could affect the functional metabolism level of the microbial community in EG through the symbiotic relationship between microalgae and bacteria,thereby enhancing the pollutant treatment capacity of the CW system.Besides,the environmental impacts of four lipid extraction technologies(soxhlet extraction(SE),ultrasonic extraction(UE),hydraulic cavitation extraction(PLE),supercritical fluid extraction(SFE))were studied.According to the results of LCA,UE was a lipid extraction technology with relatively low environmental pollution.Clean energy could be used to optimize power sources to reduce the environmental load caused by lipid extraction processes(45%-60%).Overall,these findings provide new insights for the practical application of microalgae-CWcoupling system in treating swine tailwater.