Research On Performance And Biofilm Characteristics Of UAF-BAF Process For Wastewater Treatment In Small Towns

At present,the wastewater treatment in small towns in China is still at a low level,which generally faces many obstacles,such as large changes in wastewater quality and quantity,lack of technology and poor management.Therefore,it is necessary to develop and apply an appropriate high efficiency and low consumption biofilm technology according to the typical wastewater characteristics of small towns.In addition,due to the lack of in-depth research on the micro-mechanism of biofilm and the microbial ecology,the biofilm process has insufficient operational stability.In this study,UAF-BAF process was selected to study the effects of COD/N,temperature and salinity on the performance,the biofilm characteristics and the microbial community characteristics,to deeply analyze the biofilm mechanism and the mechanism of microbial ecology in the UAF-BAF process.Finally,the applicability of UAF-BAF process in wastewater treatment of small towns was proved based on practical engineering cases.The effect of COD/N on pollutants removal and the biofilm characteristics in the UAF-BAF process was firstly discussed.Results showed that the removal rate of TN of system decreased with the decrease of COD/N.The changes of COD/N obviously affected the degradation law of pollutants along the reactor height.Based on the analysis of biofilm characteristics,with the decrease of COD/N,the biomass,biofilm thickness and dehydrogenase activity in the UAF-BAF process decreased,and the composition of EPS significantly changed.CLSM images confirmed the microscopic changes in the biofilm structure.High-throughput sequencing results showed that with the decrease of COD/N,the diversity of microbial community in the UAF reactor increased.The relative abundance of Firmicutes decreased significantly from 38.0% to 11.5%.In addition,functional prediction results showed that with the decrease of COD/N,the relative abundance of nitrogen metabolism genes decreased.In the nitrogen metabolism pathway,nitrate reductase dominated and its relative abundance decreased with the decrease of COD/N.Integrated temperature and COD/N effects on the performance of UAF-BAF process was investigated to evluate the feasibility of UAF-BAF process for wastewater treatment at low temperature of 10 ?.Results showed that the UAF-BAF process could achieve good pollutants removal,which met the first A criteria of wastewater discharge standard.Therefore,UAF-BAF process was an efficient and stable wastewater treatment technology,which was suitable for wastewater treatment at low temperature.The biofilm characteristics showed that with the decrease of COD/N,the biomass,maximum biofilm thickness and EPS content of system decreased.On the contrary,the biomass,maximum biofilm thickness and EPS content increased with the decrease of temperature.CLSM images confirmed the microscopic properties of biofilm composition at different operating stages.The results of microbial response showed that the microbial diversity and community structure changed significantly under different operating conditions.The dominant genera in the UAF reactors were Arcobacter,Tolumonas,Lactococcus,Desulfomicrobium and Flavobacterium,and the dominant genera in the BAF reactors were Janthinobacterium,Flavobacterium,Sphaerotilus,Pseudomonas and Ferruginibacter.The potential functions of microbial community changed obviously at different stages.The relative abundance of major metabolic genes increased in the UAF reactors with the decrease of COD/N,while the relative abundance of major metabolic types in B2 was higher than that in B1.The influence of salinity on the performance and the biofilm characteristics of UAF-BAF process was investigated to evaluate its adaptability to salinity inhibition.Results showed that the increase of salinity significantly inhibited the removal of NH4+-N and TN,and the system could achieve good pollutants removal when the salinity was not higher than 1.0%.The physicochemical properties of biofilm showed that with the increase of salinity,the biomass and biofilm thickness of system decreased.PS and PN content of EPS in the BAF reactor decreased,while PS and PN content of EPS in the UAF reactor increased.Furthermore,the dehydrogenase activity in the UAF reactor decreased significantly with the increase of salinity.CLSM clearly showed the response of biofilm to increased salinity.The 16 S r RNA results showed that the increase of salinity reduced the diversity of microbial community in the BAF reactor.The relative abundance of Nitrospira,which was the dominant genus and the main NOB,was significantly reduced.Nitrosomona was the main AOB,and its relative abundance increased with the increase of salinity.Function prediction showed that the functional characteristics related to metabolism changed significantly with the increase of salinity.The performance of a small town wastewater treatment plant in Heilongjiang during the long term operation was evaluated.Results showed that the UAF-BAF process had good removal efficiency for COD,NH4+-N,TN,TP and SS,which met the first A criteria of wastewater discharge standard.Based on the estimation of operation cost in the actual engineering,the waterwater treatment cost of UAF-BAF process was RMB 0.8125 per ton.Therefore,UAF-BAF process was a high efficiency and low consumption biofilm technology,which was suitable for wastewater treatment in small towns.