Research On The Performance And Mechanism Of Methane Production,Denitrification And Deodorization In Bioecological Process Of Decentralized Domestic Wastewater Treatment

Due to the dispersed population,poor wastewater collection,and weaker economy in rural areas,exploring decentralized domestic wastewater treatment technologies are of great interest for rural areas.Based on the concept of "wastewater to energy and resources",and to meet the demand of wastewater deodorization,we designed a bioecological process comprised of three-stage anaerobic wool-felt filter reactor(AWFR),an anoxic filter(ANF),four-stage water-dropping-self-rotating biological contactor(WDSRBC)and plant type construct wetland(PCW)to treat rural decentralized domestic wastewater.The AWFR units are used for the conversion of organic matter to methane at ambient temperature.A low-enery ANF-WDSRBC system is explored to denitrification and odor removal in the ANF unit via WDSRBC effluent reflux.The nutrients,such as N and P,are utilized by the ecological PCW unit via planting vegetables and food crop.The main results of this study are summaried as follows:Firstly,we used lab-scale AWFR to assess the effects of carrier filling ratio on the performance of AWFR under start-up conditions.The study showed that the removal efficiencies of COD and SS gradually increased with an increase of hydraulic retention time(HRT).Appropriately increasing carrier filling ratio in the system was beneficial to shorten the start-up time and stabilize the operation performance.Considering the proper carrier filling ratio chosen to minimize clogging in the system,we recommend carrier filling ratio of wool-felt in the system was 40%.Secondly,the pilot-scale AWFR was design to investigate the effect of HRT on the performance of AWFR in different seasons.In order to evaluate the performance of the system at ambient temperature,we used mass balance to analysis the pathway of organic matter removal.Meanwhile,the high-throughput sequencing was used to comprehensively analyze of the diversity of microbial community in the system.The results showed that COD removal efficiency increased with the extension of HRT,whereas the VFA of the effluent decreased.The system required long HRT to stabilize the performance due to a decrease of seasonal temperature.The annual performance of the system showed that COD removal efficiency and daily gas production increased significantly with an increase of seasonal temperature and a decrease of HRT.In summer,daily gas production reached a peak value of 10.7L/d.Based on COD mass balance,we found that approximately 43.5%-52.5%of influent COD was converted to methane.Seasonal temperature and HRT had a strong impact on the relative abundance of methanogens.In order to keep the system achieve a new equilibrium,the most predominant methanogens shifted from acetoclastic methanogens in summer to the hydrogenotrophic methanogens in winter,whereas Bacillus,which contributed to the fermentation,was always the most abundant genus.Thirdly,as a modified rotating biological contactor,the structure of WDSRBC units directly affects the ANF-WDSRBC denitrification and deodorization.Due to water-dropping gravity rotation of the WDSRBC,water-dropping height,HRT and influent ammonium concentration were key factors of the system.In order to optimize the performance of ammonium removal efficiency in the WDSRBC system,response surface methodology(RSM)was used to assess the effects of the three parameters and their interactions.The results showed that water-dropping height,HRT,influent ammonium concentration,the interaction of water-dropping height and HRT,the interaction of HRT and influent ammonium concentration were significant parameters.The removal efficiency of ammonium of 80%was achieved,when the system was operated with the influent ammonium concentration ranged from 20-50 mg/L,water-dropping height 0.6 m,and an HRT of 2.2 h.In order to achieve denitrification and deodorization in the ANF-WDSRBC system,WDSRBC effluent was recycled to ANF to utilize nitrate and dissolved oxygen(DO).The study showed that threshold odor number(TON)gradually decreased with the increase of reflux ratio,HRT,DO and nitrate loading rate.However,TN removal efficiency increased at first followed by a decline as reflux ratio increased.Increasing HRT and nitrate loading rate were beneficial to TN removal,whereas high DO concentration decreased TN removal owing to inhibition of denitrification.The system was used to treat the effluent of AWFR under continuous-flow operation with a total HRT of 8.8 h and a reflux ratio of 100%.The average concentrations of COD,NH4+-N,and TN effluent were 35.0mg/L,3.7mg/L and 17.1mg/L,respectively.The effluent TON was 16.2,which reached the level-two emission standard in the "Emission standards for odor pollutants"(GB 14554-93).The microbial community in different units of the system maintained diversity,The genera Thiobacillus,Thauera and Bacillus contributed considerably to denitrification,odor and nitrogen removal.The genera Nitrosomonas,Nitrosospira,Bacillus,and Nitrospira played vital roles in nitrification.Finally,the study was to investigate the performance of PCW system to utilize nitrogen and phosphorus via planting water spinach,rice,water fennel and wheat.It found that the removal efficiencies of TN and TP increased with hydraulic loading rate(HLR)decreased.Lower HLR was beneficial to improve the production of plants in system.When the PCW system was operated at an HLR of 0.2 m3/(m2·d),the contributions of water spinach and rice of the PCW system to utilized of nitrogen and phosphorus were 18.2%and 31.9%,respectively,that was higher than the contributions of water fennel and wheat(TN 15.1%,TP 23.4%,respectively).