Study On The Processes Of Nitrogen And Phosphorus Removal By Enhanced Subsurface Flow Constructed Wetland

With the rapid growth of urban population and rapid development of industry and agriculture,a large number of sewage flows into urban rivers,resulting in serious pollution of natural water in the city.As an environmentally friendly sewage treatment process,constructed wetland has been used more and more,but it still has some problems,such as small pollution load,easy congestion and poor treatment effect on special water quality.In view of the problems existing in the traditional artificial wetland,this subject uses the method of electrolysis and the method of tidal flow to enhance the subsurface constructed wetland.The electrolysis-integrated tidal flow constructed wetland(ETFCW)reactor was constructed,which treated the polluted water in urban river courses,and the removal efficiency of the pollutants was compared with the tidal flow constructed wetland(TFCW)and the subsurface flow constructed wetland(SFCW).The main operating parameters of the ETFCW were optimized,and the mechanism of ETFCW denitrification and phosphorus removal was analyzed.First,the comparative analysis of ETFCW,TFCW and SFCW on the treatment of simulated urban river wastewater was carried out.It was found that the concentration of ammonia nitrogen in the effluent from ETFCW and TFCW was not very different,but the concentration of ammonia nitrogen in the effluent of SFCW was higher,and the removal rate of ammonia nitrogen by ETFCW and TFCW was 85.6% and 83.6% respectively,and the removal rate of SFCW to ammonia nitrogen was 37.1%.The removal rate of total nitrogen and total phosphorus by ETFCW was the largest in three reactors,which were 2.01 times and 2.53 times and 2.31 times and 3.28 times that of TFCW,SFCW,and 65.71% and 84.88%.The effects of current on the physiological and biochemical conditions of wetland plants in ETFCW were also investigated.The results showed that the current was not conducive to the growth of Canna,but the damage to Canna did not affect its efficiency in the wetland.Secondly,the response surface methodology was used to optimize the operation parameters of ETFCW.Voltage,electrolytic time and starting point of electrified time as independent variables,ammonia nitrogen,total nitrogen and total phosphorus were used as the response values.The results showed that the effect of the three factors on the removal rate of ammonia nitrogen and total nitrogen was very significant,in which the interaction between the voltage and starting point of electrified time had the greatest interaction effect on the removal rate of ammonia nitrogen.The interaction between the voltage and the electrolytic time had the greatest interaction effect on the total nitrogen removal rate,and the removal rate of ammonia nitrogen and total nitrogen increases with the increase of voltage.The trend of decreasing after the large reduction would increase first and then decrease with the prolongation of the electrolysis time;it would increase first and then tend to a stable trend with the delay of the starting time.For the removal rate of total phosphorus,the effect of voltage and electrolytic time on it was very significant,but the selection of the starting point of the electrified time had little effect on the removal rate of total phosphorus.The scheme to optimize the operation parameters of the ETFCW reactor was: the voltage is 15.26 V,the electrolytic time was 4.99 h,and the electrolysis starting time was 3.34 hours after the influent time.The removal rate of ammonia nitrogen was 89.15%,the total nitrogen removal rate was 83.66%,the total phosphorus removal rate was 89.05%.The optimized ETFCW reactor was used to treat the actual river wastewater.The actual removal rate of the pollutants was similar to the model prediction results,which can provide the design reference for the practical application of the ETFCW process.The mechanism of nitrogen and phosphorus removal in ETFCW was further analyzed.The results showed that the electrochemical system in ETFCW was not oxidizing to ammonia and nitrogen,but the reduction of nitrate nitrogen was obviously effective.FTIR analysis was carried out on the precipitates near the anode of the reactor.The precipitates were rich in iron oxide structure,phosphorus oxygen bond structure,and hydroxyl polymeric iron.The results of high throughput sequencing of microbes found that when the electrochemical system was added,the species of bacteria in the reactor changed much,and the larger abundances were related to the nitrification and denitrification process,including Saccharibacteria_norank,Nitrospira,Rhizomicrobium,Rhodanobacter,Thermomonas,Comamonadaceae_unclassified,Coxiella,Mizugakiibacter and other genus of bacteria.In particular,an autotrophic denitrifying bacterium,Rhodoblastus,was found in the activated carbon layer of ETFCW with hydrogen as electron donor.