Study On The Factors Influencing Dissimilatory Nitrate Reduction To Ammonium (DNRA) And The Coupling Mechanism Of DNRA-Anammox In Activated Sludge

The dissimilar reduction of nitrate to ammonium(DNRA)in municipal sewage treatment plants reduces the denitrification efficiency by converting nitrate nitrogen to ammonia nitrogen instead of nitrogen.However,domestic and foreign studies on the effect of DNRA are mostly focused on the river bottom mud,sediment and other ecological environment,and the study on the ammonification characteristics of DNRA in activated sludge has not been in-depth enough.With the improvement of nitrogen and phosphorus discharge standards in municipal sewage treatment plants,the further improvement of biological nitrogen removal efficiency is the bottleneck of the implementation of the standards.Eliminating DNRA ammoniation in sewage treatment plants or using DNRA ammoniation effectively becomes a new way to break through the bottleneck of further improving denitrification in sewage treatment plants.Anaerobic ammonia oxidation(ANAMMOX),as a means of nitrogen removal,can save carbon source consumption and oxygen supply,avoid secondary pollution,and greatly reduce the operation cost of municipal sewage treatment plants.However,it is one of the main bottlenecks to provide stable nitrate supply at present.Combined with the action of DNRA,the coupled process of DNRA-Anammox can avoid the accumulation of ammonia nitrogen in the produced water caused by DNRA ammonification and break through the bottleneck of stable supply of Anammox nitrite,thus forming an innovative and sustainable denitrification system for urban sewage and improving the denitrification efficiency.The influencing factors of DNRA ammonification in activated sludge and the coupling mechanism of DNRA-Anammox were studied in this paper.Firstly,two groups of SBR reactors(1#intermittent flow and 2#continuous flow)were constructed to inoculate DNRA ammoniating bacteria in activated sludge for enrichment culture.The effects of different flow regimes,HRT and S2-on DNRA ammoniating bacteria in activated sludge were determined by multi-group batch test,and the optimal conditions for DNRA ammoniating enrichment were obtained.After optimized HRT,the SBR reactor was continued to enrich DNRA bacteria.By measuring the ammonification rate of DNRA and the population structure of DNRA functional microorganisms,the enrichment effect of optimized culture DNRA bacteria was analyzed.In this process,DNRA-Anammox coupling phenomenon was found in the system.Therefore,three groups of SBR reactors(different C/N,different mixing methods)were established to domesticate the activated sludge,the effects of different C/N and mixing methods on the coupling rate of DNRA-Anammox were determined by batch test.The denitrification microbial community structure was determined by 16S rRNA complete gene and denitrification,Anammox and DNRA functional gene high-throughput sequencing.The coupling effect of DNRA-Anammox was analyzed.The characteristics of DNRA-Anammox coupled sludge were analyzed in combination with the relevant parameters of sludge SEM,settling performance,EPS and SMP,and the formation mechanism of DNRA-Anammox coupling system was explored.Theoretical support for the implementation of DNRA-Anammox coupling process in sewage treatment plants was provided.The main research conclusions are as follows:(1)Batch test studies showed that,the conversion ratio of nitrate nitrogen to ammonia nitrogen in continuous flow system was generally slightly higher than that in intermittent flow system.Under the intermittent flow condition,when HRT was extended from 6 h to 48 h,the ammonification of DNRA increased with the extension of HRT,and the proportion of nitrate nitrogen reduced to ammonia nitrogen in the system increased from 0.54%to 15.2%.Under the continuous flow condition,similar results were obtained,and the proportion gradually increased from 2.54%to 25.9%.Adding S2-can effectively promote the conversion of nitrate nitrogen to ammonia nitrogen.When 20 mg/L S2-was added to the system,the conversion rates in the intermittent and continuous flow systems increased by 2.4%and 6.1%,respectively.Moreover,the total abundance of DNRA functional bacteria after S2-injection was 22.65%higher than before.Although prolonged HRT,S2-addition and continuous flow conditions were favorable for DNRA ammonification,HRT had a significant effect on DNRA ammonification.(2)Based on the optimized conditions,DNRA bacteria continued to enrich in the SBR system with influent C/N ratio of 10 and HRT of 48 h.When the effluent ammonia nitrogen concentration was stable,the maximum concentration could reach 4.34 mg/L,accounting for 14.47%of influent nitrate nitrogen,which was better than the long-term enrichment effect before changing the conditions(1.48%),but lower than the result of batch test.The maximum ammoniation rate of DNRA was 0.10 mgN(gVSS)-1h-1,which was 24%higher than that of DNRA under HRT of 12 h.The dominant phyla of enrichment culture mainly included Proteobacteria,Actinobacteria,etc.The functional bacteria genera of DNRA mainly included Anaeromyxobacter,Candidatus foremineofilum,Lacunisphaera,Actinomyces and Myxococcus.The highest abundance of functional strains of DNRA was 6.40%.(3)The maximum rate of DNRA-Anammox coupling reaction was 1.27 mgN(gVSS)-1h-1 when influent C/N=1.5 and the mixing method was mechanical stirring.The composition of the microbial community was roughly the same under different C/N and mixed methods,most of which belonged to Proteobacteria and Planctomycetes.The relative abundance of DNRA and Anammox functional genes was the highest under the influent C/N=7 and the mixing mode was oscillatory mixing,which were 10.49%and 14.48%,respectively.Although the abundance of functional genes was high under the oscillating mixing condition,the coupling rate of DNRA-Anammox was higher under the mechanical agitation,which indicated that although the mechanical agitation was not conducive to biological enrichment culture,the sludge activity was higher(DNRA-Anammox was higher).(4)Activated sludge acclimated by DNRA enrichment culture showed poor flocculation and sedimentation performance,and the protein content of extracellular polymeric substance(EPS)and soluble microorganism product(SMP)was low.Anammox bacteria had higher precipitation performance,and the synergistic symbiosis formed by the coupling of the two can not only improve the flocculation and precipitation performance of activated sludge,but also cope with the problems of insufficient carbon source and nitrite shortage.