Study On Environmental Distribution And Enrichment Culture Of Dissimilatory Nitrate Reduction To Ammonium(DNRA)Bacteria

Nitrate wastewater is widespread at present and it's main sources include urban sewage,agricultural wastewater and industrial wastewater involved in nuclear weapons manufacturing,pectin,coal products,metal processing.The discharge of nitrate wastewater would cause water eutrophication,which brings huge damage to the ecosystem and human health.The biological treatment on this kind of wastewater is an important topic of water pollution control.The traditional ways of nitrate removal are denitrification and anaerobic ammonia oxidation(Anammox),they convert nitrate to nitrogen gas releasing to the atmosphere,while dissimilatory nitrate reduction to ammonium(DNRA)keeps the nitrogen in the water and this process plays an important role in nitrate removal.Therefore,exploring the competition mechanism of DNRA and denitrification is very necessary.In this paper,DNRA bacteria community diversity and the competition conditions between DNRA and denitrifying bacteria were investigated in the Xinxue River constructed wetland and the Yellow River Estuary,which were helpful to optimize the environment parameters of DNRA enrichment culture.The research content mainly had three aspects:(1)investigated the distribution of dissimilatory nitrate reduction bacteria(DNRA,denitrifying bacteria and anammox)and study the effect of environmental conditions in the Xinxue River constructed wetland and Yellow River estuary;(2)achieved the DNRA enrichment culture using non-woven membrane bioreactor and optimized culture conditions;(3)comprehensively compared the DNRA bacterial community composition of sludge from wetland,estuary and the reactor.The main results are as follows:(1)DNRA bacteria was widespread in the natural aquatic ecosystems with different activities and abundances at different environment and different sampling sites.Generally,the activity of DNRA bacteria was positively correlated with corresponding abundance.And DNRA bacteria preferred high C/N ratio,nitrate-limited,slightly alkaline and reduced environment.DNRA bacteria abundance and community diversity in artificial wetland sediments were higher than those in the Yellow River estuary,which revealed that environment with higher organic carbon content was more suitable for DNRA.The sediments from artificial wetland are more appropriate for enrichment of DNRA bacteria.(2)Compared to the sediments,activated sludge from wastewater treatment plant was more suitable as seed sludge,and then DNRA enrichment conditions using non-woven fabric membrane bioreactor were optimized:C/N ratio was 10,temperature was controlled for 23 ?,pH was 7.1,COD was separated from ordinary water and two medium were pumped into the reactor in which carbon source of COD was sodium acetate,level controller controlled water discharge,hydraulic retention time was controlled at about 38 h,continuous mixing,N2 maintained anaerobic environment.In such conditions,DNRA bacterium were successfully enriched on 137 days' operation,and ultimately the efficiency of nitrate to ammonia was 60.65%.Eventually functional gene nrfA abundance reached 3.02 E7 copies/ng DNA.(3)Batch experiment showed that denitrification and DNRA were carried out step by step,the first step was nitrate to nitrite.Moreover,nitrate-limited culture can't be realized in batch experiment,denitrification activity was far higher than DNRA,so DNRA was more suitable for continuous flow environment.(4)Five functional populations including denitrifying bacteria,iron reducing bacteria(FeRB),sulfate reducing bacteria(SRB),sulfur oxidizing bacteria(SOB)and fermentation bacteria coexisted in the reactor,and under the catalysis of these microbes carbon,nitrogen and sulfur cycle worked well.Based on functional gene prediction,a large amount of nitrite reductase(ammonium-forming)was found and it proved that DNRA enrichment was achieved.In the reactor,main DNRA bacteria contained FeRB,some SRB and Thauera.(5)Proteobacteria,Chloroflexi and Verrucomicrobia are the most abundant phyla in each of the three systems:Xinxue River artificial wetland,the Yellow River estuary and culture sludge in the reactor.Among them,Deltaproteobacteria is the most predominant class.However,the functional genera are different in each other,for example Anaeromyxobacter is the most abundant genus in the wetland sediment while the most dominant genus in the Yellow River estuary sediment and reactor culture sludge is Geobacter.