Study On Efficient Denitrification Of Biogas Slurry Based On Ammonia Assimilation

With the continuous improvement of people’s living standards and the rapid development of the catering and the takeaway industries,the amount of wet waste is also increasing year by year.Wet waste is rich in organic matter and nutrients,which is a valuable biomass resource with high utilization value,can be easily degraded by the metabolism of microorganisms.Anaerobic digestion technology has gradually become the mainstream technology for wet waste treatment because it can simultaneously achieve reduction,harmlessness and resource utilization,but the subsequent treatment of its by-products,biogas slurry,is a challenge that needs to be given high priority.Biogas slurry is a kind of wastewater with typical characteristics such as high concentration of organic,high ammonia,low C/N,and complex pollutant components.The common denitrification technologies mainly include physicochemical methods（adsorption method,chemical precipitation method,etc.）and biological methods（traditional nitrification-denitrification,simultaneous nitrification and denitrification,short-range nitrification and denitrification,and anammox）.However,the physicochemical method methods have high operating costs and risk of secondary pollution.The mainstream biological denitrification process is susceptible to inhibition due to the complex composition of the digestate,and it will also cause the loss of gaseous nitrogen（NO,N₂O,N₂）.In contrast,nitrogen removal technology based on microbial ammonia assimilation has gradually attracted the attention of researchers because of its high efficiency,low cost,adaptability to high-salt and high-nitrogen wastewater without nitrogen loss.In this study,the centrifuged anaerobic fermentation biogas slurry from a wet waste recycling plant in Shanghai was used as the research object,and the research on the efficient denitrification of wet waste biogas slurry based on microbial ammonia assimilation was carried out.First of all,in order to grasp the actual denitrification mechanism of the multi-stage AO process used in the existing biogas slurry treatment system,an investigation was carried out on the tail water treatment system of a wet waste resource treatment plant in Shanghai.Through sampling and nitrogen balance analysis of water samples from five biological treatment tanks,as well as analysis of the evolution of microbial communities and functional genes involved in nitrogen metabolism in each unit tank,the actual denitrification mechanism in the wastewater treatment system of this plant was initially proposed.Secondly,taking the untreated biogas slurry after centrifugation in the plant as the research object,the pre-research experiments of aerobic,anoxic and sludge dosages were carried out,as well as the comparative experiments with different initial sludge concentrations under aerobic conditions.By studying the transformation of nitrogen and organic matter in the system before and after the reaction,the feasibility of denitrification by assimilation of biogas slurry was preliminarily determined,and by analyzing the nitrogen balance of the three phases of solid,liquid and gas in the reaction system,the denitrification by ammonia assimilation was clarified contribute.Finally,in order to further elaborate the mechanism of ammonia assimilation of biogas slurry from microbiological perspective.Theα-ketoglutarate in each reactor was monitored and analyzed during the reaction,and the sludge in the reactor before and after the reaction was subjected to high-throughput sequencing,and was also subjected to quantitative analysis of nitrogen removal functional genes.The conclusions of the study are as follows:（1）Research on the biogas slurry treatment from the wet waste resource treatment plant in Shanghai.The results of the field investigation revealed that the removal of organic matter and nitrogen by the biological treatment system was mainly concentrated in the primary denitrification tank and the primary nitrification A tank.High denitrification rates were monitored in both the primary denitrification tank and the nitrification tank,indicating that the high reflux ratio mixed the effluent and sludge in the anoxic tank and aerobic tank with each other,and each tank had evolved a similar biochemical environment.In addition,the accumulation of NO₃^--N and NO₂^--N was not monitored in any of the biological treatment ponds,indicating that the organic matter and nitrogen in the digestate were not removed by the nitrification-denitrification pathway although the biological treatment system was designed and operated according to the two-stage AO process.The results of high-throughput sequencing and functional gene monitoring show that each unit tank contains a large number of microorganisms and functional genes involved in ammonia assimilation,and their species and abundances were similar in each tank,indicating that the actual nitrogen removal mechanism of the biological treatment system currently operating in the plant was actually ammonia assimilation.（2）Research on the mechanism of ammonia assimilation in biogas slurry.The pre-research experiments of aerobic,anoxic and sludge dosages were carried out,and the comparative experiments of different initial sludge concentrations under aerobic conditions.It was found that the reactors under aerobic conditions could achieve simultaneous degradation of nitrogen and organic matter,and the degradation effect is significantly positively correlated with the dosing of sludge added,indicating that the degradation of organic matter and nitrogen is related to the biological activities of microorganisms under aerobic conditions.Microbial decomposition of organic matter will release alkalinity,which leads to an increase in p H of biogas slurry.Most microorganisms with ammonia assimilation function can show better adaptability in alkaline conditions,so p H and microbial degradation of pollutants have a positive impact on each other,and then the reactor gradually achieves a stable nitrogen removal performance.Phosphorus removal was positively correlated with the fluctuations of p H,and the analysis suggested that TP was mainly removed through biologically induced chemical precipitation caused by p H rise.At the end of the reaction,the sludge in each reactor multiplied,and the specific performance was that the MLSS,MLVSS and MLVSS/MLSS increased significantly,and the total EPS,PN,PS and intracellular protein concentration of the unit sludge all increased.The analysis of COD removal performance shows that the consumed organic matter and nitrogen provide nitrogen source,carbon source and energy for the assimilative growth of microorganisms,which further proves that microbial assimilation has occurred in the biogas slurry.In order to clarify the contribution of ammonia assimilation to the denitrification effect,the nitrogen balance in the system was analyzed.It was found that most of the nitrogen in the liquid phase in each reactor flowed to the solid phase through assimilation,and the removal of NH₄⁺-N in the liquid phase was positively correlated with the sludge concentration in the reactor and the total nitrogen increase in the solid phase.The total nitrogen removal efficiencies in R0,R1 and R2 rectors are47.81%,63.25%and 73.17%,respectively,among which the assimilation removal rates are30.18%,40.52%and 52.02%,respectively,which proves that the ammonia assimilation was the dominant way of nitrogen removal from the biogas slurry.（3）Microbial population analysis and determination of key genes.A certain amount ofα-ketoglutarate（α-KG）was also monitored in the control R0 reactor without additional activated sludge injection before and after the reaction,indicating that microorganisms with ammonia assimilation were originally present in the biogas slurry.As the reaction proceeded,the content ofα-KG in each reactor increased,which led to an increase in the raw material for Glu synthesis and therefore the final protein content produced by the microorganisms.The species diversity in the R0,R1,and R2 reactors decreased after the reaction,indicating that some unnecessary microorganisms were gradually eliminated,and the microorganisms that could adapt to the high-nitrogen aerobic biogas slurry were enriched.After the reaction,the OTUs overlap between the three reactors was high,and the microbial population and abundance were basically similar.The microbial phylum（Bacteroidetes,Proteobacteria,Deinococcus-Thermus,Actinobacteria,etc.）,and the microbial genus（Aequorivita,Truepera,Taibaiella,Thiopseudomonas,etc.）with ammonia assimilation function were gradually screened out as the dominant microorganism,indicating that all the reactors have similar microbial community evolution,that is,they all evolve towarded effective assimilative microorganisms.In addition,the relative copy number of functional genes encoding ammonia assimilation in each reactor was significantly higher than those involved in nitrification and denitrification at the end of the reaction,demonstrating that ammonia assimilation was the dominant nitrogen removal pathway in the three reactors.