Efficacy And Mechanism Of Assimilation And Denitrification Of Medium And High Concentration Ammonia Nitrogen Wastewater

The growing urbanisation and industrialisation processes all produce high concentrations of ammonia wastewater from a wide range of sources with complex compositions and ammonia concentrations of 150 to 2500 mg/L.Physico-chemical methods such as guano ammonia precipitation,ammonia stripping,adsorption and ion exchange techniques and membrane separation can initially achieve the recovery and removal of ammonia from medium to high concentration ammonia wastewater,but Most of them require energy supply support or consume large amounts of chemicals,which is costly and prone to secondary pollution of the environment.Traditional nitrification and denitrification technologies not only produce greenhouse gases such as N₂O,but also require high levels of aeration energy and external carbon input,resulting in high treatment costs.Heterotrophic ammonia assimilation microorganisms can convert carbon,nitrogen and phosphorus into their own organic matter,avoiding the production of greenhouse gases such as N₂O and effectively recovering nitrogen resources.The heterotrophic ammonia assimilation denitrification pathway has good sustainability and the sludge has excellent potential for resource utilization,which is in line with the national energy saving and emission reduction requirements under the dual carbon economy.In this study,a non-woven bag diffusion aeration system with regulated COD/TN was used to induce efficient ammonia assimilation denitrification of medium to high concentration ammonia-nitrogen wastewater based on the concept of no N₂O generation,effective carbon and nitrogen capture and maximisation of sludge resource utilisation.The feasibility of ammonia assimilation and denitrification of medium to high concentration ammonia nitrogen wastewater by the non-woven bag diffusion aeration system was firstly explained through a feasibility study of ammonia assimilation and denitrification of medium to high concentration ammonia nitrogen wastewater,and the ammonia assimilation mechanism of medium concentration ammonia nitrogen wastewater by the non-woven bag diffusion aeration system was initially investigated.The mechanism of ammonia assimilation removal from medium-concentration ammonia-nitrogen wastewater by the nonwoven bag diffusion aeration system was further illustrated by comparing the efficiency and mechanism of ammonia assimilation removal under different COD/TN conditions,and by combining p H and DO changes,phosphorus conversion studies and SMP changes in the effluent during the reaction,as well as the regulation of COD/TN on the efficiency of nitrogen removal by the nonwoven bag diffusion aeration system.The carbon and nitrogen balance analysis showed that the system could achieve effective carbon and nitrogen capture and uptake,quantified the carbon and nitrogen conversion relationship in the ammonia assimilation process,and further confirmed the mechanism of ammonia assimilation and nitrogen removal under this system.The mechanism of ammonia assimilation was further investigated in terms of post-reaction sludge characteristics through changes in sludge biomass,extracellular polymer and extracellular and intracellular protein andα-ketoglutarate content.Finally,the microbiological mechanisms of ammonia assimilation and denitrification under different COD/TN conditions were clarified by microbial diversity analysis,community structure analysis and nitrogen cycle functional gene assay analysis.The main conclusions of the paper are as follows:（1）Feasibility study on assimilative denitrification of medium and high concentration ammonia nitrogen wastewaterFeasibility study of ammonia assimilation denitrification for medium and high concentration ammonia-nitrogen wastewater:firstly,the concentrations of COD,TN,NH₄⁺-N,NO₂^--N,NO₃^--N and TP as well as p H and DO were monitored during the reaction process with a high concentration of simulated ammonia-nitrogen wastewater.The results show that the use of the complete mixed aeration reactor to treat high ammonia nitrogen wastewater can only make the reaction system tend to nitrification,and cannot achieve ammonia assimilation denitrification.In contrast,when the non-woven bag diffusion aeration reactor was used to treat high concentration ammonia-nitrogen wastewater,it did not accumulate NO₂^--N and NO₃^--N,but its denitrification was slow and the denitrification efficiency depended on COD/TN,and during the process of increasing COD/TN from 1.4 to 4.2,its removal rate remained low,and the highest TN and NH₄⁺-N removal rates were only 35.52%and 36.20%,which It was difficult to achieve effective removal of nitrogen from the system.The results showed that the NO₂^--N and NO₃^--N accumulated in large quantities in the complete mixed aeration group and the system tended to nitrify,and the TN removal rate was only 15.42%within 120 h,which was much lower than the 52.00%in the non-woven bag diffusion aeration group.In addition,there was almost no NO₂^--N and NO₃^--N production in the non-woven bag diffusion aeration group,which can be tentatively judged the existence of ammonia assimilation mode under this system.The degradability,chemical structure and molecular weight of the carbon source may affect microbial enzymatic activity,metabolic systems,growth and nitrogen degradation,and thus microbial denitrification efficacy.The effect of different carbon sources on the efficacy of assimilative denitrification of medium concentration ammonia-nitrogen wastewater showed that sodium acetate,sodium pyruvate,sodium citrate,sodium succinate and sodium malate mainly promoted microbial nitrogen removal by ammonia assimilation under the non-woven bag diffusion aeration system,but the sodium fumarate group produced large amounts of NO₂^--N and NO₃^--N in the reaction,which were more likely to trigger nitrification rather than assimilation under this system.The differences in TN and NH₄⁺-N removal rates between the groups also showed that the different carbon sources promoted ammonia assimilation efficiency in descending order:sodium acetate>sodium pyruvate>sodium citrate>sodium succinate>sodium malate.Therefore,sodium acetate was chosen as the exogenous carbon source in this study.The results of the ammonia oxidation blocking experiments showed that the removal of all pollutants in the non-woven bag diffusion aeration reactor was not affected by the addition of ATU,and that the changes in removal remained the same as in the blank group without the addition of ATU.It was initially confirmed that the removal of nitrogen in the non-woven bag diffusion aeration system does not rely on nitrification-denitrification,but assimilation.（2）Non-woven bag diffused aeration reactor assimilation denitrification studyA medium concentration ammonia-nitrogen wastewater in artificial configuration was used as the research object.In the four different COD/TN stages of reactor operation,the TN removal rate was 50.95%,93.66%,69.90%and 63.77%,and the COD removal rate was94.17%,98.41%,89.32%and 86.06%,respectively,as the COD/TN increased.The results showed that the ammonia assimilation efficiency was highest at COD/TN of 4.9-5.1,after which the COD/TN was increased and the ammonia assimilation efficiency decreased and the nitrogen removal efficiency became poor due to the limitation of too high carbon source for this diffusive aeration system,this experimental result was different from the conventional COD/TN increase on ammonia assimilation efficiency.In terms of COD/TN,new findings were provided for the study of the efficiency of ammonia assimilation and denitrification of medium concentration ammonia wastewater.The p H variation pattern of the system showed that microbial ammonia assimilation is a continuous process of alkalinity generation,and the p H rise rate of the system was highest at COD/TN of 4.9～5.1,confirming the best ammonia assimilation efficiency at this stage.In addition,the variation of DO in the system indicates that the regulation of COD/TN on this diffusion aeration system is mainly through the special DO gradient distribution in the reactor,which changes the microbial community structure,resulting in a corresponding change in the denitrification efficiency of the system.The variation of effluent TP content and sludge TP increase showed that the system could achieve simultaneous removal of nitrogen and phosphorus,and the best phosphorus removal effect was achieved at COD/TN of 4.9～5.1,with a removal rate of 93.42%.The results of the effluent SMP at each stage showed that the soluble microbial product fraction was mainly humic acid and the yield was low,which proved that the system has high carbon and nitrogen capture and absorption capacity.The quantitative relationship between carbon and nitrogen in nitrogen balance and carbon balance conversion shows that the carbon and nitrogen lost in the aqueous phase at each stage of the reactor is mainly absorbed by the sludge,and the ammonia assimilation process dominates the carbon and nitrogen metabolism in wastewater with medium concentration of ammonia and nitrogen.（3）Analysis of sludge characteristicsThe results of sludge biomass changes show that with the increase of COD/TN,the sludge microorganisms under the non-woven bag diffusion aeration system are continuously able to grow due to the assimilation of heterotrophic microorganisms,and the sludge production continues to grow,with the characteristics of more sludge production.At a COD/TN of 4.9-5.1,MLVSS/MLSS was the highest of the four stages,with the highest sludge activity and the best stability,showing the best denitrification state.The results of changes in sludge extracellular polymer（EPS）and intra-and extracellular protein content showed that the changes in sludge EPS content throughout the operation mainly relied on the accumulation of protein（PN）in the system,with PN occupying the major part of EPS,followed by polysaccharide（PS）and humic acid（HA）,and the system could produce a large amount of high quality protein for sludge resource utilization.the PN/PS results showed that the The results of PN/PS showed that the sludge in this system has strong stability and flocculation properties,which is conducive to efficient ammonia assimilation.α-ketoglutaric acid,as an intermediate link between carbon and nitrogen metabolism,showed that the highest content ofα-ketoglutaric acid was 169.15 mg/g of dry sludge at COD/TN 4.9-5.1,which was higher than the content ofα-ketoglutaric acid in other stages.The higher content ofα-ketoglutarate provided more substrates for carbon and nitrogen metabolism,which contributed to the efficient ammonia assimilation.（4）Analysis of microbiological mechanismsThe analysis of microbial diversity showed that at COD/TN of 4.9～5.1, microorganisms with ammonia assimilation function were enriched,while other nitrifying and denitrifying bacteria were inhibited or even died,and the diversity of microbial species and community richness in the reactor were reduced.However,Proteobacteria,Bacteroidetes and other microorganisms with ammonia assimilation function had the absolute advantage of nutrient competition and the best ammonia assimilation effect.The high relative abundance of SC-I-84,Candidatus＿Competibacter and PHOS-HE36 ensured that the ammonia assimilation process was carried out efficiently.With COD/TN regulating the specific dissolved oxygen distribution within the non-woven bag diffusion aeration system,it allows the system to sieve and select each bacterial group,thus realising the regulation of carbon,nitrogen and phosphorus removal rate by COD/TN.In addition,at a COD/TN of 4.9-5.1,the proliferation of ammonia assimilating functional microorganisms can be maximised,effectively eliminating all types of nitrifying and denitrifying bacteria,resulting in the best ammonia assimilation and denitrification efficiency of the system.The results of nitrogen cycle functional genes showed that the relative abundance of ammonia assimilation functional genes in the system was always much higher than that of nitrification and denitrification functional genes as the COD/TN increased.The relative abundance of ammonia assimilation genes was highest at COD/TN=4.9～5.1.In addition,the relative abundance of gdh A was always lower than that of gln A and glt B during the operation,indicating that the assimilation and removal of ammonia from medium concentration wastewater was mainly achieved by the GS/GOGAT pathway under the diffusion aeration system of the non-woven bag,further revealing the mechanism of ammonia assimilation and denitrification.