Effect Of Nitrogen Sources On The Performance Of Oxygen-Based Membrane Biofilm Reactor For Greywater Treatment

The allocation of resources is always a serious problem that human faced.Water,as"the source of life",will become an important factor restricting economic and social development in the future.Greywater（GW）has a high recycling value because of its light pollution and large production volume.GW reuse can bring considerable economic benefits.At present,there have been a large number of studies on GW treatment,among which oxygen-based manbrane biofilm reactor（O₂-MBfR）has attracted attention due to its unique advantages in GW treatment.Exploring the influence of different nitrogen sources on GW treatment by O₂-MBfR can provide theoretical support of engineering application for the system to treat source-deverted GW and other types of wastewater with high efficiency and low energy consumption.Therefore,in this research,an O₂-MBfR system was fabricated and applied for GW treatment.The effect of Chemical Oxygen Demand（COD）/Total Nitrogen（TN）ratios on the treatment of organics and nitrogen in GW by O₂-MBfR under different nitrogen sources（ammonia nitrogen,nitrate nitrogen,and organic nitrogen）were explored.Fourier transform infrared spectroscopy,three-dimensional fluorescence spectroscopy,16S ribosomal RNA（16S rRNA）high-throughput sequencing technology,combined with the changes of extracellular polymeric substances（EPS）of reactor biofilm,were used to analyze the changes of biofilm characteristics and microbial community in biofilm with different nitrogen sources and COD/TN ratios.The metabolic pathways of linear alkylbenzene sulfonates（LAS）and nitrogen in the system were explored.Finally,machine learning was used to fit and predict the removal of pollutants during GW treatment under different nitrogen sources in the O₂-MBfR,and the optimal model was found.It can be used to predict the effect of practical O₂-MBfR system to treat GW pollutants.The main conclusions are as follows:（1）The effect of nitrogen sources on the removal of pollutants by O₂-MBfR was shown,ammonia nitrogen and organic nitrogen systems were beneficial for the removal of COD and LAS in COD/TN=40 condition,the removal rates of COD and LAS were both above 95%and 97%.As the decreasing of COD/TN ratio,the nitrate nitrogen system have a stronger ability to remove organics.The removal effect of nitrate nitrogen system on TN and NO₃^--N were always significantly better than the other two nitrogen source reactors,its average removal rates were 88%and 92%.Under the condition of COD/TN=10,the acidic environment generated by strong nitrification in the ammonia nitrogen system significantly inhibited pollutants removal.The influence of physical and chemical properties of biofilms showed that,EPS/Total Suspended Solids（TSS）ratios of biofilms in the O₂-MBfR showed an overall upward trend with the decrease of COD/TN ratio.The residual LAS in the system promoted the secretion of protein（PN）,which can resist the biological toxicity of LAS to biofilm microorganisms.The infrared spectrum analysis results indicated that proteins,polysaccharides and benzene-related organics were existed in the biofilm during LAS biodegradation process.（2）The effect of nitrogen sources on the changes of O₂-MBfR biofilm microbial communities and predicted functional enzyme genes showed that Gammaproteobacteria and Alphaproteobacteria dominated the community at class level.These bacteria were crucial for nitrogen and carbon removal in biological treatment of wastewater.Heat map showed that when COD/TN ratio decreased,the abundances of functional bacteria related to LAS biodegradation and aerobic denitrification,including Sphingobium and f＿Burkholderiaceae et al,showed an increasing trend.This enhanced carbon and nitrogen removal ability of O₂-MBfR.Nitrifying bacteria-Flavobacterium and ammonia oxidizing bacteria-Nitrospira were more abundant in both ammonia and organic nitrogen systems.Tricarboxylic acid（TCA）circulating enzymes and LAS metabolic enzymes increased with the decreasing of COD/TN ratio,and enzymes related to nitrification were abundant in ammonia and organic nitrogen systems,while denitrification enzymes were abundant in nitrate nitrogen systems.The mechanism of carbon and nitrogen metabolism was as follows:The LAS of the three systems were mineralized by oxidase catalysis of heterotrophic microorganisms.The ammonia monooxygenase in the heterotrophic nitrification process could synergistically degrade LAS in both ammonia and organic nitrogen systems.NO₃^--N in the nitrate nitrogen system was metabolized through aerobic denitrification.NH₄⁺-N and CO（NH₂）₂ in ammonia nitrogen and organic nitrogen systems were both metabolized through heterotrophic nitrification-aerobic denitrification process.（3）The construction of O₂-MBfR system models with different nitrogen sources by machine learning showed that:pH values of all three nitrogen source systems presented a significant negative correlation with both organics and nitrogen removal efficiency in the structural equation models.And there was a significant positive correlation between LAS removal with nitrogen source and COD/TN ratio,which verified the different degrees of co-metabolic removal of organics and nitrogen in the O₂-MBfR.The best fitting model of ammonia nitrogen system for LAS and TN treatment was random forest,and the model parameters mtry was 1 and 4;The best fitting model of organic nitrogen system for LAS and TN treatment was also random forest,and the mtry was 8 and 5;The best fitting model for nitrate nitrogen system to deal with LAS was random forest,the mtry was 4,the best fitting model to deal with TN was support vector machine,and the model parameters C and σ of its kernel function Gaussian radial basis were 479.15 and 1.2257,respectively.