Effects Of Background Water Matrix On Nitrogen Removal Activity And Microbial Community Of PN/A Granular Sludge

Water quality characterization was considered as the important basis of water pollution control.The quantification of a certain water contaminants could be conducted with conventional comprehensive physicochemical indicators(such as COD_Cr),which could not provide detail evaluation of water biological inhibition as an obvious shortcoming.At present,the evaluation of water biological inhibition was commonly based on the change in sludge activity.Partial nitritation/anammox（PN/A）granular sludge was a completely autotrophic biological aggregate dominated by aerobic and anaerobic ammonia oxidizing bacteria（AOB and An AOB）,which was characterized by high nitrogen removal activity,outstanding settling property,unique brick-red appearance.It had been widely used in laboratory research and engineering applications of low carbon-to-nitrogen ratio wastewater treatment.Considering the growth characteristics of chemoautotrophic bacteria,PN/A granules exhibited the high sensitive to water matrix change,resulting in the decrease in microbial activity and the loss of biomass.In this study,a novel evaluation method of water biological inhibition was developed,on the basis of the PN/A granules responses to different water matrices in nitrogen removal activity and sludge morphology.Firstly,frozen PN/A granules was used as the seed sludge,and the continuous flow reactor was successfully started up in 34 days operation,by adopting stepwise increase in the nitrogen loading rate.Both the removal efficiency of total nitrogen over 83%and nitrogen removal loading rate of 1.67 kg·（m³·d）^-1 were obtained.During this period,the characterized size of granules was quantified using Image pro-plus software.The results indicated that the projected area of the brick-red region in the granules（A_R）had a good positive correlation with the specific nitrogen removal rate as q（TN）（R²=0.988）and the relative abundance of An AOB.Furthermore,the mature PN/A granules were employed as testing organism,and several continuous flow reactors were operated parallelly to conduct the evaluation of water biological inhibition,including tap water（control group）,surface water,municipal sewage,chemical industry wastewater and electroplating wastewater.The influent ammonia nitrogen concentration in all the water sample was fixed at 100 mg·L^-1,and the nitrogen loading rate was around 2.4 kg·（m³·d）^-1 in 30 days of operation period.The sewage and wastewater samples were the effluents from biological treatment tanks,to eliminate the interference of easy biodegradable organic matter on autotrophic nitrogen removal process.The results indicated that surface water exhibited a limited impact on the nitrogen removal performance and morphologic characteristics of PN/A granules,in which the q（TN）and A_R values were 1.8%and 1.5%lower than the control group,respectively.For three types of municipal sewage samples,30 days of continuous operation resulted in the q（TN）and A_R values decreased by 47.7%～69.8%and 56.7%～71.6%,respectively,while the density of granules was also deteriorated with the formation of porous surface.In comparison,chemical industry wastewater and electroplating wastewater showed the stronger inhibitions on the activity of granules,and the deposition of heavy metal was observed on the surface of granules with significant loss of biomass.According to the correlation analysis,the aromatic degree of organic matter,inorganic salt and heavy metal contents were the main water quality indicators influencing the sludge activity.Based on the difference in q（TN）and A_R values before and after continuous operation of reactors,the biological inhibition index（In）of different water samples as follows:electroplating wastewater（88.9±1.4%）>chemical wastewater（66.3±6.6%）>municipal wastewater（60.8±7.6%）>surface water（1.6±0.2%）.According to the results of metagenomic sequencing technology,the bacterial community structure and nitrogen metabolism pathways in PN/A granules exposure to different water matrices were analyzed.The biological inhibition of municipal sewage and industrial wastewater led to the reductions in the relative abundances of the main An AOB（Candidatus Kuenenia and Brocadia）decreased by 31.9%～89.0%,with increasing AOB（Nitrosomonas）by 1.7～6.3 times and significant growth of nitrifying bacteria（Nitrospira）and denitrifying bacteria（such as Rubrivivax,Pseudomonas,Thermomonas and other genera）.The imbalance of autotrophic nitrogen removal processes was found.As a stress response of granules to water matrix,the expression of nitrogen metabolism,quorum sensing and bacterial secretion increased by5.1%～82.4%,and the abundance of functional genes driving anammox reaction,such as hydrazine dehydrogenase（hdh）and hydrazine synthase（hzs）,decreased by 79.4%～89.6%.Notably,the potential risk of the accumulation of nitrous oxide（N₂O）as greenhouse gas increased significantly in the system,which deserved deep investigation in the further study.In summary,the water matrix has a significant impact on the nitrogen removal performance,sludge morphology,bacterial structure and nitrogen metabolism pathways of PN/A granules.According to the responses of sludge activity and morphology to different water matrices,a novel evaluation method of water biological inhibition was established,which provides new ideas for expanding the application scopes of completely autotrophic biological nitrogen removal process.