Study On The Effect Of Benzalkonium Chloride On Aerobic Granular Sludge System And Microbiological Mechanisms

As a widely used biocide,surfactant and preservative,quaternary ammonium compounds have been detected in a variety of environmental media.Due to its broad-spectrum bactericidal ability and potential ecological risks,it has attracted widespread attention in recent years.Due to hydrophobicity and low biodegradability,the quaternary ammonium compounds in the environment are mainly absorbed into the sludge by the activated sludge after entering the wastewater biological treatment system.However,when the adsorption accumulates to a certain concentration,the quaternary ammonium compounds will significantly inhibit the microbial activity of the sludge,thus affecting the biological treatment efficiency of wastewater.Aerobic granular sludge technology is an emerging wastewater biological treatment technology,which has the advantages of good settling performance,less occupation area,diverse microbial flora,high sludge concentration,and strong impact load resistance.Aerobic granular sludge thus has strong resistance to adverse environment and toxic pollutants.These advantages suggest that aerobic granular sludge technology has great potential application prospects in the treatment of wastewater containing quaternary ammonium compounds.However,at present,the influence of quaternary ammonium compounds on the operation efficiency of aerobic granular sludge system,the behavior mechanism of quaternary ammonium compounds in aerobic granular sludge and the influence of quaternary ammonium compounds on the microorganisms of aerobic granular sludge are still unclear.In view of this,this paper first reveals the effect of typical quaternary ammonium benzalkonium chloride(bac-c12)on aerobic granular sludge system at different concentrations from the aspects of pollutant removal capacity and sludge characteristics.On this basis,combined with the behavior of BAC in the system and the changes of microorganism morphology,activity and microflora structure of aerobic granular sludge under the action of BAC,the potential influencing mechanisms were revealed.This study can provide theoretical support for the application and optimization of aerobic granular sludge technology in the treatment of quaternary ammonium compounds wastewater.The main research results are as follows:(1)The aerobic granular sludge reactor without BAC has obvious sludge granulation and good pollutant removal effect.After the granulation is completed,the proportion of the particle size above 200?m,the sludge concentration and the PN/PS value increase to93%,8000mg SS/L and 3.5 respectively;VSS/SS and SVI are maintained at about 0.70and 58m L/g;the aerobic granular sludge reactor has stable removal rates of COD,NH₄⁺-N and PO₄^3-at around 96%,98%and 82%,respectively.The inhibition trend of quaternary ammonium compounds on the COD,NH₄⁺-N and PO₄^3-removal performance of the aerobic granular sludge reactor is positively correlated with its concentration,and the maximum inhibition rates are 64%,84%and 100%,respectively;the suppression of the maximum initial removal rate of COD and NH₄⁺-N can reach 96%and 100%.However,the operating efficiency of the aerobic granular sludge reactor can restore to more than 85%after adding BAC and running for 34 days,showing the adaptability to BAC inhibition.BAC can reduce the particle size,and can also promote the aggregation of microorganisms and increase the particle size;with the increase of the concentration of BAC,the structure of aerobic granular sludge gradually becomes loose and large cavities appear inside until disintegration.The addition of BAC significantly decreased MLSS and VSS/SS first,then gradually returned to the initial level.Low-concentration BAC can increase the EPS content of aerobic granular sludge mainly by promoting the secretion of PN;high-concentration BAC can reduce the EPS content mainly by inhibiting the secretion of PS.Under the action of BAC,the aerobic granular sludge PS is distributed not only on the outside,but also on the inside;PN is distributed in all sections of the aerobic granular sludge particles;living cells are mainly distributed on the surface layer of aerobic granular sludge particles,and are also distributed inside the particles under the loose structure.(2)The fitting degrees of the adsorption behavior of aerobic granular sludge to quaternary ammonium compounds under the Langmuir,Freundlich,and Temkin models are 0.93,0.93,and 0.793,respectively.BAC is easily adsorbed by aerobic granular sludge.This process involves monolayer adsorption,multi-molecular layer adsorption and chemical adsorption under the action of electrostatic force.Both intra-particle diffusion and membrane diffusion play an important role in the adsorption process of BAC;the adsorption process is more in line with the quasi-second-order kinetic model,which is a physical-chemical adsorption dominated by chemical action.The initial adsorption rate and equilibrium adsorption amount are positively correlated with the initial concentration of BAC.The deprotonated carboxyl C=O of EPS in aerobic granular sludge,amino-NH in EPS and protein,C-O-C in polysaccharide and other active groups may play a major role in the adsorption process.Aerobic granular sludge has a biodegradation effect on BAC,but it is mainly adsorption,and the BAC removed by microbial degradation accounts for about 20%.The aerobic granular sludge reactor can remove BAC with a concentration below 50mg/L stably and effectively for a long time.(3)BAC has a relatively obvious inhibitory effect on filamentous bacteria and bacilli;after exposure to BAC,microorganisms show obvious agglomeration;under the action of high concentrations of quaternary ammonium compounds,supporting microorganisms are inhibited and the sludge gradually disintegrated.BAC can inhibit the microbial activity of aerobic granular sludge,and the inhibition trend is positively correlated with the concentration of BAC.The results of oxygen uptake rate showed that the exogenous nitrification activity of autotrophic nitrifying bacteria in aerobic granular sludge reactors operated with 20 mg/L and 50 mg/L BAC were completely inhibited.With the increase of the concentration of BAC,the fluorescence intensity ratio of the living and dead cells of aerobic granular sludge microorganisms dropped sharply.After exposure to 50mg/L BAC,the ratio decreased from 2.2 to 0.30,and the decrease reached 86%.High-throughput sequencing results show that BAC will continuously reduce the diversity and complexity of microbial species in aerobic granular sludge.Proteobacteria,the dominant bacteria in granular sludge,can quickly respond to the stimulation of BAC,and finally formed a better adaptability and tolerance.Thauera and Flavobacterium were the dominant bacteria at the genus level,which were tolerant to BAC,and there might be a competitive relationship between them.The variation trend of COD,NH₄⁺-N and PO₄^3-removal rates during the operation of aerobic granular sludge reactor is consistent with the variation trend of the relative abundance of 11 species of bacteria involved in the denitrification process and 3 species involved in the phosphorus removal process in aerobic granular sludge.