| To avoid the inactivation of the disinfectant,the bacteria in drinking water distribution systems(DWDSs)generally adheres to surfaces of suspended particles,rarely existing as a free individual.Exploring the process of bacteria accumulation on suspended particles is the basis for the study of biofilm formation in DWDSs.However,the current understanding of this process is still scarce.This study aims at investigating the impacts of chlorine concentration,the concentration and the types of suspended particle and extracellular polymer substances(EPS)on bacterial surface characteristics and aggregation process in a DWDS through a simulated experimental platform,and reveals the mechanism of how suspended particulates influence bacterial aggregation and disinfection efficiency,thus providing theoretical guidance for bacterial growth control in DWDSs.Firstly,to analyze the effects of residual chlorine concentration on the bacterial aggregation onto suspended particle surfaces,the adhesion experiment of goethite at different residual chlorine concentrations was carried out.The results showed that the low concentration of chlorine(0-0.5 mg/L)promoted bacterial attachment on the surface of goethite,while the high concentration of chlorine(0.5-3.0 mg/L)inhibited the bacterial surface attachment.Through quantitative determination and confocal laser scanning microscope observations,when the chlorine concentration was 0.5 mg/L,the total bacterial number and extracellular polymer substances of the aggregates reached the maximum.The study revealed that the residual chlorine concentration regulates the secretion of bacterial EPS,which in turn affects the adherence and aggregation of bacteria on the surface of the particulate matter in the DWDSs.Secondly,the effect of particle concentration on the attachment of bacteria to the surface was analyzed from the specific surface area and sedimentation of particulates,bacterial EPS and surface characteristics.The results showed that,in the presence of chlorine,increasing goethite concentration promoted bacterial attachment.Meanwhile,the presence of goethite reduced the efficiency of chlorine disinfection for DWDS bacteria,and its protective effect was correlated positively with goethite concentration.The EPS secreted by bacteria and Zeta potential varied with goethite concentration.Both may have less influence on the aggregation and attachment of bacteria than the adsorption and sedimentation of particles.Finally,the effects of two kinds of soil particles(Na-montmorillonite and kaolin)on adhesion and aggregation of bacteria at various chlorine concentrations were studied.The results showed that the number of attached bacteria on Na-montmorillonite was higher than that on kaolin under the same chlorine condition.This may result from smaller particle size and larger specific surface area of Na-montmorillonite,and more EPS excreted by the bacteria attaching onto the surface of the Na-montmorillonite shaped by chlorine disinfection.When the chlorine concentration was 0.5 mg/L,the number of bacteria attached to both surfaces reached maximum,further confirming that the low concentration of chlorine(0-0.5 mg/L)in the DWDSs promotes to the attachment of bacteria to the surface of suspended particles. |
