Composition And Role Of Extracellular Polymer Substances In The Aggregation Of Activated Sludge

As one of the mature sewage treatment technologies,the activated sludge process has been used in the field of sewage treatment for more than 100 years.However,problems associated with inefficient aggregation of microorganisms,such as sludge bulking,poor solid-liquid separation etc,are still frequently found in the application of activated sludge processes.Extracellular Polymeric Substances?Extracellular Polymeric Substances,EPS?,as a high-molecular-weight mixture of polymers,coated on the surface of microbial cells,has a significant impact on the aggregation process of microorganisms.Studying the composition and function of EPS in the process of activated sludge aggregation will contribute to improve the aggregation performance of microorganisms and provide theoretical guidance for solving the problem of inefficient aggregation of microorganisms in activated sludge process.In this study,activated sludge was used as the research object,and an EPS extraction method with high yield and rich varieties of EPS was established at the beginning.Based on the extraction method,the aggregation performance of sludge microorganisms and the changes on EPS species under different environmental factors were investigated,and the best aggregating culture conditions were finally established.Further,Microorganisms dispersed by activated sludge were cultured under optimal aggregating conditions.The role and mechanism of different kinds and components of EPS in the process of activated sludge aggregation was studied in depth.Through experiments and analysis,the conclusions are as follows.The extraction method of EPS in activated sludge showed that EPS can be divided into soluble EPS?Soluble EPS,S-EPS?,loosely bound EPS?Loosely Bound EPS,LB-EPS?and tightly bound EPS?Tightly Bound EPS,TB-EPS?;tightly bound EPS can be further divided into ionic TB-EPS and hydrophobic TB-EPS.The extraction order of EPS was determined as:S-EPS?LB-EPS?ionic TB-EPS?hydrophobic TB-EPS.The extraction method of EPS was as follows:the liquid phase environment used in all EPS extraction processes was phosphate buffer saline?phosphate buffer saline,PBS?containing 1.5 M NaCl;S-EPS was obtained by centrifugation at 5000 g for 10 min at4°C.The extraction of LB-EPS was carried out by heating in a water bath at 80°C for 5min.The extraction of ionic TB-EPS was that 250 mg Na₂S·9H₂O/g Volatile Suspended Solids?Volatile Suspended Solids,VSS?was added to sludge firstly and the sludge was oscillated processing for 1 h at 4°C and 400 rpm under nitrogen atmosphere secondly,170 mg of NH₄F/g VSS was added to sludge thirdly,and finally sludge was stirred at 600rpm and 4°C for 1 hour.The extraction method of hydrophobic TB-EPS was that after adding 140 mg/g VSS SDS to sludge,sludge was stirred at 600 rpm and 4°C for 45 min.The new method is an excellent EPS extraction method because it has higher yield and justifiable cell breakage than water bath heating,Cation Exchange Resin?Cation Exchange Resin,CER?,and formamide-NaOH method,and has more abundant EPS species.The effects of different environmental conditions on the aggregation of sludge microorganisms showed the following conclusions.The best environmental conditions for aggregation were trisodium citrate as carbon source,the organic load of 2 kg COD/m³,nitrogen source of ammonia nitrogen,and ammonia nitrogen concentration of 160 mg/L,the calcium ion concentration of 10 mM,and the growth time of 12 hours.Microorganisms tended to aggregate in unfavorable growth environments such as growing into the end of exponential growth,encountering toxic or complex carbon sources and carbon or nitrogen restrictions.The substrate concentration had a greater effect on aggregation than the type of substrate.Changes in environmental factors altered the composition of EPS,and changes in the composition of EPS were closely related to microbial aggregation performance.Study on the functional analysis of EPS in sludge aggregation process found that S-EPS and LB-EPS had a negative impact on sludge accumulation through the form of electrostatic repulsion,which accounted for the high proportion of uronic acid in two kinds of EPS and the relatively small proportion of the favorable structure for aggregation in the secondary structure of the protein.LB-EPS had more negatively charged groups,thus possessing higher inhibition of sludge aggregation than S-EPS.Ionic TB-EPS played a major role in the sludge aggregation process,and hydrophobic TB-EPS had a weak promotion effect on sludge aggregation.Ionic TB-EPS was the main constituent form of EPS.Its rich-OH and some C=OO-groups provided abundant and sufficient cationic binding sites.Meanwhile,the polysaccharide part of the ionic TB-EPS was distributed on the filamentous bacteria extending to the floc.the secondary structure of the protein of ionic TB-EPS was relatively loose and the favorable structure of aggregation maintained relatively high proportion.The structural characteristics of ionic TB-EPS made its cation binding sites more exposed to the outer layer,thus playing a more important role in sludge aggregation.The weaker condensing effect of hydrophobic TB-EPS on sludge aggregation was mainly due to the higher hydrophobic groups in the protein,but the content of hydrophobic TB-EPS was less and the protein structure was relatively tight.