Effect Of Extracellular Polymeric Substances On Sludge Deep Dewatering Performance And The Transformation Mechanism Of Water

With the rapid rise of sewage amounts and treatment rates,subsequent sludge treatment and disposal problems were becoming increasingly apparent.Therefore,it was necessary to realize the reduction of sludge volume and the subsequent disposal costs by sludge deep dewatering.The main reason for the difficulty of sludge dewatering were the complex composition,high organic matter content and water molecules absorbed in the network structure formed by extracellular substances(EPS).To explore the effect of EPS on the deep dewaterability and water transformation mechanism of sludge,municipal sludge and paper mill sludge with difference in EPS concentration were used as the research object.The changes of sludge physicochemical properties,EPS concentration and composition,and dewaterability were examined after physical,chemical and biological conditioning method.After freezing-thaw,chitosan hydrochloride(CTSCL)and lysozyme(LZM)conditioning,the sludge floc structure was destroyed,the value of?_y decreased from 1.23 Pa to 0.13 Pa,and the viscoelastic behavior of the network structure was weakened.Additionally,SRF,CST and the water content of the dewatered sludge were decreased,while the water content of the dewatered sludge after LZM conditioning was the lowest.To be specific,the protein(PN)concentration in S-EPS rose to 51.64 mg/g DS from 1.15 mg/g DS,and the polysaccharide(PS)concentration increased from 1.84 mg/g DS to 37.22 mg/g DS.The kinetic model was used to further analyze the EPS transformation mechanism,which suggested the change of TB-EPS should be described by a piecewise linear function.According to the vapor liquid equilibrium principle,a new method to determine the sludge water distribution was constructed based on the MHE-GC technique.The results showed that the MHE-GC method has a better measurement repeatability(RSD<18.60%)and sensitivity(limit of quantitation=0.028%).This method was adopted to analyze the water bound with EPS.Moreover,the water transformation mechanism of sludge was investigated based on the XDLVO theory and EPS viscoelasticity.As a result,the capillary water ratio of loosely bound EPS(LB-EPS)was up to 99.99%,and the capillary water ratio of S-EPS and TB-EPS was59.16%and 54.77,respectively.After LZM conditioning,the proportion of capillary water in S-EPS and TB-EPS was increased,while that of adsorbed water declined.This was mainly attributed to the reduction of EPS viscosity,which increased the fluidity of water molecules.Meanwhile,the attraction potential energy between S-EPS and microbial cells was increased,and the repulsion potential energy between TB-EPS and microbial cells was decreased,signifying a weakened adsorption capacity to water.Compared with other conditioning methods,the volume of sludge filtrate was the greatest after LZM conditioning,with the transmissivity of 1.65%.When the pressure filtration time increased to 30 min,the free water in sludge was completely dewatered,and the proportion of capillary water decreased to 56.14%.Afterwards,the pore throat model was utilized to visualize the sludge drainage channels was built,and the mechanism of EPS in the sludge deep dewatering was clarified.It was found that after LZM conditioning,the pore volume decreased to 0.072 m L/g and the porosity decreased to 3.17%.This might be because the drainage channels were blocked due to the release of organic substances with pressure filtration time.Based on the aforementioned,the paper mill sludge was conditioned by freezing-thaw,CTSCL and LZM conditioning,so as to examine the changes in sludge dewatering performance and physicochemical properties.The results discovered after conditioning,the concentration of PN and PS in EPS slightly increased,and the changes were lower than those in municipal sludge.Additionally,the dewatering and settling performance of paper mill sludge were better than that of municipal sludge.After LZM conditioning,the proportion of bound water of paper mill sludge decreased to 0.32%.In the deep dewatering process,there were more drainage channels in sludge after LZM conditioning.At the pressure filtration time of 3 min,5 min and 30 min,the pore volumes were 0.84 m L/g,0.63 m L/g and 0.30 m L/g,respectively.In addition,the fluid flow was relatively dense inside the sludge,and the inner porosities were 33.59%,30.13%and13.28%,respectively.The above-mentioned changes might be ascribed to the low contents of microbial cells and EPS in paper mill sludge,and the slightly increase of PN and PS facilitated sludge dewatering.Moreover,LZM mainly exerted a function of electrical neutralization in paper mill sludge,which provided more drainage channels during the filter-pressing process.The multivariate regression equation of sludge deep dewatering performance was established by multivariate statistical analysis combined with PLS regression.According to the results,the deep dewatering performance of municipal sludge was affected by zeta potential and PNS,while the deep dewatering performance of paper sludge was affected by D1,PSL and PST.Additionally,the PN in S-EPS(PNS)and PS in S-EPS(PSS)were positively correlated with the water content of the dewatered municipal sludge,while they were negatively correlated with that of paper mill sludge.This phenomenon was consistent with the experimental results,which further clarified the effect of EPS on sludge dewatering performance.