Research On Conditioning Optimization And Dewatering Performance Of Surplus Sludge

With the development of economy, increasing amount of sludge was generated bymunicipal wastewater treatment plant (MWTP), water reduction is the key point ofsludge disposal and a crucial step to avoid secondary pollution. Chemical conditioningand thickening are often used in sludge dewatering pretreatment technique, owning tothe complex composition, the selection of suitable chemicals and dynamics controloptimization have restrict the effects of sludge dewatering. Chemical optimization,dynamics control and influence of storage time of surplus sludge on the sludgedewatering efficiency have to be studied aiming at a specific sludge characteristics. Thesurplus sludge from Harbin wenchang MWTP was studied, using PAC and FeCl3asconditioner, the morphology, dynamics control of sludge flocculation process weredescribed. The change of sludge settling and dewatering properties caused by thevariation of inner nature of sludge were analyzed, and the effect of storage time onsludge dewatering properties and subsequent chemical conditioning were alsoinvestigated.PAC and FeCl3were employed in the reagent optimization experiment; dynamicstirring and sedimentation experiment were conducted, respectively. The results showedthat sludge self-flocculation can improve the dewatering performance to a certain extent.CST value decreased by12.3%at a two-stage stirring, first stirring at a rate300rpm for90s, then60rpm for12min. Take the index: sludge particle size, fractal dimension,CST value, moisture content of the cake into account, the orthogonal test found out thebest match of the two level energy input: the first level Fr=0.061, second level Fr=0.021;the first level energy h1=9.74J, second levels of energy h2=3.12J. PAC and FeCl3couldsignificantly improve the sludge filtration and sedimentation performance, but thelowered range of moisture content of the cake was less than3%. CST value, moisturecontent of the cake, SVI, Zeta potential can be used as evaluation index of the optimumdosage of the reagent, but different indicator often leads to a different optimal dosage.The optimal dosage determined by SVI: PAC0.3g/L, FeCl30.6g/L, Zeta potentialleads to an optimal dosage which to be the highest: PAC0.5g/L, FeCl31g/L. However,sludge viscosity and bound water content does not indicate the optimal dosage well.The dewatering of sludge pretreated with PAC and FeCl3were investigated, themoisture content of the cake was used to define the degree of dehydration of sludge, andfiltering speed was measured by CST value, SVI represented for settling performance.Zeta potential, pH, particle size, viscosity, EPS structure and composition, the fractaldimension, bound water content and microstructure of the sludge were tested after the chemicals were added; the experimental results are as follows: Zeta potential, particlesize, LB-EPS and protein of this layer, the fractal dimension could well indicate thesludge dewaterability, the filtration performance improved when Zeta potential washigher than-4mV, the coefficient of correlation R2between Zeta potential and CST was0.944when PAC was employed and0.987when FeCl3was used. The growth of smallparticles (the increase of D(0.1), D(0.5)) would deteriorate filtration; the decrease ofD(0.9) had a negative effect on settling. The settling and dewatering performanceimproved as the content of LB-EPS decreased, the protein acted as the most significantpart in the LB-EPS layer, while the TB-EPS, EPS basically played a limited role in theprocess. The lower2-fractal dimension, the higher moisture content of cake; pH, sludgeviscosity and bound water content did not show a clear correspondence to dewateringperformance.The laboratory sedimentation experiment simulated concentration tank was carriedout, the changes of dewatering nature in48h was discussed. The study found thatprolong storage time would increase the CST value; CST at48h was63.7s, increasedby118.1%when compared to the initial.24h would cost another more40%PAC than18h, even the PAC consumption doubled could not make the sludge easy for filtrationin32h or48h. The miniaturization of the particle size was irreversible when thestorage time lasted too long. Take the proper solid content of mechanical equipment anddewatering properties into consideration, we suggest a18h storage time would bereasonable.This research results can further improve the mechanisms of sludge dewateringprocess in Harbin Wenchang MWTP, and provide theoretical support for enhancing thesludge dewaterability and achieving the energy-saving and consumption-reducingduring sludge dewatering process, also the social and economic profits can be gained.