Effectiveness And Mechanisms Of Sewage Sludge Conditioning With Chitosan And Modified Coal Fly Ash

A large amount of sewage sludge is produced in municipal wastewater treatmentsystem. The sludge contains high moisture content and displays poor dewaterability,furthermore, the common methods of sludge conditioning show some disadvantages,such as high cost, unsatisfactory effectiveness, and secondary pollution. To solvethese problems, single and dual sludge conditioning with low-cost modified coal flyash and non-toxic chitosan were investigated, and the corresponding mechanismswere analysed.Firstly, the excess sludges from sludge thickening tank of a wastewatertreatment plant were collected in one year, and the sludge properties were evaluated.Besides, the dewaterability change of sludge samples stored in differentcircumstances were measured to optimize the sludge preservation conditions. Resultsshowed that the raw sludge was nearly neutral, high organic content, easily corrupt,high water content and high compressibility, difficult to be dewatered, and showed astable colloid system due to the strong electronegative property. The dewateringproperties of sludge samples insignificantly changed at4℃for5days.Buchner filtration test and cylinder settling test were conducted to measurespecific resistance to filtration (SRF), time to filter (TTF), filter cake moisture(FCM), and sludge settling ratio (SV30), and to evaluate the dewaterability of sludgeconditioned with single conditioner and dual conditioners. The single conditionerincluded chitosan, liquid for coal fly ash modification (MCFAl), and solid ofmodified coal fly ash (MCFAs). The dual conditioners included the combination ofMCFAland chitosan, and combination of MCFAsand chitosan. Results showed thatall of the sludge conditioning methods were effective. The conditioned sludgesshowed a lower SRF, a shorter TTF, a lower FCM and a smaller SV30, and thefilterability and the settleability of the sludge were improved. In the case of sludgeconditioning with chitosan, the sludge conditioning effectiveness was affected by thechitosan concentration, dosage and sludge pH. Under the optimum conditions forsingle chitosan conditioning, the sludge SRF reduced by89.2%, and filter cakemoisture was84.5%. In the case of sludge conditioning with single MCFAl, theMCFAldosage and reaction time were the important factors. Under the optimumconditions for single MCFAlconditioning, the sludge SRF reduced by79.5%, and filter cake moisture was71.8%. In the case of sludge conditioning with singleMCFAs, the MCFAsdosage was the main factor. Under the optimum conditions forsingle MCFAsconditioning, the sludge SRF reduced by90.4%, and filter cakemoisture was72.8%. In dual conditioning with MCFAland CTS, the sludge waspretreated by MCFAland then conditioned with chitosan. The highest SRF reductionwas80.3%, and filter cake moisture was69.1%. In dual conditioning with MCFAsand chitosan, the conditioning effectiveness was affected by conditioner additionsequence and dosage. Under the optimum conditions for dual MCFAsand chitosanconditioning, the sludge SRF reduced by98.7%, and filter cake moisture was69.0%.The dual conditioning was more effective than the single conditionings under thesame conditions. Furthermore, the consumption of chitosan reduced in dualconditioning.The mechanisms of sludge conditioning were analyzed. The mechanisms ofchitosan improving sludge dewaterability were to build bigger size flocs because ofelectrostatic neutralization and adsorption bridging, which resulted in lower colloidstability and stronger flocculation effect. Moreover, adsorption bridging played adominant role. The actions of MCFAlwere to dissolve the sludge extracellularpolymeric substances, disrupt sludge structure, reduce sludge electronegativity andrelease bound water in the sludge. The enhancement of sludge dewatering withMCFAswas caused by electrostatic neutralization and skeleton function. As skeletonbuilder, MCFAshelp to build up a more porous, permeable and rigid lattice structurein the filter cake. The mechanisms of sludge conditioning with MCFAl chitosan (orMCFAs chitosan) were based on the respective single conditioning mechanisms, andthe synergistic interaction between MCFAland chitosan (or between MCFAsandchitosan) functioned.The release of heavy metals in sludge conditioning process was investigated.The heavy metals were released when the sludge was conditioned with modified coalfly ash, however, this problem can be sloved through dual conditioning with MCFAand chitosan. The speciation of heavy metals in sludge cake changed after sludgeconditioning and dewatering, but the impacts of sludge conditioning were differentfor different heavy metals. The sludge filter cakes after conditioning and dewateringneed further treatment for agricultural utilization.This study provides a variety of effective, nonpolluting, low-cost and practical sludge conditioning, which may supply theoretical basis for sludge conditioningpractices.