Research For A Novel Conditioning Process For Enhancing Dewaterability Of Dredged Sediment And The Conditioning Mechanism

In recent years,the rapid development of the environmental protection dredging industry has produced a large amount of dredged sediment,which needs to be properly disposed of.The high moisture content and a large volume of the sediment are not conducive to subsequent storage,transportation and disposal,so dewatering is the first logical treatment approach.The extracellular polymer of sediment（EPS）plays a vital role in the dehydration of sediment.Tannic acid（TA）can combine with the protein in EPS to form insoluble complexes,Fe（Ⅱ）and persulfate（PDS）can react to form oxidizing sulfate radicals（SO₄^-·）,which has the potential to crack EPS by oxidation.Therefore,we chose the sediment of Jingyue Lake in Songjiang District,Shanghai as the research object,and selected two new types of sediment dewatering conditioners-TA and Fe（Ⅱ）activating persulfate system,to explore the changes in the dewatering performance of the sediment after conditioning and the mechanism.The changes of the dewatering performance of the sediment can be revealed through the changes in the filterable dehydration rate of the sediment,the specific resistance（SRF）during the vacuum filtration process and the water content（Wc）of the cake after the filtration.In addition,the changes in the properties of the sediment after conditioning,including p H,zeta potential and viscosity;protein and polysaccharide content in EPS;bound water content;particle size;surface area and pore structure;sediment cake morphology and structure;functional group,etc.,are studyed to explore the conditioning mechanism.This research has obtained the following conclusions:（1）TA and Fe（Ⅱ）activating persulfate system can effectively improve the dewatering performance of the sediment.The optimal dosage of TA is 0.20 mmol/g TS.Under these conditions,TA reduced the Wc of the cake by 12.29%and SRF by 70.6%,and increased the filterable dehydration rate by 9%.When Fe（Ⅱ）is used in combination with PDS,the best additive dosage is1.3 mmol/g TS PDS,1.17 mmol/g TS Fe（Ⅱ）,n(Fe²⁺)/n(S₂O₈^2-)=0.9.Under this condition,16.3%of the Wc of the cake and 92.3%of the SRF can be reduced,and the filterable dehydration rate can be increased by 15.9%.（2）The properties of the sediment,including EPS content,viscosity,zeta potential,bound water content,particle size,surface area and pore structure,are closely related to the dewatering performance of the sediment.（3）A two-step mechanism was proposed to explain the conditioning mechanism of TA.First,under the low-dose TA treatment,the colloidal structure of the sediment was dissociated,the particle size of the sediment became smaller and the viscosity increased.Then,TA acted as a coagulant/flocculant to flocculate the sediment particles into large agglomerates.In this process,TA converted bound water into free water by reducing the EPS content,and the viscosity,surface area and pore diameter gradually decreased.In conclusion,TA altered the properties of the sediment by combining with protein,thereby effectively improving the dewatering performance of the sediment.TA had a series of positive effects on the sediment so that it could be used as a conditioning agent in sediment dewatering.What’s more,the mechanism of Fe（Ⅱ）activating persulfate system was analyzed.Fe（Ⅱ）and PDS can react to form SO₄^-·with strong oxidizing,which can break the sediment flocs,destroy microbial cells and release organic matter,to accelerate the decomposition of EPS and release intracellular bound water.At the same time,Fe（ⅡI）produced by the reaction of Fe（Ⅱ）and PDS can agglomerate sediment particles into larger groups,which is more conducive to dehydration.These results confirm the feasibility of using TA and Fe（Ⅱ）activating persulfate system as rapid dewatering conditioners for sediments,and provide an effective reference for selecting suitable sediment dewatering conditioners.