Study On Deep Dehydration And Dewatering Mechanism Of Sludge Treated With Modified Phosphogypsum

Sewage sludge,which has a higher moisture content and massive toxic substances such as parasitic eggs,organics,heavy metals etc.,is a solid waste produced in the sewage treatment section of municipal sewage treatment plant.Generally,the moisture content of dewatered sludge is between 75%and 80%,which is still too high.Besides,higher water content not only increases the transportation cost,but also restricts the subsequent treatment and disposal of sludge.Therefore,it is particularly important to dehydrate sludge in depth seek to reach the requirements of comprehensive utilization of sludge.At present,the commonly used dehydration methods have the characteristics of high cost,unsatisfactory dewatering effect,single treatment object and secondary pollution.In this paper,solid waste phosphogypsum（PG）was used as raw material to prepare sludge dewatering agent,which was signed as modified phosphogypsum（MPG）.Simultaneously,the dewatering efficiency and mechanisms of sludge under pretreatment with MPG were investigated.Moreover,effects of MPG on toxicity reduction of sludge were also studied.Particularly,several existing problems of MPG used in the dewatering section of wastewater treatment plant were also explored preliminarily.Through vacuum filtration experiments,sludge sedimentation experiments and determination of sludge indices,the effects of single addition of MPG,combined addition of MPG and HAC on sludge dewatering performance,i.e.combined water release,organics variation,sludge properties etc.,were analyzed.Results showed that the dewatering and filtration properties of sludge were significantly improved after pretreatment.After treatment with MPG,moisture content of sludge decreased from99.5%to 62.58%,and the specific resistance（SRF）of sludge decreased from 2.25×10¹³to 3.70×10¹² m/kg.Under treatment with combined MPG and HAC,the water content was reduced to 50.22%,achieving the goal of deep dehydration.And SRF of treated sludge decreased to 2.5×10¹² m/kg,illustrating that water can be removed from sludge flocs easily through filtration after pretreatment.This further demonstrated that those two methods could significantly improve dewatering performance of sludge,especially for the combined method which could realize deep dewatering of sludge.Moreover,model of bound water release was established,finding that the release of bound water was closely related to SRF,variation of extracellular polymers substances（EPS）and size of sludge flocs.In particular,under pretreatment with MPG/HAC,bound water release was also related to the degradation of organics in sludge,and organics reduction will promote release of bound water from sludge solid phase.In addition,the reaction mechanisms of single MPG treatment,combined MPG and HAC pretreatment for sludge dewatering were investigated.The results indicated that addition of MPG may lead bound water to release from the stable colloid network through compressing double-electro-layer and neutralizing charge,thus the released bound water becoming free water（which can be easily removed from solid phase in sludge）in supernatant.Subsequently,sludge particles released from the network were aggregated into larger flocs（which were easy to settle）through bridging adsorption and porous adsorption of MPG.Simultaneously,MPG also provided high-strength framework support for sludge flocs in order to improve structural strength of sludge flocs,which could resist the shear stress in filtration process and can be easily separated with water.Additionally,on the basis of MPG,adding HAC could promote degradation of organics in sludge,and then reduce the viscosity and perishability of sludge,thus enhancing strength of sludge flocs.Besides,HAC,as a cell growth inhibitor,can cause protein denaturation on cell membrane,inhibit cell growth,increase cell membrane permeability,and make intracellular substances such as intracellular bound water,microbial by-products etc.easy to flow out,finally promoting release of intracellular water from cells.In comparison with single MPG addition,combined MPG/HAC showed higher efficiency in bound water release,flocs flocculation and dewatering performance of sludge.Effects of those two pretreating methods on morphology distribution and environmental risk level of heavy metals in sludge were analyzed.It was found that the content of unstable heavy metals in supernatant and sludge filter cake decreased significantly after pretreatment,and the proportion of stable heavy metals in filter cake increased significantly,thus reducing the migration risk of heavy metals.Furthermore,compared with single MPG addition,MPG/HAC could immobilize more unstable heavy metals,i.e.the environmental risk of heavy metals was more effectively weakened.In addition,the migration and transformation model of heavy metals and the reduction rate model of heavy metals were established.The results showed that the migration and transformation of heavy metals in sludge treated with MPG were closely related to the oxidation-reduction potential（ORP）and variation of EPS in sludge.And dissolution of unstable heavy metals was corelated with flocs size,EPS variations and SRF of sludge.Moreover,changes of heavy metal in sludge under treatment with MPG/HAC was related to the degradation of organics,in addition to the above-mentioned indies.From the application experiment results,it was found that the dose of pretreating agents used in the laboratory cannot adapt to the larger sludge load in the dehydration section.Meanwhile,analysis of environmental benefit and cost indicated that environmental benefits obtained by those two methods in small-scale experiment and the application-oriented experiment were greater than the economic costs,which showed that the environmental benefit of those two methods were good.In this paper,solid waste phosphogypsum was used to produce dewatering agent for deep dehydration of sludge,which could significantly improve sludge dewaterability.Besides,phosphogypsum is cheap and easy to obtained,which could greatly reduce costs.In addition,method of simultaneous reduction of sludge moisture content and toxicity was put forward,which made up for the defect of single effect of dehydrating agent in sludge treatment at present.Particularly,it embodied the environmental protection idea of treating waste with waste,and promoted the interactive use of wastes.