Electro-dewatering Of Sewage Sludge Conditioned With Electrolytes And Its Mechanism

The sewage sludge production is increasing rapidly.High water content of sludge is one of the main factors limiting its treatment and disposal.For conventional mechanical dewatering(centrifugation or belt filter press),the final water content of dewatered sludge cake is still as high as 80 wt%.The objectives of this thesis were to investigate the electro-dewatering(EDW)of mechanical dewatered sludge so as to further decrease the water content,with the application of self-developed EDW device.Particularly,the effects of different valent cationic electrolytes on sludge EDW process and the related migration mechanism were investigated.The main contents are as follows:1.Optimization of the EDW parametersThe effects of voltage,current,mechanical pressure and sludge loading rate on EDW process were investigated.The results indicated that with increasing the voltage,the dry solid content of sludge cake increased first and then decreased,and reached its maximum of40.6 wt%at 30 V.As the current increased,the dewatering efficiency increased.However,it would decrease if continuously increasing the current.At current of 1.0 A,the solids content of dewatered sludge was maximum(38.6 wt%),but higher energy consumption was observed at this high current.Therefore,constant voltage condition was considered as more suitable for EDW process in terms of less energy consumption.With increased mechanical pressure,the efficiency of EDW increased then remained constant.For mechanical pressure of 3.11 kPa,the solids content of dewatered sludge cake was 40.5 wt%.With the decreased sludge loading rate,the dewatering efficiency and energy consumption increased.When the sludge loading rate was 40 g,the solid content was 40.2 wt%,which met the requirement of landfill(<60 wt%),and less energy consumption was achieved.Considering the dewatering effect and energy consumption,the optimal parameters were voltage of 30 V,current of 1.0A,mechanical pressure of 3.11 kPa and sludge loading rate of 40 g.2.Optimization of different valent cationic electrolytes conditioning in EDWThe effects of sodium sulfate,ferrous sulfate and ferric sulfate on EDW efficiency were investigated.The results showed that compared to sodium sulfate conditioning system,ferrous sulfate and ferric sulfate can effectively reduce the electrolysis and the specific energy consumption.The efficiency of EDW process can be improved but energy consumption was also improved when dosage of sodium sulfate was 0.1 mol/kg DS;the addition of ferrous sulfate can improve the filtrate mass per energy consumption,however,it failed to improve dewatering efficiency;the ferric sulfate conditioning system can effectively improve the EDW dewatering efficiency and the related dewatering rate,and decrease energy consumption at the dosage of 0.1 mol/kg DS.After EDW process,the heavy metal ions in sludge can be removed to a certain extent.For sodium sulfate and ferric sulfate,ferrous sulfate electrolytes,the highest removal rate of copper was respective 5.6%,4.8%,and 13.5%;the highest removal rate of zinc was respective 5.6%,4.8%,and 21.3%;the highest removal rate of lead was respective 14.2%,9.8%,and 11.5%.3.Study on the compounds migration during the EDW processThe related materials(i.e.water,organic matters,protein,polysaccharide,pH,Zeta potential and heavy metals)migration during EDW process was investigated in layered tests conditioned with sodium sulfate.Results indicated EDW process followed a S shape curve.The S shape curve can be divided into three stages:desalination stage,dewatering stage and dewaterability limit.Within the initial few minutes of electro-dewatering,the rate of electroosmosis was extremely slow while there was intense electromigration of ions like Na~+.After this period,the dewatering efficiency increased dramatically until the maximum value was achieved,possibly due to the increased electric resistance due to water removal.The dewatering process followed a S shape curve,especially at high sodium sulfate dosage.The results of layered experiments indicated organic matters in sludge moved gradually from the cathode to the anode during EDW process.pH of cathode sludge cake increased,while pH of anode sludge cake decreased.Zeta potential of raw sludge was affected by pH.Zeta potential of conditioned sludge may be affected by ion strength.For common metals,potassium ion and sodium ion migrated from cake to filtrate quickly;calcium ion and magnesium ions moved from cathode to anode,and accumulated near the cathode;iron and aluminum ions showed unobservable migration.For heavy metals,EDW showed slight removal efficiency.The effects of different valent cationic electrolytes on sludge EDW process and the related material migration behavior were investigated.The results gained from this thesis provided theoretical basis for the subsequent application of EDW process.