Process Preferences And Removal Characteristics Of Different Forms Of Heavy Metals From Dewatered Sludge

Dewatered sludge can be used as a soil fertiliser to provide nutrients such as nitrogen and phosphorus,and as a soil conditioner to improve soil structure,but the presence of heavy metals in dewatered sludge has become a major factor limiting its resourceful use.This thesis addresses the problem of excessive heavy metal content in dewatered sludge,proposes a new method of combined pretreatment to transform heavy metal forms,investigates the influence of conventional physicochemical indicators on heavy metal forms in dewatered sludge during pretreatment,and carries out research on the removal of heavy metals from dewatered sludge by combined pretreatment with chemical drenching and electrokinetic techniques,providing a reliable technology for the removal of heavy metals from dewatered sludge and its resource utilisation.This study provides reliable technical support for the removal of heavy metals from dewatered sludge and its resource utilization.The main contents include:(1)Eight combined acid-pyrolysis pretreatment processes were proposed based on the transformation of heavy metal forms in dewatered sludge by different pretreatment processes,and the results showed that low-temperature pyrolysis pretreatment could increase the concentration of oxidizable state of heavy metals in dewatered sludge,and the concentration of oxidizable state of Zn,Cd,Ni and As increased by 76.7%,14.8%,12.3% and 10.3%,respectively,after 4 h of pyrolysis at120℃,The length of pretreatment did not affect the unstable state mobility of heavy metals under acidic conditions,with Cu,Cr,Pb and Cd increasing by 440.5%,29.1%,27.6% and 97.5% after 48 h of acid treatment under aerobic conditions,and Zn,Ni and As increasing by 62%,27.6% and 63.3% after 96 h,respectively;the anaerobic environment could more significantly increase the The anaerobic environment could more significantly increase the unstable state migration rate of heavy metals.After 96 h of anaerobic treatment under acidic conditions,the dewatered sludge had the highest unstable state growth of Cu,Cd,Zn and Ni,with increases of 207.8%,120%,78.1%and 39.4%,respectively.(2)The results showed that the p H of dewatered sludge tended to increase after the eight pretreatment processes;acid pretreatment led to an increase in the oxidation potential(ORP)of dewatered sludge by about 200 m V,which increased the oxidation of dewatered sludge,thus promoting the migration of heavy metals to the unstable state,but pyrolysis pretreatment led to a decrease in the ORP of dewatered sludge to about-100 m V,which increased the reduction of dewatered sludge and promoted the migration of heavy metals to the stable state.-The content of total phosphorus(TP),total nitrogen(TN)and Kjeldahl nitrogen(TKN)in dewatered sludge before and after different pretreatments ranged from 20.1±0.5 mg/kg,46.3±8.6 mg/kg and 42.9±4mg/kg,respectively,indicating that different pretreatment processes had no effect on the content of nutrients.The protein and polysaccharide content of the dewatered sludge increased by 31.6% and 142.6% under pyrolysis conditions,and decreased from0.4 mg/g and 0.2 mg/g to 0.2 mg/g and 0.1 mg/g under acidic conditions,respectively,while the aerobic/anaerobic/anoxic environment had little effect on the protein and polysaccharide content.(3)The results showed that the simultaneous aerobic pickling process combined with electrokinetic treatment had the best removal effect on Pb,Ni and Zn,with removal rates of 65.1%,79.1% and 99.3% respectively;the pyrolysis-anaerobic-pickling process combined with electrokinetic treatment had the highest removal rates on As and Cu,with removal rates of 98.9% and 14.7%respectively;the pyrolysis-anaerobic-pickling process and the simultaneous anaerobic pickling process combined with electrokinetic treatment had the best removal effect on Cd and Cr,with removal rates of 28.1% and 14.7% respectively.The removal rates of Cd and Cr were 28.1% and 24% for the pyrolysis-anaerobic-acid washing process and the simultaneous anaerobic acid washing process combined with chemical drenching technology,respectively.