Removal Of Heavy Metals From Sludge Through Alkaline Hydrolysis And Electrolysis Process

Sewage treatment plants produce hundreds of millions of tons of excess sludge every year,which contains a variety of essential nutrients including nitrogen,phosphorus,potassium,amino acids,etc.,for crops,holding a great potential in agricultural utilization.However,the excess sludge also involves a certain amount of harmful substances,for instance,heavy metals（e.g.,chromium,cobalt,nickel,copper,zinc and lead）.Because of the huge yield of excess sludge and the accumulation of heavy metals,the sludge reclamation offers a potential risk,especially in the field of agriculture utilization,which may improperly enter soil and groundwater,causing secondary pollution.More seriously,the heavy metal-polluted animal and plant species may further affect or damage the health and safety of human beings and livestock through the food chain.As a result,to effectively reduce the content of heavy metals in excess sludge is recognized as one of the keys in sludge reclamation.This study used a alkaline method to break the microbial cells in excess sludge,and explored the optimal experimental conditions to achieve the maximum release of intracellular substances and to prepare the sludge hydrolysate.Afterwards,this study adopted an electrochemical approach to treat the prepared sludge hydrolysate,inducing the transportation of heavy metals under the electric field and finally accumulated on the electrodes to achieve their effective removal.Main findings are summarized as follows:This study first explored the optimal experimental parameters of the alkaline hydrolysis method to treat the excess sludge.The COD dissolution rate,and the concentrations of protein and polysaccharide were used as indictors to evaluate the performance of hydrolysis.The results showed a good hydrolysis performance and economic efficiency with Na OH severing as the alkaline agent,and the optimal experimental conditions were summarized as p H=13,temperature of 30℃,and a reaction duration of 12 h.Under such conditions,the results showed that the dissolution rate of COD increased by 60%,the protein concentration increased by 13 times,and the polysaccharide concentration increased by 22 times.And the sludge hydrolysate was prepared for further utilization.Based on the above results,an electrochemical method was applied to treat the prepared sludge hydrolysate,to reduce the content of heavy metals.This study set the kinds of additional electrolyte,the gradient of applied voltage,and the electrolysis time as three optimization conditions,respectively,to explore the removal performance of heavy metals through electrolysis treatment.The experimental results illustrated that the optimal conditions were confirmed as the voltage gradient of 2 V/cm and the electrolysis duration of 36 h with KH₂PO₄ as the electrolyte.The removal rates of Ti,V,Cr,Mn,Co,Ni,Cu,Zn,As and Pb were 22.1%,20.0%,30.1%,7.2%,27.8%,22.0%,61.1%,42.8%,29.2%,and 50.7%under the optimal conditions.The electrolysis method can enrich the heavy metals near the electrode,effectively reduce the heavy metal concentration in the sludge hydrolysate,which is conducive to the subsequent agricultural recycling of sludge hydrolysate.This study provides a theoretical and practical support for the removal of trace heavy metals from excess sludge and the promotion of sludge reclamation.