Study On Sludge Reduction By Combined Alkali Hydrolysis And Electrolysis

The excess sludge produced from sewage treatment plant contains a large amount of poisonous and pernicious substances such as virus,pathogenic bacterium,parasite and heavy metals,etc.Furthermore,the excess sludge is also accompanied with obnoxious odor,which will directly cause pollution to the environment if it was discharged without any treatment.In recent years,increasing amounts of sludge was discharged from the Municipal Sewage Treatment Plants.It has become a new solid waste pollution in the environment and needs special treatment.The cellular structures can be broken and dissolved in the process of excess sludge hydrolysis,which would increase the sludge bio-degradability.However,the hydrolysis of sludge will also cause the release of metals.Our research is based on the combination technique of alkaline hydrolysis and electrolysis.This not only could extract protein by hydrolysis,but also could removal the heavy metals by electrolysis.Additionally,through improving and optimizing the technological process and the operating parameters,the data could be used to guide further engineering application.Applying the excess sludge of Longshan Sewage Treatment Plant in Wafangdian,a pilot-scale plant which combined cell breakage and electro-chemical reinforcement of sludge degradation was established.First,sodium hydroxide as hydrolysis agent destroyed the membranaceous structures of cell and organelle.The organic matter and the heavy metal within the cells or stick on the outside of the cells will be released into the liquid phase of the sludge.Then the heavy metals could be deposited by electrochemical oxidation.At the same time,the organic macromolecular will be further oxidized and degraded into smaller and easily biodegradable molecules on the electrode.The hydrolysate can be separated into aqueous phase and thickened sludge.The aqueous phase could be returned into sewage treatment system.As a result,the aims of sludge volume reduction,using as resources and innoxiousness could be achieved.Through batch experiments,we found that NaOH is a better hydrolytic agent than Ca?OH?₂?the increasing rate of total chemical oxygen demand?TCOD?is only 25.3%?.KOH is also a good hydrolytic agent,but its price is very expensive.pH of the sludge was adjusted from 7 to 14 by NaOH and the sludge was broken for 24 h.It was found that the higher the alkalinity was,the better the breaking effect was on the sludge hydrolysis.The highest increasing rate of TCOD was 57.2%.The pH of the sludge changed because the alkaline agent was consumed during the alkaline hydrolysis.The efficiency could be improved through adding NaOH to adjust pH in the process of the sludge hydrolysis.Polysaccharide,nucleic acid and protein were detected in the alkalinized sludge.And the concentrations of these organic substances were significantly increased,which further verified the cracking effects of NaOH.Most of the heavy metals in the sludge were removed by electrolysis,among which Cu and Pb showed the best removal effects?64.2%and 53.9%?.However,negative removal rate was obtained for Mn due to precipitation or enveloped sludge particles.Through the continuous experiments,it was found that the optimal retention time of sludge hydrolysis was 6 h.The removal of heavy metals in sludge could also be realized through the electrolysis device.As the sludge particles became smaller after cracking and were not easy to settle,an ultrafilter was added to speed up the solid-liquid separation of the sludge.The separated water could be re-entered into the sewage treatment system.The success of dynamic experiment provided good data support for the practical application of this technology.