Process And Mechanism Of Sludge Dewatering By Hydrothermal Treatment

Reduction is one of the three major aims(harmlessness,reduction,stabilization)for sludge treatment and disposal,and the dewatering is the key.Dozens of methods have been raised to enhance the dewaterability of sludge in recent decades.Nevertheless,there are some difficulties in dewatering and applications.Hydrothermal carbonization is a new sludge treatment method that has been concerned by more and more people because of its dewatering efficiency and the recycling of the biochar.The research was conducted by analyzing the sludge cake produced at 130℃,160℃,190℃,220℃and 250℃for 15min,30min,1h,1.5 and 2h of municipal sludge to investigate the effects of hydrothermal reaction temperature and duration on dewatering performance.The specific resistance to filtration(SRF),the sludge volume index and the moisture content of sludge cake were measured to examine the dewatering performance of sludge.The research showed that the reaction temperatures had a larger effect on the dewatering performance of sludge than the duration.The dewatering performance was becoming worse first and then becoming better with the increase of the reaction temperature.The dewatering performance began to show a positive at temperature 190℃,the moisture content in the dewatered cake decreased from 80.62%to 70.13%,the SRF of the hydrothermal sludge decreased from 1.01×10~8s~2/g to 3.39×10~8s~2/g,the SVI of the hydrothermal sludge decreased from 35.40mL/g to 14.51mL/g.The improvement of dewaterability began to fatten at temperature more than 190℃.The optimal condition determined by experiment and the energy consumption was that treating at 190℃for1.5h.Physico-chemical properties of hydrothermal sludge,hydrochar and filtrate were determined in order to illustrate the dewatering mechanisms of hydrothermal sludge.Volatile suspended solids(VSS)had an important effect on dewatering performance of sludge,the result showed that the rate of VSS/SS decreased by the hydro thermal treatment,which can effectively improve the stability and the dewaterability.The surface morphology was observed by the scanning electron microscope(SEM),the sludge cell disruption was observed when temperature exceed 190℃,which caused the release of internal water and the improvement of dewaterability.The increasing amount of chemical oxygen demand(COD)and the ammonia nitrogen in the hydrothermal filtrate were caused by the degradation of the organic matter.The filtrate color was resulted from the Maillard reaction.The Extracellular polymeric substances(EPS)content had a high correlation with the SRF and the moisture content of the dewatered cake.The surface properties of sludge were altered by the hydrothermal treatment,the sludge viscidity were improved,which was closely correlated to the improvement of dewaterability.Contents and the speciation distribution of heavy metals(Ni,Cu,Pb,Cd,Zn)in sludge were analyzed during the hydrothermal carbonization process.The results showed that the contents of heavy metal in hydrochar were higher than that in the original sludge,and the heavy metals had transformed to oxidizable and residual forms from the exchagable and reducible forms during the hydrothermal treatment.The contents of heavy metals basically met the standards of pollutants in sludge for agricultural use(CJT 309-2009)and standards of sludge used in gardens or parks(GBT 23486-2009).The risk assessment code(RAC)were considered to evaluate the potential ecological risk.In general,the hydrothermal carbonization showed obvious advantages in stabilization of heavy metals and showed harmlessness to the ecological environment.Based on the previous experiment,the medium-sized experiment were performed for further application.The original sludge(moisture context was 97.5%)in Foshan Zhen’an sewage treatment plant was treated by hydrothermal carbonization with temperatures ranging from 160℃to 190℃,then treated sludge was dewatered by the plate and frame type filter press.The sludge cake and filtrate were identified to illustrate the dewatering process.The research showed that hydrothermal treatment could realize deep dewatering of sewage sludge,the moisture content in the dewatered cake decreased to 65.46%.The energy-economic analysis was shown that treating 1 ton of sewage sludge,the cost of hydrothermal treatment was 313.26 Yuan.Besides,the volume of the sewage sludge had decreased by 67.78%.In general,the hydrothermal treatment was performing better in sludge harmlessness and reduction,both energy consumption and cost in hydrothermal carbonization treatment had to be studied further.In general,the analysis results showed that hydrothermal treatment not only improved the dewaterability of sludge but also stabilized the sludge,which were valuable for further application in sewage treatment industry.