Experimental Study On The Treatment Of Compressed Leachate From Waste Transfer Stations By Progressive Freeze-melt Method

In waste transfer stations,the process of compressing and reducing the volume of domestic waste inevitably produces compressed leachate with complex composition and high-concentration of contaminants.Compressed leachate is characterized by fluctuating and complex water quality,small water volume,and high COD levels,leading to drawbacks in conventional treatment processes.For example,high pollution levels and small water volumes make it difficult for the biological method to operate stably;the treatment cost of the physical and chemical method is high.Therefore,this study was carried out to investigate the treatment of compressed leachate generated from waste transfer stations by the progressive freeze-melt method.Progressive freezing was used.Humic acid,a difficult-to-degrade macromolecular organic compound,was used as a target contaminant in the simulation experiments.The effects of freezing temperature,icing ratio and initial concentration on humic acid removal in the process of progressive icing and the effects of initial concentration and ice crushing on humic acid melting and removal in the process of melting were investigated.The structure of ice formed at different temperatures was observed by microscopy,and the mechanism of ice"capture"of humic acid was discussed.On the basis of this study,the vertical distribution of contaminants in the ice and the treatment effect of tertiary freezing were investigated,and the release of"captured"contaminants from the ice bodies by ice crushing melting and centrifugal melting were also investigated.The main findings are as follows.The experiments of humic acid removal by progressive icing showed that the humic acid removal rate was 83.41%at the icing ratio of 40%at-4°C.As the freezing temperature decreased,the ice growth rate increased,resulting in a decreasing trend of humic acid removal rate.The humic acid removal rate tended to decrease with the increase of icing ratio.The humic acid removal rate was 86.75%at the initial concentration of 125 mg/L,while it decreased to 64.69%～66.32%at the initial concentrations of 250 mg/L～2000 mg/L,with small changes.The microscopic ice structure indicated that humic acid was aggregated in the form of“salt cells”or void channels rather than uniformly distributed.Ice melting experiments showed that humic acid melted out more in the early stage of ice melting process and gradually decreased in the middle and late stage.The humic acid removal rate increased with the increase of ice melt percentage.The higher the initial concentration,the higher the humic acid concentration in the initial meltwater,which indicates that the more contaminated ice are more suitable for melting method to improve the removal rate.Crushed ice melting could effectively promote the large release of humic acid at the initial stage of melting.The humic acid removal rate was 85.07%when the percentage of melting ice at the time of crushed ice melting was 30%,which was higher than that of natural melting at 67.83%.During the progressive icing of the compressed leachate,the contaminants"captured"by the ice gradually increased from top to bottom.The removal rates of conductivity,TDS,COD,TOC,Ca²⁺,and Mg²⁺ranged from 86.05%to 95.77%after tandem three-stage freezing.In both the natural melting method and the crushed ice melting method,the contaminants in the ice showed more melting out at the beginning of melting and then gradually decreased.Crushed ice melting not only accelerated the melting process but also promoted the melting out of contaminants at the initial stage of melting.The removal rate of TDS,COD,TOC,Ca²⁺and Mg²⁺by crushed ice melting method ranged from 79.09%to81.33%at 30%melting ratio,which was higher than that of 68.29%to 72.85%by natural melting method.In the experiment of treating compressed leachate by progressive icing-ice crushing-centrifugal melting method,the percentage of ice melt increased continuously with the increase of centrifugal time and centrifugal speed,while the removal rates of conductivity,TDS,COD,TOC,Ca²⁺and Mg²⁺also increased gradually.After centrifugation at 2000r/min for 60 s,the percentage of meltwater was 30.57%,and the average removal rates of TDS,COD,TOC,Ca²⁺,Mg²⁺ranged from 90.14%to 95.28%,which were higher than those of natural melting method and crushed ice melting method.This study enriches the research on the treatment of organic wastewater from humic acid and compressed leachate by progressive icing-melting method,which provides a new idea for the rational disposal of the compressed leachate from transfer stations.The decentralized waste transfer stations can reduce the treatment cost and help to reduce carbon emissions to achieve carbon neutrality if they combine the use of natural refrigeration,solar energy and other green energy for freezing treatment.