Pretreatment of landfill leachate membrane concentrate by coagulation-softening process
To investigate the pretreatment efficiency of coagulation-softening process on leachate membrane concentrate from a southern city's landfill,a pilot-scale study was conducted to compare commonly used coagu-lants(PAC,PAFC,FeCl3),flocculants(PAM),and softening agents(Ca(OH)2+Na2CO3,NaOH+Na2CO3).By opti-mizing the dosage of chemicals and the initial pH value,the removal efficiency of pollutants(primarily COD,Ca2+,Mg2+)and the cost of chemicals in the pretreatment process were analyzed.The optimal matching point of the combined process was determined,and an analysis on the organic matters removal mechanism in the pretreatment process was also carried out.The test results indicated that FeCl3 was a suitable coagulant,the optimal parame-ters for the coagulation stage were FeCl3 dosage of 2.5 g/L,PAM dosage of 60 mg/L,and initial pH value of 7.5(unadjusted),resulting in effluent COD mass concentration of 880 mg/L and chroma of 300 PCU.For the soften-ing stage,the optimal combination and parameters were as follows:adopting caustic soda-soda ash method(NaOH+Na2CO3),the dosage of NaOH and Na2CO3 were 0.3 and 0.5 g/L respectively,resulting in a total hardness of 92.1 mg/L in the effluent water.The analysis on the pollutants removal mechanism showed that,the coagulation-soft-ening process was most effective in removing humic acid substances from dissolved organic matters,as well as large molecular esters,amine compounds,and aromatic volatile organic compounds.
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