The Research Of Concentrate Leachate Treatment By Air Humidification-dehumidification Technology

The nanofiltration(NF)concentrate produced by the landfill site contains high concentrations of pollutants,which can cause serious harm to the environment without reasonable treatment.In particular,the landfill concentrate of coastal cities,if it is directly discharged,which will cause incalculable damage to the marine environment.More and more attention has been paid to the treatment of concentrated liquid by evaporation method.However,there are some disadvantages in the evaporation method like that the ammonia nitrogen cannot up to emission standard,the equipment is easy to scale and the heat transfer efficiency is low.In order to solve the problems existing in the evaporation method,the paper firstly introduces the treatment of landfill leachate from a NF unit,using an air humidification-dehumidification(HD)process,and the effects of different pH values on the removal efficiencies for NH3-N,TN,COD,TOC,UV254 and conductivity were studied.Firstly,the experimental device was set up and the air-water direct contact heat and mass transfer in humidifying tower was studied.The results demonstrated that the volumetric heat transfer velocity and water production rate could reach up to 236 kW/m3 and 8.11 kg/h when the NF concentration and air flow rates were 240 L/h and 200 m3/h.Then,on the basis of analyzing the water quality of NF concentrate,the pH of NF concentrate was adjusted to acidic(about 3 and 5)and alkaline(about 9)by adding NaOH or H2SO4 solution.When the heat and mass transfer efficiency of the humidifying tower was the highest,the concentration of different pH value is treated,and found that,the removal rates for NH3-N,TN,TOC and conductivity under acidic condition was higher than the alkaline condition,while the alkaline condition was beneficial to the removal of COD.More specifically,the removal rate of UV254 was above 98.00%at various pH values,and the removal rate of TOC reached its maximum value after 1 h at pH 3.11.The maximum removal rates for COD,NH3-N,TN,TOC,and UV254 were 97.60%,97.90%,98.90%,99.00%,and 99.00%,and the concentrations of NH3-N,TN,COD,TOC,and the absorbance of UV254 in the condensate dropped to 1.95 mg/L,3.15 mg/L,32.00 mg/L,7.00 mg/L,and 0.037 cm-1,respectively.The maximum removal rate of conductivity was 99.38%,the value dropped from the original 18.7 ms/cm t0 116.3μs/cm.Therefore,the condensate water quality can attain acceptable discharge limits.The economic feasibility of the evaporation and air HD methods was compared theoretically,and the latter was lower in the costs of equipment and operation.Lastly,the common air dehumidification technologies were summarized and compared.It was concluded that the heat pump cooling dehumidifying technology was the best choice.Compared to the evaporation method,this air HD technology exhibits high economic feasibility,and accordingly the HD process is thought to be an effective way to treat NF concentrate.