Mechanism And Effects Study Of Evaporation Process For Treating High Salinity Wastewater At Low Temperatures And Ambient Pressure

In this study,a lab scale unit of evaporation system for treating high salinity wastewater at low temperatures and ambient pressure was designed and installed,which was based on the evaporation theory of direct contact between water and air.Real wastewater and simulated wastewater containing sodium chloride and glucose were used in this study.Analysis in laboratory revealed the effects of influential factors on the quality of condensed water and evaporation rate of the system.A pilot project of the application study was also conducted in the fine chemical factory in Shanghai.The effects of system structure,evaporation packing,operating conditions and raw wastewater on the quality of condensed water and evaporation rate of the system were studied in the laboratory.The heat and mass transfer during the evaporation process was also discussed.The results showed that the evaporation rate of counter flow system was nearly 1.5 times as high as cross flow system under the same operating conditions.The total dissolved solids and the chemical oxygen demand removal efficiencies could reach up to 99.9%and 99.8%through counter flow and cross flow evaporation system.The recrystallized salts obtained from the separator at the bottom of the evaporation chamber had 15.6%moisture content.The total dissolved solids and the chemical oxygen demand removal efficiencies both could reach up to 99.9%and 99.8%in the evaporation studies with three types of evaporation packing.The evaporation rate with structured plate packing was higher than grid packing.There were positive correlations between evaporation rate and input wastewater temperature,wastewater flow rate and air flow rate.The evaporation rate increased faster with higher input wastewater temperatures.For the simulated wastewater,the temperature of input air had little effect on the quality of condensed water.High inlet air speed?>2.5 m/s?and wastewater flow rate?>1.0 m³/h?could lead to the result of high TDS and COD_Cr concentrations in condensed water.The TDS and COD_Cr removal efficiencies for wastewaters with different salt concentrations were 99.5%and 99.8%at least through the evaporation system.Evaporation rate was affected significantly by the density,viscosity,and saturated vapor pressure of wastewater.The equation of the mass transfer coefficient considering the density,viscosity,and saturated vapor pressure of wastewater was built to predict the mass transfer efficiency for the evaporation process.A case study using real wastewater from a pharmaceutical factory illustrated that the system performed well in practical situations.The TDS and COD_Cr removal efficiencies were as high as99.8%and 95.8%through the evaporation system,although the COD_Cr concentration in condensed water was 1823 mg/L.The predicted mass transfer coefficient agreed well with the measured value,with 6.1%relative difference.Based on the studies in laboratory,a pilot project of the application study was also conducted in the fine chemical factory in Shanghai.The capacity of the system was 15m³/d?evaporation rate?.The COD_Cr and TDS concentrations of input wastewater were146500 mg/L and 52700 mg/L,respectively.The COD_Cr and TDS removal efficiencies could reach up to 98.70%and 99.20%through the evaporation system.The biodegradability of wastewater was improved significantly after the evaporation process.The economic analysis of the evaporation system showed that the treatment costs was about 65 kW?h/t and it was an energy saving way for high salinity wastewater treatment.In this study,the mechanism and effects of evaporation process for treating high salinity wastewater were investigated.The results provide a theoretical basis for treating high salinity wastewater through the developed evaporation system to achieve Zero Liquid Discharge.The developed evaporation system,which operates at low temperature and ambient pressure,is feasible in technology and economic.It has good prospects for the treatment of high salinity wastewater.