Advanced Treatment Of Biologically Treated Coking Wastewater By Membrane Distillation

Coking wastewater is a highly concentrated organic wastewater,which is usually generated in the process of high-temperature carbonation,coal gas purification,and chemical refining in coking plants.Due to the presence of refractory and inhibitory contaminants,the effluents from biological treatment system still contain a relatively high concentration of non-biodegradable organic pollutants,inorganic salts and a small amount of micro-organisms.Therefore,advanced treatment of the biologically treated coking wastewater（BTCW）is becoming increasingly important,especially for reuse purpose.In theory,it can retain all nonvolatile pollutants to achieve distilled-water like water quality at lower temperature.More importantly,membrane distillation（MD）is an energy conservation technology which can make use of waste heat and other renewable energy.Previous researches on membrane distillation were mainly focused on sea water desalinization and treatment of highly concentrated saline water.Attempts have been made using MD to treat industrial wastewater in recent years.However,due to the presence of volatile organic compounds,inorganic salts and other components in biologically treated coking wastewater,the application of MD on these waters were impeded because of the wastewater’s influence on the water quality,thermal efficiency of membrane process and the flux decline.To our best knowledge,advanced treatment of the biologically treated coking wastewater by membrane distillation has not yet been reported.The present study aims to determine the optimum operating conditions of membrane distillation during advanced treatment of the BTCW by comprehensive analysis of the water quality,thermal efficiency of membrane process and the flux decline phenomenon.The effects of pretreatment on the removal of pollutants permeate quality and flux decline were also studied.In addition,membrane fouling of different conditions was investigated comprehensively to determine composition of the membrane foulant and possible fouling mechanism which is important for control of the flux decline.The main results are as follows:（1）The advanced treatment of BTCW using direct contact membrane distillation（DCMD）system was studied and the influence of recirculation flow rate on water quality,thermal efficiency and the flux attenuation were analyzed.The results showed that the recirculation flow rate had little influence on water quality and the removal rate of color,inorganic salt,COD and ammonia nitrogen were all above 99.5%、99.5%、97.8%and 82.1%,respectively.The thermal efficiency increased first then then decreased with the increasing of flow rate,and the highest thermal efficiency is obtained at 0.3m/s.In addition,the higher flow velocity can mitigate the membrane fouling.Higher flow velocity can reduce the frequency of membrane cleaning and prolong the life of membrane,but it will increase the cost of operating.The optimal flow velocity is 0.3m/s considering all these factors.Under this operating condition,the initial flux was 17.90 kg·m^-2·h^-1 and the main pollutant index of the permeate effluent,such as COD,color,conductivity and ammonia were 6.5mg/L,1°,37.0μs/cm and 1.08mg/L,respectively.Additionally,the membrane flux decline was only about 2.5 kg·m^-2·h^-1 after operating for 8h.（2）The effect of pre-coagulation on the removal of pollutants in coking wastewater and the process of advanced treatment of the biologically treated coking wastewater by membrane distillation was investigated with poly-aluminum chloride（PAC1）and polyacrylamide（PAM）.Results showed that pre-coagulation with poly-aluminum chloride（PAC1）was found to be effective for significantly reducing the contaminant level in BTCW and the removal rates of color,COD and UV254 were 39.5%,40.1%and 32.2%,respectively,which can effectively reduce fouling during membrane distillation.Use of polyacrylamide（PAM）as a coagulant aid could further decreased the contaminant level in BTCW,but it may lead to even severer membrane fouling.Scanning electron microscopy（SEM）,fourier transform infrared spectroscopy（FTIR）and energy-dispersive spectrometer（EDS）analyses revealed that PAC1 coagulation can alleviate the membrane fouling by preventing the formation of calcium carbonate on the membrane surface,whereas the addition of PAM facilitated the accumulation of aggregates on membrane surface,which lead to increase of the mass transfer resistance.（3）The influence of adsorption and coagulation/adsorption pretreatment on dissolved organic matter（DOM）removal and process performance were studied with D301R resin.Results showed that pre-adsorption had better removal efficiency in terms of DOM,especially the chromophore group and auxochrome group,and the removal rate of color and COD can reach 85.9%and 62.5%,respectively.After coagulation/adsorption pretreatment,the removal efficiency of color,COD and UV254 further increased to 91.5%,76.1%and 94.1%.The amount of resin was also saved by about 1/3,which showed that PACl coagulation pretreatment can significantly reduce the amount of non-soluble suspended substances,colloids and part of organic matter,and D301R resin can remove DOM especially for those with brown color.Membrane distillation experiments showed that the trace low molecular weight volatile aromatic substances were dominant in the organic residuals.Moreover,after 12h operation of membrane distillation,flux decline was less than 5%.It is demonstrated that coagulation/adsorption pretreatment could remove inorganic and organic pollutants significantly from membrane surface,which had a positive effect on the long-term operation of membrane distillation for advanced treatment of BTCW.