Research On Treatment Efficiency Of Phenol Degradation And Methane Production In Coal Gasification Wastewater By Enhanced Anaerobic Process

Coal gasification produces a large number of coal gasification wastewater during gas washing,cooling and purification process.The characteristic of coal gasification wastewater are high concentrations of phenolic compounds,refractory organic compounds and toxic pollutants.The production of these pollutants has a relationship with the structure of gasifier,operating conditions,coal and so on.Nowadays,the unsatisfied effluent treatment efficiency,instability in system,high energy consumption existed in the treatment of coal gasification wastewater.According to the characteristics of the coal gasification wastewater,the anaerobic filters?AF?was chosen as our research object for advanced anaerobic process.Also the development process of the effluent recirculation and anaerobic co-digestion were investigated in treating coal gasification wastewater.Relevant operation parameters were found out and the treatment efficiency of adding biochar in batch test under anaerobic condition was also checked.At last the stability of AF treating coal gasification wastewater was studied in the present research.When the raw coal gasification wastewater as influent flowed into the reactor,it had an adverse effect on the activity of anaerobic microorganisms due to high concentration of phenolic compounds and other toxic pollutants present in coal gasification wastewater.Thus,diluted coal gasification wastewater was adopted in the influent.The result indicated that AF received good performance in treating diluted coal gasification wastewater,but step-wise increasing organic loading rate to the raw wastewater,the system showed instability.Hence,the effluent recirculation was adopted in treating coal gasification wastewater,under the optimal recirculation rate of 0.5:1,which can greatly improve the treatment efficiency,the average effluent COD and total phenol were reached 234.0 and 14.2 mg/L with the average methane production rate reached 169.0 mL CH₄/gCOD/d.In order to further improve the treatment efficiency of coal gasification wastewater and facilitate it to resource utilization.The glucose solution and starch solution were added into the influent,respectively and the COD was maintained at 1000 mg/L,respectively.The result showed that when the low concentration of coal gasification wastewater was maintained in the influent,the system was in stable operation,but raw wastewater in influent leads to the unsatisfied result.Increasing the COD of starch solution to 1500 mg/L,the system exhibited good performance with the removal rate of COD?62%?and total phenol?60%?,respectively.Moreover,methane production rate increased to 260 mL CH₄/gCOD/d.Then the two stage AF under co-digestion was investigated in treating coal gasification wastewater,which kept the first AF adding starch solution of the COD=500 mg/L,HRT of the first AF was of 18 h,24 h,36 h and 48 h,respectively.The HRT of the second AF was kept about 48 h.The treatment efficiency of coal gasification wastewater in two-stage AF was studied.The result indicated that although adding starch solution as co-substrate could not provide enough carbon source and energy to support the cooperative metabolism of anaerobic micro-organism.At last the total removal of COD and total phenol reached 50%and 70%,respectively.But the methane production rate was not very high.Thus,the removal of the pollutants in the system were mainly depended on the sludge absorption or transition to other form and could not completely degraded by anaerobic micro-organisms.Because the biochar has well-developed pore structure,surface area and adsorption.Therefore,moderate biochar could be added in anaerobic biofilters to form the biochar-anaerobic activated sludge system treating coal gasification wastewater.The shaking-bottle incubating test was applied to investigate the treatment efficiency of biochar-anaerobic activated sludge system treating coal gasification wastewater.The result indicated that the COD and total phenol removal in mild alkali conditions?pH=8?were superior to partial acidic condition.Moreover,when the dosage of biochar was 20 g/L,the total phenol was removed completely.At last the experimental data was fitted by kinetic equation,the result indicated that the fitting result of the COD removal was a little better than that of the total phenol removal which further demonstrated that the removal of COD and total phenol were depended on the absorption of biochar.Hence,biochar-anaerobic activated sludge system can be applied to remove the refractory compounds in coal gasification wastewater.In order to explore the stability of AF,the AF was stopped for 2months then again restarted,the result demonstrated that comparing with the continuous operation,the treatment efficiency of AF was almost unchanged when the AF was restarted and after short-time stop the activity of anaerobic micro-organism could be recovered immediately.Moreover,the system has strong endurance to pH change in short time.In order to further enhance the treatment efficiency of AF,the concentration of ethyl alcohol?5%?was added as co-substrate in influent.The result indicated that it was unfeasible to add ethyl alcohol in influent as co-substrate.After operating for one week,addition of ethyl alcohol was stopped and glucose solution was used with COD=1500 mg/L in influent to recover the activity of the anaerobic micro-organism.The result showed that the inhibition of low concentration of ethyl alcohol to anaerobic micro-organism was reversible and treatment efficiency in mesophilic conditions was better than in ambient temperature condition.It is necessary to explore the inhibition to anaerobic micro-organism under different phenol concentrations as coal gasification wastewater contains almost 50%phenol.The result of batch test indicated that the phenol concentration under 280 mg/L can stimulate the growth of methanogens.In the lab-scale operation,the HRT=48 h was kept,adding glucose solution with the COD=3000 mg/L in influent as co-substrate,which facilitate the quick start-up of AF?60 days?and effluent COD and total phenol reached 1200,100 mg/L,respectively.Also methane production rate reached about 200 mL CH₄/gCOD/d.Moreover,with the increasing concentration of phenol in system,the concentration of exocellular protein in EPS was increased and the anaerobic granular sludge was accumulated in the range of 0-400 um.High through-put sequencing results demonstrated the abundance of the Chloroflexi,Planctomycetes were inhibited by toxicity of CGW and the pollutants removal had some relationship with the main microbial community shift.At last,compared with the treatment efficiency of these advanced process of AF,the result indicated that adopting effluent recirculation?effluent recirculation rate=0.5:1?in AF has an obvious advantage on treatment efficiency.In economic benefit consideration,the result indicated that AF with effluent recirculation has low cost and satisfied treatment efficiency which comparing with the AF,AF with effluent recirculation process and AF with co-digestion process.