Anaerobic Membrane Bioreactor(AnMBR) For Municipal Wastewater Treatment And Energy Balance Analysis

Along with the growth of economic and population,the discharge and treatment capacity of municipal wastewater(MWW)also present a steady upward trend.Currently activated sludge process is widely used in MWW treatment,but faced with the issues of high energy consumption,low sludge production,carbon emissions,and limited resource recovery.With the growing shortage of water resources and utilization of reclaimed water,based on the concepts of energy conservation emissions reduction,low carbon and sustainable development,MWW treatment processes should produce effluents with qualified water quality,meanwhile recover the bioenergy and bioresource as much as possible.Anaerobic membrane bioreactor(AnMBR)is regarded as a promising technology for MWW treatment and reclamation,but at room temperature conditions AnMBR still face the technical bottlenecks of membrane fouling and low energy recovery efficiency,needing in-depth research.Therefore,the actual MWW chosen as the objective,based on a continuously running AnMBR device,the following aspects have been investigated,including the effects of optimal operation conditions on AnMBR performance,the gas-liquid phase equilibrium of biogas and the solubility of methane as well as COD balance and energy balance analysis.The main research results are shown as follows:(1)A 20-L submerged AnMBR was operated for the treatment of real MWW over a 200 d operation period,according to the variations of operational flux(5,8 and 11 L/m2h)and intermittent filtration mode(filtration to relaxation time(F/R)ratios of 3:1,4:1,5:1 and 9:1).The AnMBR results show that the change of operating conditions at normal temperature(25℃)has no obvious effect on the removal of organic matter and gas production.The removal rate of COD removal(85%～90%),satisfactory methane production(0.07～0.10 LCH4/L-wastewater),and a high methane content of 80%regardless of filtration mode.At low-flux conditions(5～8 LMH),the transmembrane pressure(TMP)remained at low values of 0.5～1.0 kPa,and was not affected by changed F/R ratios.However,at the higher flux of 11 LMH,the TMP increasing rate was obvious higher at F/R ratios of 3:1 and 9:1,because a higher instantaneous flux and shorter relaxation time resulted in substantial irreversible fouling caused by a high-flux induced drag force and insufficient detachment of foulants,respectively.(2)The changes of sludge properties in the AnMBR were analyzed from the perspectives of sludge concentration,particle size distribution,excitation-emission matrix(EEM)spectra and microbial community structure.The ratio of MLVSS/MLSS in the reactor was stable between 80%～90%,and the sludge particle size was mainly distributed in the range of 10-100 μm.EEM spectra analysis showed that the main membrane foulants originated from SMP and EPS.In addition,high-throughput sequencing was used to detect the evolution of microbial populations.The bacteria mainly included Firmicutes,Bacteroidetes,Proteobacteria and Chloroflexi,which played an important role in the biodegradation process of organic matter.Methanogenic archaea mainly included Methanosaeta and Methanobacterium at the genus level,and the relative abundance of Methanosaeta species were significantly higher than that of Methanobacterium,indicating that the methanogenic pathway was dominated by acetoclastic methanogenesis rather than hydrogenotrophic methanogenesis.(3)According to the experimental results and theoretical calculation,the COD mass balance and energy balance have been conducted.The COD balance analysis showed that the contribution of the effluent COD to the total influent COD was a constant 13%.In the case of sludge production,the proportions rose from 7%during Phase Ⅰ,11%during PhaseⅡ,to 16%during Phase Ⅲ,which could be attributed to the increased accumulation of slowly biodegradable organics derived from influent wastewater at shorter HRT and higher hydraulic loading.The makeup of total methane production in the COD balance decreased from 80%to 76%and 72%,and soluble methane accounts for 15%,23%and 21%of total methane production,respectively.Energy balance analysis shows that biogas cycle is the main component of energy consumption(>62%)at room temperature.AnMBR process could achieve a net energy production when a combined heat and energy(CHP)system could be applied for recovering heat and power from biogas produced,while the energy associated with biogas sparging contributed to more than 80%of the total energy consumption.However,it is still needed to improve the energy balance status under low temperatures.