Energy Saving And Operation Optimization Of Membrane Bioreactors For Municipal Wastewater Treatment

Membrane bioreactor?MBR?,an efficienttechnology for wastewater treatment and reclamation,has been applied in practical engineering in China over the past decade.However,membrane fouling and high energy consumption are two stubborn obstacles for wider application of MBRs.This study aimed at reducing energy consumption and operation optimization in large-scale MBRs for municipal wastewater treatment.Seasonal variation of mixed liquor physicochemical characteristics and its impact on membrane fouling were systematically investigated.Calibrated wastewater treatment model of A²/O-MBR process and aeration control strategy were established to perform a real-time aeration control in aerobic tanks.Pulse-mode aeration was applied to reduce energy consumption and control membrane fouling in membrane tanks.Optimized membrane cleaning strategy was designed to improve the cleaning efficiency of fouled membrane by chemical reagents.Actual membrane fouling in large-scale MBRwas correlated with temperature significantly.Besides the impact on filtrate viscosity and then on filtration resistance,low temperature condition induced more soluble microbial products from microorganisms into MBR supernatant to increase organic concentration,resulting in severe membrane fouling.Organics in supernatant played an important role in membrane fouling in actual MBRs.Humic substances was the predominant foulant.Hydrophilic and large-molecule polysaccharides and proteins also showed correlation with membrane fouling.Biological wastewater treatment model was established based on a large-scale A²/O-MBR process.Key parameters were calibrated according to experimental data obtained from a continuous monitoring of the A²/O-MBR plant.The calibrated model performed accurate dynamic simulation of ammonia N and total nitrogen removal and acceptable prediction of COD variation trend.An aeration control strategy,which consisted of an “ammonia-DO-aeration” two-step cascaded PI feedback control and an “influent flow rate-aeration” feedforward offset control,was designed based on the model.Simulation indicated an aeration flow rate reduction of 13% 18% in aerobic tanks.The control strategy was applied in the large-scale MBR,reducing the actual blower power of aerobic tanks by 15%.The annual average specific energy consumption was predicted to be cut off by 0.02 kWh/m³.Pulse-model aeration was achieved in membranetanks via modification of aeration system,resulting in a 31% reduction of aeration flow rate without deteriorated membrane fouling.The specific energy consumption was reduced by 0.11 kWh/m³.The increase of total aeration flow rate led the mixed liquor to an unfavorable condition?smaller particle size and high supernatant organic concentration?for membrane fouling control.The optimal aeration flow rate in membrane tanks was 15000 m³/h.Membrane cleaning efficiency was improved by means of setting acid cleaning procedure before NaClO cleaning,replacing citric acid by oxalic acid,and adding assistant reagent?EDTA-Na₂?into Na ClO solution.The efficiency of acid cleaning was dominated by the acidity of reagent,and could be impacted by the complex capacity of reagent.The induction of EDTA-Na₂ ensured simultaneous removal of organic and metal foulants from membrane fouling layer.The optimal dosage of EDTA-Na₂ was 150 mg/L.The replacement of citric acid by oxalic acid reduced the membrane cleaning cost?on unit membrane surface?by 0.12 RMB/m²,while the dosage of EDTA-Na₂ increased the cost by 0.13 RMB/m².The two approaches possessed better membrane cleaning efficiency at a similar cost level,comparing with traditional membrane cleaning strategy.