Effects Of Air/water Ratio On Performance And Carbon Footprint Of Membrane Bioreactors (MBR)

With the application of membrane bioreactor (MBR), membrane fouling is always the key issue puzzling its efficiency and cost. As one of the measures in alleviating membrane fouling, the air/water ratio, which had a close relationship with membrane fouling and subsequent dehydration of sludge, had important impact on properties of mixed liquor, such as relative hydrophobicity, SVI, Zeta potential, the content of extracellular polymeric substances (EPS) and its composition. Moreover, it was closely related to the energy consumption and carbon footprint. Combined with the concept of carbon footprint, the effect of air/water ratio on pollutant removal, sludge characteristics and membrane fouling in MBR system was investigated and its carbon emissions were preliminary calculated in this study. The main contents and results were summarized as follows:Firstly, the effect of air/water ratio on effluent quality of MBR was investigated. The air/water ratio had little impact on COD removal (more than 90%) due to the degradation of organic matters by heterotrophic bacteria. On the other hand, the membrane cutoff would keep most large organic molecules in MBR so that the effluent COD kept stable. Ammonia removal increased with aeration intensity increased, and the removal rate of TN in the dissolved oxygen concentration of 0.8 mg·L-1 was the highest. With the increase of air/water ratio, TP removal was not high. The higher air/water ratio could alleviate membrane fouling.Secondly, EPS content and its distribution under different air/water ratio was studied. The results showed that, with the air/water ratio increased, LB-EPS contents in the supernatant and sludge were decreased, but the total EPS content changed little. Both EPS in sludge and supernatant were mainly composed of polysaccharides. In addition, the dissolved and total EPS contents in the membrane fouling layer increased, and proteins were dominative. When the air/water ratio was 75:1, the ratio of proteins and polysaccharides in dissolved EPS in the membrane fouling layer was between 1.1-2.0, and the ratio of proteins and polysaccharides in total EPS was between 1.0-1.8.Thirdly, the effect of air/water ratio on the properties of MBR sludge was investigated. Air/water ratio had little effect on the sludge concentration. However, with the air/water ratio increased, MLVSS/MLSS ratio increased from 0.83 to 0.88. In addition, the dehydrogenase activity and SOUR of sludge also increased. The sludge Zeta potential showed upward trend, rising from -38.5 mV to -32.1 mV, the sludge volume particle size in average increased from 60μm to 120μm. Zeta potential of sludge and membrane fouling showed a good negative correlation with air/water ratio.Finally, the carbon footprints of MBR system treating simulated municipal wastewater under different air/water ratio were primarily calculated in this study. The preliminary results showed that the carbon footprint increased with the increase in air/water ratio of MBR system, and CO2 emissions were mainly from the energy consumption. When the air/water ratio of MBR was 45: 1, there was the smallest carbon footprint, which was 4.67 g·gCOD-1.