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External membrane bioreactor for the anaerobic treatment of low strength municipal wastewater Di, Donghong

Abstract

In this study, the workability of a newly designed anaerobic external membrane bioreactor (AnMBR) was examined. During the acclimatization period, the system was operated at a temperature of 25 to 42 °C and a pH of 6.8 to 7.5. The influent COD was 510 (±13) mg/L. After the acclimatization, the system was maintained at 25 °C and a pH of 6.3 to 7.0. The influent COD was 290 (±18) mg/L. The hydraulic retention time (HRT) was decreased in a stepwise manner from an initial value of 61 hours to 2 hours. As the HRT was decreased, the corresponding volumetric organic loading rate (OLR) was increased from 0.22 ± (0.04) kg COD/m³day to 2.84 ± (0.11) kg COD/m³-day. Results showed that the anaerobic external MBR was capable of treating low strength municipal wastewater under controlled conditions. The COD removal ef ficiencies were 84 (±4) % and 60 (±14) % for the acclimatization period and the steady-state operation period, respectively. The decrease in COD removal efficiency may have been caused by short retention time of organic material in the reactor due to the short HRTs, the inhibition of biomass activity by the high OLR, a decrease of the soluble COD portion of the influent, and the accumulation of VFAs in the reactor. An HRT of 8 hours and OLR of 1.6 kg COD/m³•day was identified as the optimum operational conditions for the system. The permeability of the membrane limited the system treatment capacity. Inorganic membrane permeability was superior to the organic membrane permeability. When operated at trans-membrane pressure (TMP) of 207 kPa and cross flow velocity (CFV) of 3.3 m/s, the organic membrane exhibited high and stable permeate flux. Membrane permeability decreased with an increase of sludge concentration at TMP of 276 kPa, but was independent of sludge concentration at TMP of 83 kPa. Organic membrane cleaning with sodium hypochlorite resulted in an effective flux recovery.

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