Article pubs.acs.org/est
Contribution of Liquid/Gas Mass-Transfer Limitations to Dissolved Methane Oversaturation in Anaerobic Treatment of Dilute Wastewater Hyeongu Yeo,† Junyeong An,† Robertson Reid,† Bruce E. Rittmann,‡ and Hyung-Sool Lee*,† †
Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario Canada, N2L3G1 ‡ Swette Center for Environmental Biotechnology, The Biodesign Institute at Arizona State University, P.O. Box 875701, Tempe, Arizona 85287-5701, United States
Downloaded by STOCKHOLM UNIV on August 26, 2015 | http://pubs.acs.org Publication Date (Web): August 13, 2015 | doi: 10.1021/acs.est.5b02560
S Supporting Information *
ABSTRACT: The mechanisms controlling the accumulation of dissolved methane in anaerobic membrane bioreactors (AnMBRs) treating a synthetic dilute wastewater (a glucose medium) were assessed experimentally and theoretically. The AnMBR was maintained at a temperature of 24−26 °C as the organic loading rate increased from 0.39 to 1.1 kg COD/m3-d. The measured concentration of dissolved methane was consistently 2.2- to 2.5fold larger than the concentration of dissolved methane at thermodynamic equilibrium with the measured CH4 partial pressure, and the fraction of dissolved methane was as high as 76% of the total methane produced. The low gas production rate in the AnMBR significantly slowed the mass transport of dissolved methane to the gas phase. Although the production rate of total methane increased linearly with the COD loading rate, the concentration of dissolved methane only slightly increased with an increasing organic loading rate, because the mass-transfer rate increased by almost 5-fold as the COD loading increased from 0.39 to 1.1 kg COD/m3-d. Thus, slow mass transport kinetics exacerbated the situation in which dissolved methane accounted for a substantial fraction of the total methane generated from the AnMBR.
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INTRODUCTION Energy-efficient wastewater treatment is gaining attention for lowering costs and the carbon footprint. Municipal wastewater treatment plants (WWTPs) mainly have used the activated sludge process for almost a century.1 However, activated sludge is expensive due to the large operations and maintenance (O&M) costs of aeration and sludge disposal, and it has a large carbon footprint.2 To reduce O&M costs and improve sustainability, anaerobic biotechnologies are being considered as alternatives to activated sludge.3 Furthermore, anaerobic biotechnologies allow the recovery of substrate electron equivalents as valuable products, such as methane, hydrogen, H2O2, or electric power.4−7 These merits already have driven the growth of anaerobic treatment of high strength organic wastes and wastewaters (e.g., animal manure, industrial wastewater, and primary and waste activated sludge), for which achieving a low effluent concentration of biochemical oxygen demand (BOD) is not a constraint, since anaerobic treatment often is used as pretreatment, not for meeting effluent standards.8,9 In contrast, anaerobic treatment of dilute wastewater, such as domestic sewage, is rarely used due to deficiencies in effluent quality and process reliability. However, combining membrane separation with anaerobic treatment (called anaerobic mem© XXXX American Chemical Society
brane bioreactors (AnMBRs)) can improve effluent quality and reliability due to the perfect separation of suspended solids.3,10,11 For example, membrane separation can lower the effluent concentration of BOD enough (
