Operating an anaerobic digester at low pH Could offer several advantages over operation at neutral pH Most wastewater streams targeted for anaerobic digestion are inherently acidic, requiring alkalinity supplementation (at added expense) to buffer the pH at neutral. Additionally, previously published work completed by the authors using batch systems suggested that lowering the system pH could increase methane production by as much as 30%. The goal of this research was to evaluate the feasibility of sustaining methanogenesis at low pH in a semi-continuous laboratory-scale fermentor. Significant methane production was achieved in a system ranging in pH from approximately 4.0-5.3. Results show that, if the consortium is allowed to Sufficiently acclimate to acidic conditions, methanogenesis can be maintained under acidic pH conditions, resulting in overall chemical oxygen demand (COD) reduction and methane production comparable to that achieved in a neutral pH system. (C) 2008 Elsevier Ltd. All rights reserved.