Comparing Membrane Bioreactors and Conventional Activated Sludge Processes for Low Nutrient Limits

Engineers now regularly apply membrane bioreactor (MBR) technology at wastewater treatment facilities when effluent regulations require efficient liquid/solids separation. MBR applications can provide distinct treatment advantages at specific sites but frequently at increased capital and operating cost compared to traditional conventional activated sludge (CAS). Utility managers who re responsible for managing capital programs are most interested in capital, operation and maintenance (O&M), and the associated life cycle costs. The fiscal responsibilities associated with managing public funds often drive projects through an extensive conceptual or preliminary design phase where engineers evaluate numerous plant process options and technologies. Three recent case studies demonstrate the higher capital and life cycle costs seen with MBR technology, which typically drive the process selection toward conventional activated sludge (CAS) treatment systems.This paper reviews recent detailed technical evaluations of advanced wastewater treatment plant expansions in Virginia, North Carolina, and Florida considering both MBR and CAS processes to reach stringent effluent nutrient limits for nitrogen and phosphorus. In each case study, thorough evaluations of both MBR and CAS processes resulted in selecting the CAS option. Although the benefits of an MBR ranked this technology at or near the top of the process alternatives evaluated for each project, the associated capital and life cycle costs of the MBR were typically 10% to 30% higher than the CAS alternatives.