Influence of anaerobic and anoxic hydraulic retention time (HRT), a commonly used parameter by treatment plant design engineers and operators, on nitrogen and phosphorus removal was studied in an anaerobic/ anoxic/oxic membrane bioreactor with fixed recycle rates. Anaerobic HRTs were varied between 0.5 and 3 h, anoxic HRTs were varied between 1 and 5 h while the aerobic HRT was fixed at 8 h. Total nitrogen removals ranged from 76% to 89% and were found to increase with increasing anoxic HRTs but were minimally impacted by increasing anaerobic HRTs except when the anoxic HRT was at 5 h. Phosphorus removals ranged from 40% to 82% and were found to increase with increasing anaerobic HRTs from 0.5 to 2 h but decreased for an anaerobic HRT of 3 h and with an increase in anoxic HRTs. The study shows that optimal removals for both nitrogen and phosphorus require balancing the conflicting needs of a longer anoxic HRT for increased nitrogen removal but decreased phosphorus removal and an optimal anaerobic HRT for phosphorus removal without impacting the nitrogen removal. The most favorable HRTs for combined optimal nitrogen and phosphorus removal were found to be 2-h anaerobic and 4-h anoxic. (C) 2010 Elsevier B.V. All rights reserved.