Active biomass retention is a technical challenge in anaerobic digester treating dilute animal manure that contains solids particles. A strategy was tested using fibers in the dairy manure as biomass carriers by controlling settling time. Settling time ranging from 0.5 to 60 min were applied to eight anaerobic sequencing batch reactors to investigate their effects on active biomass retention in anaerobic digestion of flushed dairy manure. Results revealed that there existed a critical settling time at 2 min at which only minimum amount of active biomass was retained, and as settling time increased or decreased from this threshold, more active biomass could be retained. Gravity settling and selection pressure theories were suggested to account for the results. A model integrating these two effects was developed and verified with the experimental data. Knowledge derived from this study may lead to innovative bacterial retention technology for cost-effective anaerobic digestion of dairy wastes. (C) 2010 Published by Elsevier Ltd.