A 75.6 liter per minute (lpm, 20 gallons per minute) anaerobic selector pilot plant was operated at the King County Department of Metropolitan Services East Division Reclamation Plant (EDRP) between August 1992 and April 1993. The pilot test verified the EDRP Enlargement III design for producing a low sludge volume index (SVI) mixed liquor suspended solids (MLSS) at relatively short solids residences times (SRT) with an anaerobic selector. The pilot plant operated successfully under both summer (3-day SRT at 20 degrees C) and winter (4-day SRT at 14 degrees C) design conditions. Stirred SVIs averaged 67 and 69 milliliter per gram (mL/g), respectively. Phosphorus-removal organisms (poly-P) washout occurred at aerobic SRTs of 1.9 days. (12 to 14 degrees C) and 1.3 to 1.4 days (20 to 22 degrees C). The washout rate of poly-P organisms and nitrifiers was nearly equal at 20 to 22 degrees C. Storm flows and/or prolonged aeration of the anaerobic zone did not hinder the reestablishment or effectiveness of the poly-P biomass. Off-gas testing suggested the selector increased alpha-factors 15 percent above those resulting from conventional treatment. Enlargement III design will increase EDRP capacity by 50 percent, with a 33 percent increase in aeration tank volume and a 25 percent increase in secondary clarifier volume.