The historical mixed liquor settling performance of the City of Portland, Oregon’s Columbia Boulevard Wastewater Treatment Plant (CBWTP) has been erratic at times with sludge volume index (SVI) values periodically greater than 400 mL/g. Tests were conducted at full-scale during winter operating conditions to test the feasibility of improving biological selector performance to lower SVI below historical records and postpone the need for additional secondary clarifiers. One of eight parallel activated sludge treatment trains (the pilot) was isolated from the remaining seven trains (the control) to measure the effect on SVI of operating the pilot in a different manner from the control and to identify design features that would ensure operation at a low SVI.