The Clarifier Research Technical Committee's draft protocol for evaluating secondary clarifier performance was field applied at the Metro Wastewater Reclamation District's (MWRD's) Central Treatment Plant. Two clarifiers in the MWRD's South Complex were tested. These clarifiers were identical circular units, 42.7 m (140 ft) in diameter, except for sludge removal mechanisms. South Secondary Clarifier No. 7 (SSC7) was equipped with a scraper/plow sludge removal mechanism that moved settled solids to a hopper located at the tank's center. The other unit tested, SSC9, was equipped with a draft tube sludge removal system; the draft tubes removed sludge from the clarifier's bottom at ten locations along the continuously rotating rake arms.