The Ann Arbor Waste Water Treatment Plant was originally designed as a conventional activated sludge plant. In 1992 it was modified to operate as an A/O plant. Initial operations were difficult and the required phosphorus removal could be obtained only through heavy additions of ferric chloride. Our study focused on the application point of the ferric chloride, and reexamined the major operating parameters of the plant. Paying close attention to parameters such as sludge blanket levels and oxygen concentrations in the aeration tanks lead to an improved phosphorus effluent level of 0.5 mg/l on a continuous basis. Close attention to the oxygen levels in the anaerobic sections also resulted in a better operation. The addition of ferric chloride has been greatly reduced, and thus has lowered the operating costs and the sludge disposal costs.

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