An activated-sludge treatment plant without primary sedimentation has two aeration basins and two clarifiers. The waste-activated sludge is stabilized and thickened in aerobic digesters, dewatered by belt filter presses, and hauled to land disposal. Each aeration basin is prismoidal in shape and the length is twice the width, with two pedestal-mounted mechanical aerators. Since the plant is currently over design load, a proposed expansion is to add another identical aeration basin and clarifier. The original design loadings were 10,000m3/d with 300g/m3 •d of BOD (200 mg/l); the current maximum-month loading is 13,000m3/d with a BOD concentration of 260 mg/l. Although the BOD and suspended solids in the effluent are below the specified maximums of 20 mg/l and 20 mg/l, the state regulatory agency has established a new effluent quality standard of a maximum of 5 mg/l of ammonia nitrogen during the maximum month upon completion of the plant expansion. Even though the wastewater temperature is usually 20 C or greater throughout the year, the original designer did not think that extensive nitrification would occur. Nevertheless, partial nitrification is occurring as the total nitrogen (ammonia plus organic nitrogen) in the effluent is reduced to 21 mg/l.
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