Challenge

The Raleigh, NC treatment plant is a conventional activated sludge plant with both diffused air and jet aeration. Although their clarifiers were operating very well, the plant wanted to determine the performance characteristics of one of the large-diameter Rim-Flo® clarifiers under normal and high flow conditions, with particular emphasis on the overall hydraulic characteristics.

Solution

During the tests:

• The activated sludge basins were operated in their normal service mode.
• The sludge blankets were cross-sectioned at four locations along the bridge, using a Royce® solids meter.
• The flow curve/detention time dye tests were conducted at both normal and high flow conditions.
• FSS and DSS tests were conducted.

Using three core samplers at multiple locations along the walkway, the flow current patterns were defined along this axis.

Results

• Flow curve characteristics, under normal flow conditions, were better than the flow curve characteristics in similarly-sized center-feed clarifiers; overall hydraulic efficiency of 65%.
• Morril Index of 4.9 indicates that there is less dispersion in this clarifier than in most.
• Flow curve, under high flow conditions, compared favorably with respect to the overall hydraulic characteristics; hydraulic efficiency of 66%.
• The clarifier’s capacity was at least two times more than the design’s average limits. It was possible to maintain, over eight hours, an overflow rate of up to 1,300 GPD/sq. ft. with a solids loading of up to 50 lbs/sq. ft. per day.
• Effluent suspended solids levels can be maintained below 20 ppm at hydraulic loading up to 1,300 GDP/sq. ft./day.
• Effluent SS levels below 15 ppm were achieved at levels of 800 GPD/sq. ft.