## Imhoff Tank Design

2. Sludge storage at 5 ft3 per capita = 5 x 150 = 750 ft3.

3. Sludge drying beds at 1.25 ft2 per capita = 150 x 1.25 = 188ft2.

4. Trickling filter loading at 4000lb of BOD/acre-ft = 0.25lb/yd3. Loading based on 0.17 lb of BOD/capita with 35 percent removal in primary settling = 150 x 0.17 x 0.65 = 16.6 lb/day.

400 16.6 3

H 43,560 x

Hence, the required filter diameter,

FIGURE 3.14 Section through Imhoff tank, with design details.

7T X 20 X 20 = __X— = 2 080,000 gpd / acre on a 4 43,500

6-ft deep filter.

15,000 3

5. Final settling provides 2-hour detention. ——— x 2 = 1250 gal =167 ft

With a surface

2 180 x tank depth settling rate = 500 gpd / ft2 =-—; tank depth = 5.6 ft.

### 2-hour detention

6. If the BOD in the raw sewage is 200mg/l, and the Imhoff tank removes 35 percent, the applied BOD = 0.65 x 200 = 130mg/l. According to the National Research Council Sanitary Engineering Committee formulas,* a filter loaded at 400 lb of BOD per acre-ft will produce an average settled effluent containing 14 percent of that applied, or 0.14 x 130 = 18mg/l.

High-Rate Trickling Filter Plant with Imhoff Tank

1. Flowing through channel same as with standard rate filter = 209 ft3.

2. Sludge storage at 8ft3/capita = 8 x 150 = 1,200ft3.

3. Sludge drying beds at 1.50ft2/capita = 150 x 1.50 = 225 ft2.

4. Trickling filter loading at 3000lb of BOD/acre ft = 1.86lb/yd3. Loading based on 0.17 lb of BOD per capita with 35 percent removal in primary settling = 150 x 0.17 x 0.65 = 16.6 lb/day. The BOD in the raw sewage

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