## Treatment Ponds

The types of treatment ponds (described in Chapter 4) include oxidation ponds, facultative ponds, controlled-discharge ponds, partial-mix aerated ponds, complete-mix ponds, proprietary approaches, and modifications to conventional approaches. The area estimate for pond systems will depend on the effluent quality required (as defined by biochemical oxygen demand [BOD] and total suspended solids [TSS]), on the type of pond system proposed, and on the climate in the particular geographic location. A facultative pond in the southern United States will require less area than the same process in Canada. The equations given below are for total project area and include an allowance for roads, levees, and unusable portions of the site.

### Oxidation Ponds

The area for an aerobic pond assumes a depth of 3 ft (1 m), a warm climate, a 30-day detention time, an organic loading rate of 80 lb/ac-d (90 kg/ha-d), and an effluent quality of 30 mg/L BOD and >30 mg/L TSS. The planning area required is calculated using Equation 2.1:

where

Apm = Total project area, (ac; ha). k = Factor (3.0 x 10-5, U.S. units; 3.2 x 10-3, metric). Q = Design flow (gal/d; m3/d).

### Facultative Ponds in Cold Climates

The area calculation in Equation 2.2 assumes an 80-day detention time, a pond 5 ft (1.5 m) deep, an organic loading of 15 lb/ac-d (16.8 kg/ha-d), an effluent BOD of 30 mg/L, and TSS > 30 mg/L. The area required is:

where

AfC = Facultative pond site area (ac; ha). k = Factor (1.6 x 10-4, U.S. units; 1.68 x 10-2, metric). Q = Design flow (gal/d; m3/d).

### Facultative Ponds in Warm Climates

Assume more than 60 days of detention in a pond 5 ft (1.5 m) deep and an organic loading of 50 lb/ac-d (56 kg/ha-d); the expected effluent quality is BOD = 30 mg/L and TSS > 30 mg/L. The area required is:

where

Aw = Facultative pond site area, warm climate (ac; ha). k = Factor (4.8 x 10-5, U.S. units; 5.0 x 10-3, metric). Q = Design flow (gal/d; m3/d).

### Controlled-Discharge Ponds

Controlled-discharge ponds are used in northern climates to avoid winter discharges and in warm climates to match effluent quality to acceptable stream flow conditions. The typical depth is 5 ft (1.5 m), maximum detention time is 180 days, and the expected effluent quality is BOD < 30 mg/L and TSS < 30 mg/L. The required site area is:

where

Acd = Controlled-discharge pond site area (ac; ha). k = Factor (1.32 x 10-4, U.S. units; 1.63 x 10-2, metric). Q = Design flow (gal/d; m3/d).

### Partial-Mix Aerated Pond

The size of the partial-mix aerated pond site will vary with the climate; for example, shorter detention times are used in warm climates. For the purpose of this chapter, assume a 50-day detention time, a depth of 8 ft (2.5 m), and an organic loading of 89 lb/ac-d (100 kg/ha-d). Expected effluent quality is BOD = 30 mg/L and TSS > 30 mg/L. The site area can be calculated using Equation 2.5:

where

Apm = Aerated pond site area (ac; ha). k = Factor (2.7 x 10-3, U.S. units; 2.9 x 10-3, metric). Q = Design flow (gal/d; m3/d).

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