Secondary Treatment - Wastewater Operators - page 265

7.2.2.7 Organic Loading

Organic loading can be expressed as pounds of BOD 5 per day per
acre (most common), pounds BOD 5 per day per acre-foot, or people per
day per acre.

BOD mg/L) × Influent Flow (MGD) × 8.34

Organic Loading
(lbBOD /day/ac)

5

= (7. 8)

Pond Area (ac)

5

Note: Normal range is from 10 to 50 lb BOD 5 per day per acre.

 Example 7.2

Problem: A wastewater treatment pond has an average width of 380 ft
and an average length of 725 ft. The influent flow rate to the pond is 0.12
MGD with a BOD concentration of 160 mg/L. What is the organic loading
rate to the pond in pounds per day per acre (lb/day/ac)?

Solution:

1ac
43,560 ft

725 ft × 380 ft × = 6.32 ac

2

0.12 MG DD160 mg/L × × 8.34 lb/gal = 160.1lb/day

160.1l b/day

6.32 ac

= 25.3lb/day/ac

7.3 TriCKling filTers

Trickling filters have been used to treat wastewater since the
1890s. It was found that when settled wastewater passed over rock sur-
faces slime grew on the rocks and the water became cleaner. Today, we
still use this principle, but in many installations we use plastic media
instead of rocks. In most wastewater treatment systems, the trickling
filter follows primary treatment and includes a secondary settling tank
or clarifier, as shown in Figure 7.2. Trickling filters are widely used
for the treatment of domestic and industrial wastes. The process is a
fixed-film biological treatment method designed to remove BOD 5 and
suspended solids.

A trickling filter consists of a rotating distribution arm that sprays
and evenly distributes liquid wastewater over a circular bed of fist-sized
rocks, other coarse materials, or synthetic media (see Figure 7.3). The
spaces between the media allow air to circulate easily so aerobic condi-
tions can be maintained. The spaces also allow wastewater to trickle
down through, around, and over the media. A layer of biological slime
that absorbs and consumes the wastes trickling through the bed covers