Wastewater Calculations - Mathematics and Statistics for the Environment - page 679

293.6 lb BOD/day
25,434 ft 3

× 1000
1000

Regrouping the numbers and the units together,

293.6 lb BOD/day × 1000 lb BOD/day

1

lb BOD/day
1000 ft

× =× 11 .

3 3 3

25,434 ft 000 ft

24.5.1.3 BOD and SS Removed
To calculate the pounds of BOD or suspended solids removed each day, we need to know the mg/L
BOD or SS removed and the plant flow.

■ EXAMPLE 24.26
Problem: If 120 mg/L suspended solids are removed by a trickling filter, how many pounds per day
suspended solids are removed when the flow is 4.0 MGD?

Solution:

SS (mg/L) × Flow (MGD) × 8.34 lb/gal = 120 mg/L × 4.0 MGD × 8.34 lb/gal = 4003 lb/day

■ EXAMPLE 24.27
Problem: The 3,500,000-gpd influent flow to a trickling filter has a BOD content of 185 mg/L. If the
trickling filter effluent has a BOD content of 66 mg/L, how many pounds of BOD are removed daily?

Solution:

BOD removed = 185 mg/L - 66 mg/L = 119 mg/L

BOD removed (lb/day) = BOD removed (mg/L) × Flow (MGD) × 8.34 lb/gal

BOD removed = 119 mg/L × 3.5 MGD × 8.34 lb/gal = 3474 lb/day

24.5.1.4 Recirculation Flow
Recirculation in trickling filters involves the return of filter effluent back to the head of the trickling
filter. It can level flow variations and assist in solving operational problems such as ponding, filter
flies, and odors. The operator must check the rate of recirculation to ensure that it is within design
specifications. Rates above design specifications indicate hydraulic overloading; rates under design
specifications indicate hydraulic underloading. The trickling filter recirculation ratio is the ratio of
the recirculated trickling filter flow to the primary effluent flow. The trickling filter recirculation
ratio may range from 0.5:1 (.5) to 5:1 (5); however, the ratio is often found to be 1:1 or 2:1:

Recirculated flow (MGD)

Recirculationratio = (24.20)

Primary effluent flow (MGD)

■ EXAMPLE 24.28
Problem: A treatment plant receives a flow of 3.2 MGD. If the trickling filter effluent is recirculated
at the rate of 4.50 MGD, what is the recirculation ratio?

Solution:

Recirculated flow (MGD) 4.5 MGD

3.2 MGD

Recirculationratio = = =1.4

Primary effluent flow (MGD)