Wastewater Calculations - Mathematics and Statistics for the Environment - page 688

24.7.1.4.1 Required MLVSS Quantity
The pounds of MLVSS required in the aeration tank to achieve the optimum F/M ratio can be deter-
mined from the average influent food (BOD or COD) and the desired F/M ratio:

Primaryeffluent BOD/COD low(M × GD) × 8.34 lb/gal

MLVSS(lb) = (24.35)

DesiredF/M ratio

The required pounds of MLVSS determined by this calculation can then be converted to a concen-
tration value by

DesiredMLVSS (lb)
Aeration vol

MLVSS(mg/L) = (24.36)

ume(MG) × 8.34 lb/gal

■ EXAMPLE 24.45
Problem: The aeration tank influent flow is 4.0 MGD and the influent COD is 145 mg/L. The
aeration tank volume is 0.65 MG. The desired F/M ratio is 0.3 lb COD per lb MLVSS. How many
pounds of MLVSS must be maintained in the aeration tank to achieve the desired F/M ratio?

Solution: Determine the required concentration of MLVSS in the aeration tank:

145 mg/L × ×

lb CODper lb MLVSS

4.0 MGD 8.34 lb/gal

MLVSS(lb) = = 16,124 lb

0.3

16,124 lb
0.65 MG

MLVSS(mg/L) = = 974 mg/L 2

× 8.34 lb/gal

24.7.1.4.2 Calculating Waste Rates Using F/M Ratio
Maintaining the desired F/M ratio is accomplished by controlling the MLVSS level in the aeration
tank. This may be achieved by adjustment of return rates; however, the most practical method is by
proper control of the waste rate:

Waste volume solids (lb/day) = Actual MLVSS (lb/day) - Desired MLVSS (lb/day) (24.37)

If the desired MLVSS is greater than the actual MLVSS, wasting is stopped until the desired level is
achieved. Practical considerations require that the waste quantity be converted to a required volume
of waste per day. This is accomplished by converting the waste pounds to flow rate in million gal-
lons per day or gallons per minute:

Waste volatile(lb/day)

Waste (MGD) = (24.38)

Waste vo latile concentration(mg/L) × 8.34 lb/gal

Waste (MGD) 1,000,000 gpd/MGD
1440

×

Waste (gpm) = (24.39)

min/day

Key Point: When the F/M ratio is used for process control, the volatile content of the waste activated
sludge should be determined.