Wastewater Calculations - Mathematics and Statistics for the Environment - page 691

Waste (lb/day)
WASconcentration

Waste (MGD) = (24.43)

(mg/L) × 8.34

Waste (MGD) ×

40 min/day

1,000,000 gpd/MGD

Waste (gpm) = (24.44)

14

■ EXAMPLE 24.50
Problem: Given the following data, determine the required waste rate to maintain a MCRT of 8.8
days:

MLSS 2500 mg/L

Aeration volume 1.20 MG

Clarifier volume 0.20 MG

Effluent TSS 11 mg/L

Effluent flow 5.0 MGD

Waste concentration 6000 mg/L

Solution:

2500 mg/L × (1.20 + 0.20) × 8.34

− ( 11 mg/L × 5.0 MGD × 8.34)

Waste (lb/day) =

8.8 days

= 3317 lb/d ay-459 lb/day

= 2858 lb/day

2858 lb/day
6000 mg/L

Waste (MGD) = = 0.057 MGD

× 8.34 gal/d ay

0.057 MGD 1,000,000 gpd/MGD
1440 min

×

Waste (gpm) = = 40 gpm

/day

24.7.1.7 Estimating Return Rates from SBV 60
Many methods are available for estimation of the proper return biosolids rate. A simple method
described in the Operation of Wastewater Treatment Plants: Field Study Programs , published by
California State University, Sacramento, uses the 60-min percent settled biosolids (sludge) volume.
The %SBV 60 test results can provide an approximation of the appropriate return activated biosolids
rate. This calculation assumes that the SBV 60 results are representative of the actual settling occur-
ring in the clarifier. If this is true, the return rate in percent should be approximately equal to the
SBV 60 . To determine the approximate return rate in million gallons per day (MGD), the influent
flow rate, the current return rate, and the SBV 60 must be known. The results of this calculation can
then be adjusted based upon sampling and visual observations to develop the optimum return bio-
solids rate.

Note: The %SBV 60 must be converted to a decimal percent and total flow rate. (Wastewater flow
and current return rate in million gallons per day must be used.)

[ ] ×

Est. return rate (MGD) = Influentflow(MGD) + Current return flow (MGD)%SBV 60 (24.45)