Activated Sludge Process - Wastewater Operators - page 329

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

 Example 8.8

Problem: Given the following information, determine the required waste
rate in gallons per minute to maintain an F/M ratio of 0.17 lb COD/lb
MLVSS.

Primary effluent COD = 140 mg/L
Primary effluent flow = 2.2 MGD
MLVSS = 3549 mg/L
Aeration tank volume = 0.75 MG
Waste volatile solids concentration = 4440 mg/L

Solution:

Actual MLVSS = 3.549 mg/L × 0.75 MG × 8.34 = 22,199 lb

140 mg/L × ×

7lbCOD/lbMLVSS

2.2MGD 8.34

Required MLVSS = = 15,110 lb

0.1

Waste(lb)22,1 = 99 lb − 15,110 lb = 7089 lb

/day
4440 mg/L

7089 lb

Waste(MGD) = = 0.19 MGD

× 8.34

0.1 9MGD 1,000,000 gpd/MGD

1440 min/day

×

Waste(gpm) = = 132gp m

8.14.6 Mean Cell residence Time (MCrT)

mean cell residence time (MCRT), sometimes referred to as sludge
retention time , is a process control calculation used for activated sludge
systems. The MCRT calculation illustrated in Example 8.9 uses the
entire volume of the activated sludge system (aeration and settling).

⎡ Aeration Volume (MG) ⎤

MLSS (mg/L) × +

⎥ ×

8.34 lb/MG/mg/L

⎢

Clarifier Volume (MG)

MRCT
(days)

⎣ ⎦

=

(8.16)

⎡ ⎣ ⎤ ⎦

WAS (mg/L) × WAS Flow (MGD) × 8.34 lb/MG/mg/ L

⎡ ⎣⎣ ⎤ ⎦

+ TSS out (mg/L) × Flow × 8.34 lb/MG/mg/L

Note: MCRT can be calculated using only the aeration tank solids inven-
tory. When comparing plant operational levels to reference materials,
it is important to determine which calculation the reference manual
uses to obtain its example values. Other methods are available to deter-
mine the clarifier solids concentration; however, the simplest method
assumes that the average suspended solids concentration is equal to
the solids concentration of the aeration tank.