Activated Sludge Process - Wastewater Operators - page 332

Effluent TSS = 11 mg/L
Effluent flow = 5.0 MGD
Waste concentrations = 6000 mg/L

Solution:

⎡
⎣ ⎢

⎤
⎦ ⎥

2500 mg/L × (1.20 + 0.20) × 8.34

Waste (lb/day) =

8. 8 days

− ( 11 mg/L × 5.0 MGD × 8.34)

= 3317 l b/day - 459 lb/day = 2858 lb/day

2858 lb/day
6000 mg/L

Waste (MGD) = = 0.057 MGD

× 8.34

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

×

Waste ( gpm) = = 40 gpm

ay

8.14.7 Mass balance

mass balance is based on the fact that solids and BOD are not lost
in the treatment system. In simple terms, the mass balance concept
states that “what comes in must equal waste that goes out.” The con-
cept can be used to verify operational control levels and to determine if
potential problems exist within the plant's process control monitoring
program.

Note: If influent values and effluent values do not correlate within 10 to
15%, it usually indicates either a sampling or testing error or a process
control discrepancy.

Mass balance procedures for evaluating the operation of a settling
tank and a biological process are described in this section. Operators
should recognize that, although the procedures are discussed in refer-
ence to the activated sludge process, the concepts can be applied to any
settling or biological process.

8.14.7.1 Mass Balance for Settling Tank Suspended Solids

The settling tank mass balance calculation assumes that no sus-
pended solids are produced in the settling tank. Any settling tank oper-
ation can be evaluated by comparing the solids entering the unit with
the solids leaving the tank as effluent suspended solids or as sludge
solids. If sampling and testing are accurate and representative, and pro-
cess control and operations are appropriate, the quantity of suspended
solids entering the settling tank should equal (±10%) the quantity of
suspended solids leaving the settling tanks as sludge, scum, and efflu-
ent total suspended solids.