Wastewater Calculations - Mathematics and Statistics for the Environment - page 693

BVI Expected Conditions

Less than 100 Old biosolids; possible pin floc

Effluent turbidity increasing

100-250 Normal operation; good settling

Low effluent turbidity

Greater than 250 Bulking biosolids; poor settling

High effluent turbidity

24.7.1.9 Mass Balance: Settling Tank Suspended Solids
Solids are produced whenever biological processes are used to remove organic matter from waste-
water. Mass balance for anaerobic biological process must take into account both the solids removed
by physical settling processes and the solids produced by biological conversion of soluble organic
matter to insoluble suspended matter organisms. Research has shown that the amount of solids
produced per pound of BOD removed can be predicted based on the type of process being used.
Although the exact amount of solids produced can vary from plant to plant, research has developed
a series of K factors that can be used to estimate the solids production for plants using a particular
treatment process. These average factors provide a simple method to evaluate the effectiveness of
a facility's process control program. The mass balance also provides an excellent mechanism to
evaluate the validity of process control and effluent monitoring data generated.

24.7.1.9.1 Mass Balance Calculation

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

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

Solids produced (lb/day) = [BOD in (lb) - BOD out (lb)] × K

TSS out (lb/day) = TSS out (mg/L) × Flow (MGD) × 8.34 lb/gal (24.48)

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

Solids removed (lb/day) = TSS out (lb/day) + Waste (lb/day)

( Solids produced − solids remo ved ) × 100

%Massbalance =

Solidsproduced

24.7.1.10 Biosolids Waste Based on Mass Balance

Waste rate (MGD) = Solids produced (lb/day)/(waste concentration × 8.34 lb/gal) (24.49)

■ EXAMPLE 24.53
Problem: Given the following data, determine the mass balance of the biological process and the
appropriate waste rate to maintain current operating conditions.

Extended aeration (no primary)

Influent flow 1.1 MGD Effluent flow 1.5 MGD Waste flow 24,000 gpd

BOD 220 mg/L BOD 18 mg/L TSS 8710 mg/L

TSS 240 mg/L TSS 22 mg/L