chapter  13
6 Pages

Mass Balance and Measuring Plant Performance

The mass balance for the settling tank calculates the quantity of solids entering and leaving the unit.

To get a better feel for how the mass balance for settling tanks procedure is formatted for actual use, consider Figure 13.1 and the accompanying steps provided below. We will use an example computation to demonstrate how mass balance is actually used in wastewater operations:

• Step 1: Solids in = pounds of influent suspended solids • Step 2: Pounds of effluent suspended solids • Step 3: Biosolids solids out = pounds of biosolids solids pumped per day • Step 4: Solids in — (solids out + biosolids solids pumped)

Example 13.1


A settling tank receives a daily flow of 4.20 MGD. The influent contains 252 mg/L suspended solids, and the unit effluent contains 140 mg/L suspended solids. The biosolids pump operates 10

min/h and removes biosolids at the rate of 40 gpm. The biosolids content is 4.2% solids. Determine if the mass balance for solids removal is within the acceptable 10 to 15% range.