Mass Balance and Measuring Plant Performance - Basic Mathematics for Water and Wastewater Operators - page 187

13 Mass Balance and

Measuring Plant
Performance

The simplest way to express the fundamental engineering principle of mass balance
is to say, “Everything has to go somewhere.” More precisely, the law of conservation
of mass says that when chemical reactions take place, matter is neither created nor
destroyed. What this important concept allows us to do is track materials (e.g., pol-
lutants, microorganisms, chemicals) from one place to another. The concept of mass
balance plays an important role in treatment plant operations (especially wastewater
treatment) where we assume that a balance exists between the material entering and
leaving the treatment plant or a treatment process: “What comes in must equal what
goes out.” The concept is very helpful in evaluating biological systems, sampling
and testing procedures, and many other unit processes within the treatment system.
In the following sections, we illustrate how the mass balance concept is used to
determine the quantity of solids entering and leaving settling tanks and mass balance
using biochemical oxygen demand (BOD) removal.

MASS BALANCE FOR SETTLING TANKS
The mass balance for the settling tank calculates the quantity of solids entering and
leaving the unit.
Note: The two numbers—in (influent) and out (effluent)—must be within 10 to
15% of each other to be considered acceptable. Larger discrepancies may indicate
sampling errors, increasing solids levels in the unit, or undetected solids discharge
in the tank effluent.

To get a better feel for how the mass balance for settling tanks procedure is format-
ted for actual use, refer to Figure 13.1 and the steps provided below, which are used
in Example 13.1 to demonstrate how mass balance is actually used in wastewater
operations.

Step 1. Solids in = Pounds of influent suspended solids
Step 2. Solids out = Pounds of effluent suspended solids
Step 3. Biosolids out = Pounds of biosolids pumped per day
Step 4. Balance = Solids in - (Solids out + Biosolids pumped)

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