Find Mass Balance of Your Settling Tanks

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. This important concept allows us to track materials (that is, pollutants, microorganisms, chemicals, and other materials) from one place to another.

The concept of mass balance plays an important role in treatment plant operations (especially wastewater treatment), where we assume a balance exists between the material entering and leaving the treatment plant or a treatment process: “What comes in must equal what goes out.”

This concept is very helpful in evaluating biological systems, sampling and testing procedures, and many other unit processes within the treatment system.

See the below illustrate how the mass balance concept is used to determine the quantity of solids entering and leaving settling tanks and mass balance using Biological Oxygen Demand (BOD) removal.

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

Key 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 or 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 formatted for actual use, consider Figure below figure 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)


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 10min/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.


• Step 1: Solids in = 252 mg/L x 4.20 MGD x 8.34 = 8827 lb/d
• Step 2: Solids out = 140 mg/L x 4.20 MGD x 8.34 = 4904 lb/d
• Step 3: Biosolids solids = 10 min/hr x 24 hr/day x 40 gpm x 8.34 x 0.042 = 3363 lb/d
• Step 4: Balance = 8827 lb/day – (4904 lb/day + 3363 lb/day) = 560 lb, or 6.3%

Next >> Mass Balance Using BOD Removal

Article by Sivanandan


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