Example 352

Problem: If a tank is 20 feet in diameter and 25 feet deep, how many gallons of water will it hold?

Figure 3.8 Circular or cylindrical water tank.

Hint: In this type of problem, calculate the surface area first, multiply by the height, and then convert to gallons.


r = D - 2 = 20 ft - 2 = 10 ft A = n x r2 = n x 10 ft x 10 ft = 314 ft2 V = A x H = 314 ft2 x 25 ft = 7850 ft3 x 7.5 gal/ft3 = 58,875 gal


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 (e.g., pollutants, 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 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, as well as BOD removal.

Guide to Alternative Fuels

Guide to Alternative Fuels

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