Pumping Calculations - Basic Mathematics for Water and Wastewater Operators - page 193

14 Pumping Calculations
Pumping facilities and appurtenances are required wherever gravity cannot be used
to supply water to the distribution system under sufficient pressure to meet all service
demands. For wastewater, pumps are used to lift or elevate the liquid from a lower
elevation to an adequate height at which it can flow by gravity or overcome hydrostatic
head. There are many pumping applications at a wastewater treatment facility. These
applications include pumping of (1) raw or treated wastewater, (2) grit, (3) grease and
floating solids, (4) dilute or well-thickened raw biosolids or digested biosolids (bio-
solids or supernatant return), and (5) chemical solutions. Pumps and lift stations are
used extensively in the collection system. Each of the various pumping applications
is unique and requires specific design and pump selection considerations.
With few exceptions, the pumps used in water and wastewater treatment are the
same. Because the pump is so perfectly suited to the tasks it performs, and because
the principles that make the pump work are physically fundamental, the idea that
any new device would ever replace the pump is difficult to imagine. The pump is the
workhorse of water/wastewater operations. Simply, pumps use energy to keep water
and wastewater moving. To operate a pump efficiently, the operator and/or mainte-
nance operator must be familiar with several basic principles of hydraulics. In addi-
tion, to operate various unit processes, in both water and wastewater operations at
optimum levels, operators should know how to perform basic pumping calculations.
BASIC WATER HYDRAULICS CALCULATIONS
W eight of W ater
Because water must be stored or kept moving in water supplies and wastewater must
be collected, processed, and discharged (outfalled) to its receiving body, we must
consider some basic relationships with regard to the weight of water. One cubic foot
of water weighs 62.4 pounds and contains 7.48 gallons. One cubic inch of water
weighs 0.0362 pounds. Water 1 foot deep will exert a pressure of 0.43 pounds per
square inch (psi) on the bottom area (12 in. × 0.062 lb/in. 3 ). A column of water 2 feet
high exerts 0.86 psi, one 10 feet high exerts 4.3 psi, and one 52 feet high exerts
52 ft × 0.43 psi/ft = 22.36 psi
A column of water 2.31 feet high will exert 1.0 psi. To produce a pressure of 40
psi requires a water column calculated as follows:
40 psi × 2.31 ft/psi = 92.4 ft
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