Page 156 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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Chapter 4

Quantities of Water

and Wastewater Flows

Knowledge of the required quantities of water and wastewater flows is fundamental to
systems design and management. In the United States the volumes of water supplied to
cities and towns or removed from them are expressed in U.S. gallons per year (gal/yr),
month (gal/mo), day (gpd), or minute (gpm). The U.S. gallon occupies a volume of
3
0.1337 cubic feet (ft ) or 3.785 liters (L) and weighs 8.344 pounds (lb) or 3.785 kilograms
3
(kg). The fundamental metric (SI) unit in engineering work is the cubic meter (m ), weigh-
ing l,000 kilograms (kg) or 1 metric tonne (T) and equaling 1,000 liters (L). In the United
States, annual water or wastewater volumes are conveniently recorded in million gallons
(MG) or million liters (ML). Daily volumes are generally expressed in million gallons per
day (MGD) or million liters per day (MLD) if more than 100,000 gpd (378,500 L/d). The
gallons per capita daily volumes are stated in gpcd while the liters per capita daily volumes
are stated in Lpcd. Connected or tributary populations and numbers of services or dwelling
units may take the place of total populations.
Per capita and related figures generalize the experience. They permit comparison of
the experience and practices of different communities and are helpful in estimating future
requirements of specific communities. Fluctuations in flow are usefully expressed as ratios
of maximum or minimum annual, seasonal, monthly, weekly, daily, hourly, and peak rates
of flow to corresponding average rates of flow.
Most water and wastewater systems include massive structures (dams, reservoirs,
and treatment works) that have long construction timelines and are not readily
expanded; they also include pipes and other conduits sunk into city streets, which
disrupts traffic while they are being laid. Accordingly, the principal system components
are purposely made large enough to satisfy community needs for a reasonable number
of years. For this reason, selecting the initial or design capacity is not simple. It calls
for skill in interpreting social and economic trends and sound judgment in analyzing
past experience and predicting future requirements. Among needed estimates are the
following:
1. The number of years, or design period, for which the proposed system and its com-
ponent structures and equipment are to be adequate
2. The number of people, or design population, to be served
3. The rates of water use and wastewater release, or design flows, in terms of per
capita water consumption and wastewater discharge as well as industrial and
commercial requirements

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