Info

Source: Metcalf and Eddy, Inc., 1991. Note: l = gal X 3.7854.

Source: Metcalf and Eddy, Inc., 1991. Note: l = gal X 3.7854.

tion, and other nondomestic sources such as industrial wastewater discharges. Environmental engineers may need to use population forecasting to project future rates of wastewater generation in the service area of a wastewater treatment plant. Some mathematical or graphical methods used to project population data to a design year include (Qasim 1985):

• arithmetic growth

• geometric growth

• decreasing rate of increase

• mathematical or logistic curve fitting

• graphical comparison with similar cities

• employment forecast

Water usage exhibits daily, weekly, and seasonal patterns; and wastewater flow rates can also exhibit such patterns. Figures 7.1.1 and 7.1.2 show typical hourly, daily,

Time of day

FIG. 7.1.1 Typical pattern of hourly variations in domestic wastewater flow rates. (Reprinted, with permission, from Metcalf and Eddy, Inc., 1991.)

Time of day

FIG. 7.1.1 Typical pattern of hourly variations in domestic wastewater flow rates. (Reprinted, with permission, from Metcalf and Eddy, Inc., 1991.)

Time (day)

FIG. 7.1.2 Typical patterns of daily and weekly variations in domestic wastewater flow rates. (Reprinted, with permission, from Metcalf and Eddy, Inc., 1991.)

Time (day)

FIG. 7.1.2 Typical patterns of daily and weekly variations in domestic wastewater flow rates. (Reprinted, with permission, from Metcalf and Eddy, Inc., 1991.)

and weekly wastewater flow rates, respectively (Metcalf and Eddy, Inc. 1991).

Wide variations of wastewater flow rates can occur within a municipality. For example, minimum to maximum flow rates range from 20 to 400% of the average daily rate for small communities with less than 1000 people, from 50 to 300% for communities with populations between 1000 and 10,000, and up to 200% for communities up to 100,000 in population (Water Pollution Control Federation and American Society of Civil Engineers 1977). Large municipalities have variations from 1.25 to 1.5 average flow. When storm water runoff goes into municipal sewerage systems, the maximum flow rate is often two to four times the average dry-weather flow (Water Pollution Control Federation and American Society of Civil Engineers 1977).

Flow rate information needed in designing a wastewater treatment plant includes (Metcalf and Eddy, Inc. 1991):

AVERAGE DAILY FLOW—The average flow rate occurring over a 24-hr period based on total annual flow rate data. Environmental engineers use average flow rate in evaluating treatment plant capacity and in developing flow rate ratios.

MAXIMUM DAILY FLOW—The maximum flow rate occurring over a 24-hr period based on annual operating data. The maximum daily flowrate is important in the design of facilities involving retention time, such as equalization basins and chlorine-contact tanks.

PEAK HOURLY FLOW—The peak sustained hourly flow rate occurring during a 24-hr period based on annual operating data. Data on peak hourly flows are needed for the design of collection and interceptor sewers, waste-water pumping stations, wastewater flowmeters, grit chambers, sedimentation tanks, chlorine-contact tanks, and conduits or channels in the treatment plant.

MINIMUM DAILY FLOW—The minimum flow rate that occurs over a 24-hr period based on annual operating data. Minimum flow rates are important in sizing conduits where solids deposition might occur at low flow rates.

MINIMUM HOURLY FLOW—The minimum sustained hourly flow rate occurring over a 24-hr period based on annual operating data. Environmental engineers need data

TABLE 7.1.5 PHYSICAL, CHEMICAL, AND BIOLOGICAL WASTEWATER CHARACTERISTICS CONSIDERED FOR DESIGN

Physical Chemical Biological

Solids Organics Plants

Temperature Proteins Animals

Color Carbohydrates Viruses

Odor Lipids

Surfactants Phenols Pesticides Inorganics pH

Chloride Alkalinity Nitrogen Phosphorus Heavy metals Toxic materials Gases Oxygen

Hydrogen sulfide Methane

Source: Water Pollution Control Federation and American Society of Civil Engineers, 1977, Wastewater treatment plant design (Washington, D.C.), 10.

on the minimum hourly flow rate to determine possible process effects and size wastewater flow meters, particularly those that pace chemical-feed systems.

SUSTAINED FLOW—The flow rate value sustained or exceeded for a specified number of consecutive days based on annual operating data. Data on sustained flow rates may be used in sizing equalization basins and other plant hydraulic components.

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