Wastewater Treatment
Wastewater is water that has been used for various purposes around a community, including sewage, stormwater, and all other water used by residences, businesses, and industry. Wastewater requires treatment before it returns to lakes, rivers, and streams to protect the health of the waterbody and community.
Early in the nation’s history, people living in cities and the countryside used cesspools and privies to dispose of municipal wastewater. Cities began to install wastewater collection systems in the late nineteenth century because of increasing awareness of waterborne disease and the popularity of indoor plumbing and flush toilets.
“water issues are the most immediate environmental concern facing the world right now. This translates to three main areas: improving access to safe drinking water, preventing marine litter, and strengthening water infrastructure. The U.S. will continue to take a leadership role in this area. Right now, over 2 billion people around the world lack access to safe drinking water and, as a result, lack access to proper sanitation. This lack of access to safe drinking water is the cause of up to three million deaths every year. According to the United Nations, nearly a thousand children die each day due to preventable water and sanitation-related diseases.” said former EPA Administrator Andrew Wheeler.
The primary purpose of wastewater treatment is to protect the health and well-being of the community by removing pollutants from the wastewater before it is released to a local body of water. The extent or completeness of treatment necessary to accomplish this purpose is governed by legislation and will vary.
The use of wastewater treatment and collection systems has brought dramatic improvements in public health. Today, approximately 16,000 municipal wastewater treatment facilities in the United States serve over 75 percent of the population and process approximately 34 billion gallons of wastewater every day.
Municipal wastewater is treated using a centralized or public wastewater system. This type of system treats wastewater in a single, centralized location. The wastewater from homes, businesses, and industries travels underground using a collection system of pipes and is delivered to a treatment plant for processing. After the wastewater is treated to set standards, it is reused or discharged to a local body of water.
Wastewater treatment plants handling less than 1.0 million gallons per day (mgd) are generally considered small treatment systems. The principles of design are usually similar as those in found larger systems. Smaller treatment plants inherently have less operational flexibility; however, they are capable of performing effectively and efficiently.
Wastewater treatment processes have been classified as preliminary, primary, secondary, and advanced.
Preliminary Treatment
Preliminary treatment of wastewater includes screening, grinding, grit removal, flotation, equilization, and flocculation. Screens, grinders and grit removal are provided for the protection of other equipment in the treatment plant. Air flotation and flocculation aid in the removal of suspended solids and oil in the primary clarifier and reduce the biological loading on secondary treatment processes. Prechlorination or pre-aeration may be required to prevent odor problems and to eliminate septic conditions where wastewater has abnormally long runs to the plant. Equalization structures are used to dampen diurnal flow variations and to equalize flows to treatment facilities.
Bar Screens
a. Description and function. The primary function of coarse screening is protection of downstream facilities rather than effective removal of solids from the plant influent. All screens used in sewage treatment plants or in pumping stations may be divided into the following classifications:
(1) Trash racks, which have a clear opening between bars of 1½ to 4 inches and are usually cleaned by hand, by means of a hoist or; possibly, by a power-operated rake.
(2) Standard, mechanically cleaned bar screen with clear openings from ½ to 1½ inches (fig 10-1).
(3) Fine screen with openings ¼ inch wide or smaller.
b. Design basis. Screens will be located where they are readily accessible. An approach velocity of 2 feet per second, based on average flow of wastewater through the open area, is required for manually cleaned bar screens. For a mechanically cleaned screen, the approach velocity will not exceed 3.0 feet per second at maximum flows.
(1) Bar spacing. Clear openings of 1 inch are usually satisfactory for bar spacing, but ½ to 1½-inch openings may be used. The standard practice will be to use 5/16-inch x 2-inch bars up to 6 feet in length and 3/8-inch x 2-inch or 3/8-inch x 2½-inch bars up to 12 feet in length. The bar will be long enough to extend above the maximum sewage level by at least 9 inches.
(2) Size of screen channel. The maximum velocity through the screen bars, based on maximum normal daily flow, will be 2.0 feet per second. For wet weather flows or periods of emergency flow, a maximum velocity of 3.0 feet per second will be allowed. This velocity will be calculated on the basis of the screen being entirely free from debris. To select the proper channel size, knowing the maximum storm flow and the maximum daily normal flow, the procedure is as follows: the sewage flow (million gallons per day) multiplied by the factor 1.547 will give the sewage flow (cubic feet per second). This flow (in cubic feet per second) divided by the efficiency factor obtained from table 10-1 will give the wet area required for the screen channel. The minimum width of the channel should be 2 feet and the maximum width should be 4 feet. As a rule, it is desirable to keep the sewage in the screen channel as shallow as possible in order to keep down the head loss through the plant; therefore, the allowable depth in the channel may be a factor in determining the size of the screen. In any event, from the cross-sectional area in the channel, the width and depth of the channel can be readily obtained by dividing the wet area by the depth or width, whichever is the known quantity.
(3) Velocity check. Although screen channels are usually designed on the basis of maximum normal flow or maximum storm flow, it is important to check the velocities which would be obtained through the screen for minimum or intermediate flows. The screen will be designed so that, at any period of flow, the velocities through the screen do not exceed 3 feet per second under any flow condition.
(4) Channel configuration. Considerable attention should be given to the design of the screen channel to make certain that conditions are as favorable as possible for efficient operation of the bar screen. The channel in front of the screen must be straight for 25 feet. Mechanical screens with bars inclined at an angle of 15 degrees from the vertical will be installed.
(5) Screenings. The graph shown in figure 10-2 will be used to predict the average amount of screenings that will be collected on the bar screen. The information required to make this estimate is flow and bar spacing. Grinding of the screenings (and returning them to the wastewater flow), incineration, and landfilling are satisfactory methods for disposal of the screenings.
figure 10-2
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