EXPLANATION OF THE WASTEWATER TREATMENT PROCESS

The Regional Water Resource Agency (RWRA) is the local provider of wastewater services for Owensboro and Daviess County, Kentucky.

In an effort to help you, as a citizen, understand the history of wastewater, the treatment process involved, and the importance of wastewater treatment in protecting the welfare of citizens and the environment, RWRA is providing this information.

THE WASTEWATER TREATMENT PROCESS
Wastewater treatment processes involve the collection, treatment, and sanitary disposal of liquid and water-carried wastes from households, commercial establishments, and industrial plants.

History
Sanitary sewers have been found in the ruins of the prehistoric cities of Crete and the ancient Assyrian cities. Storm-water sewers built by the Romans are still in service today. Toward the end of the Middle Ages, below-ground privy vaults and cesspools were developed. A few centuries later the construction of storm sewers again developed, mostly in the form of open channels or street gutters, and by the 19th century communities recognized that health could be improved by discharging human waste into storm sewers for rapid removal. Development of municipal water-supply systems and household plumbing brought about the beginning of modern sewer systems. In the early 20th century, a few cities began constructing sewage treatment facilities. The septic tank was introduced to treat domestic sewage from individual households.

During the 1950’s and 1960’s, the United States government provided funds for the construction of municipal waste-treatment plants and water-pollution research. However, expanding industrial and economic growth caused pollution difficulties to increase. In the 1970’s the U.S. government became more active in controlling and treating wastewater.

Transportation of Wastewater
Wastewater is carried to treatment facilities that are classified according to the type of wastewater flowing through them. If the system carries both domestic and storm-water sewage, it is called a combined system. These usually serve the older sections of urban areas, such as those in North Owensboro.  As cities expanded, sanitary sewage was separated from storm sewage by a separate pipe network. Households are usually connected to the sewer mains by clay, cast-iron, or polyvinyl chloride (PVC) pipes. Sewer mains are generally located along the centerline of a street or alley about 1.8 m (about 6 ft.) or more below the surface. Wastewater flows through sewer pipes by gravity rather than by pressure; thus pipes must be sloped adequately so that the solid material does not settle. Urban sewer mains generally discharge into interceptor sewers, which usually discharge into large pump stations. Wastewater can then be pumped directly or indirectly through a series of pump stations to wastewater treatment facilities.

Nature of Wastewater
Wastewater is commonly classified as domestic sewage, industrial waste, infiltration, and storm water drainage. Domestic sewage results from common human activities, such as bathing, bodily elimination, and food preparation. Industrial wastewater depends on the type of industry, the management of its water usage, and the degree of treatment the wastewater receives before it is discharged. Infiltration occurs when sewer lines are placed below the water table or when rainfall percolates down to the depth of the pipe. Storm water drainage results primarily from rainfall.

The composition of wastewater is analyzed using several measurements, the most common of which are the measurements of solids, biochemical oxygen demand (BOD5), and pH. Solid waste includes dissolved and suspended solids. The concentration of organic matter is measured by the BOD5 analysis. The BOD5 is the amount of oxygen used over a five-day period by micro-organisms as they decompose organic matter at a temperature of 20 degrees Celsius. The pH is a measure of the acidity or alkalinity of a wastewater sample. The composition of industrial waste cannot be generalized because its makeup depends on the type of manufacturing process involved. Infiltration and storm-water drainage also vary in composition.

Wastewater Treatment
The processes involved in wastewater treatment plants are usually classified as being part of primary, secondary, and tertiary treatment. Primary treatment involves the removal of solids from wastewater by sedimentation, flotation, chemical coagulation and flocculation, and by digestion. In sedimentation, organic materials are allowed to settle out of liquid before being drawn off for disposal. In flotation, air is forced into the wastewater; the rising air bubbles cause suspended solids to rise to the surface, where they are removed. Coagulation is the precipitating of suspended solids with chemicals, and flocculation causes the suspended solid particles to adhere to each other. Digestion is a microbiological process that converts the organic sludge to methane, carbon dioxide, and an inoffensive humus-like material.

After 40 to 60 percent of the suspended solids are removed in primary treatment, Secondary treatment reduces the organic material that remains in the liquid stream. In secondary treatment, bacteria in the presence of oxygen convert organic matter to stable forms such as carbon dioxide, water, nitrates, and phosphates, as well as other organic materials. Tertiary treatment is generally used to remove phosphorus and dissolved solids. Wastewater is then disinfected through methods such as ozone, ultraviolet light contact, chlorination, or other methods, in order to destroy harmful bacteria and viruses. If chlorination is used, the remaining chlorine must be neutralized before returning the water to the environment.

Ultimately the treated liquid stream is discharged into a receiving stream or lake, although treatment agencies in some areas are reusing treated wastewater for groundwater recharge, irrigation of non-edible crops, industrial processing, and recreation.

Industrial Wastewater
Industrial wastewater, because of its nature, may contain contaminants that would harm the environment, if it is discharged into receiving streams. In an effort to protect the environment, the Environmental Protection Agency (EPA) has developed a national program to monitor industrial wastewater.

Because industrial wastewater can vary so much in composition, EPA has established federal standards regulating pollutants, based on the level of toxicity (harmful or poisonous effect to plants, animals, or the environment). It is the responsibility of wastewater treatment plants to regulate industries and the amounts of the pollutants discharged into the wastewater system. Regulation of these industries is typically accomplished by establishing a “Pretreatment Program”.

When wastewater exceeds established levels of toxicity, it is necessary to reduce these levels of pollutants. Industries that fall into this category must pre-treat their waste, which means that they (the industries) are responsible for finding a way to reduce their pollutant levels before discharging to the wastewater system. The wastewater authorities work with industries to achieve this goal.

Citizen Involvement
You, as a private citizen, can help protect the environment by being careful about the material you might dispose of into the wastewater system. Things such as used motor oil, antifreeze, gasoline (or anything flammable), paint, weed killers, insecticides, or any other household chemicals when dumped into the sewer cannot always be removed by the wastewater treatment process. If these items are “dumped” into the sewer system, they could harm the treatment plant, the sewer system, wastewater personnel, and would probably enter the waterways (rivers, lakes, streams, creeks, etc.) untreated, which would be harmful to the environment.

If you have any additional questions about wastewater, the proper disposal method for any material or would like to schedule a group tour of facilities, please call 270-687-8440 for assistance.


    
(c) 2012 - Regional Water Resource Agency - 1722 Pleasant Valley Road - (270) 687-8440