https://www3.epa.gov/npdes/pubs/primer.pdfSEWER: Waste Water Treatment – a Stealth Silent Weapon
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Heat reduces the capacity of water to retain oxygen. In some areas, water used for cooling is discharged to streams at elevated temperatures from power plants and industries. Even discharges from wastewater treatment plants and storm water retention ponds affected by summer heat can be released at temperatures above that of the receiving water, and elevate the stream temperature. Unchecked discharges of waste heat can seriously alter the ecology of a lake, a stream, or estuary. |
In nature, bacteria and other small organisms in water consume organic matter in sewage, turning
it into new bacterial cells, carbon dioxide, and other by-products. The bacteria normally present in water must have “oxygen” to do their part in breaking down the sewage. In the 1920s, scientists observed that these natural processes could be contained and accelerated in systems to remove organic material from wastewater. With the addition of oxygen to wastewater, masses of microorganisms grew and rapidly metabolized organic pollutants. Any excess microbiological growth could be removed from the wastewater by physical processes.
Today, more than 16,000 publicly-owned wastewater treatment plants operate in the United States and its territories. The construction of wastewater treatment facilities blossomed in the 1920s and again after the passage of the CWA in 1972 with the availability of grant funding and new requirements calling for minimum levels of treatment. Adequate treatment of wastewater, along with the ability to provide a sufficient supply of clean water, has become a major concern for many communities.
Sewer collection systems serve over half the people in the United States today. EPA estimates that there are approximately 500,000 miles of publicly- owned sanitary sewers with a similar expanse of privately-owned sewer systems. Sewers were designed and built to carry wastewater from domestic, industrial and commercial sources, but not to carry storm water. Nonetheless, storm water enters sewers through cracks, particularly in older lines, and through roof and basement drains. Due to the much smaller volumes of waste water that pass through sewer lines compared to combined sewers, sewer systems use smaller pipes and lower the cost of collecting wastewater.
Asset Management
America’s public water- based infrastructure – its water supply, wastewater, and storm water facilities, and collection/distribution systems – is integral to our economic, environmental and cultural vitality.
Much of this country’s public wastewater system infrastructure has crossed the quarter-century mark, dating back to the CWA construction grant funding of the 1970s. Many of our collection systems date from the end of World War II and the population boom of the post war era. The oldest portions of the collection system pipe network exceed 100 years of service. Significant parts of this infrastructure are severely stressed from overuse and the persistent under-funding of repair, rehabilitation, and replacement. In an increasing number of communities, existing systems are deteriorating, yet the demand for new infrastructure to accommodate growth presses unabated. A revitalized approach to managing capital wastewater assets for cost effective performance is emerging in this country. This asset management approach focuses on the cost effective sustained performance of the wastewater collection and treatment system assets over their useful life.
A vast array of chemicals are included in this category. Examples include detergents, house- hold cleaning aids, heavy metals, pharmaceuticals, synthetic organic pesticides and her- bicides, industrial chemicals, and the wastes from their manufacture. Many of these sub- stances are toxic to fish and aquatic life and many are harmful to humans. Some are known to be highly poisonous at very low concentrations. Others can cause taste and odor prob- lems, and many are not effectively removed by conventional wastewater treatment.
Carbon, nitrogen, and phosphorus are essential to living organisms and are the chief nutri- ents present in natural water. Large amounts of these nutrients are also present in sewage, certain industrial wastes, and drainage from fertilized land. Conventional secondary bio- logical treatment processes do not remove the phosphorus and nitrogen to any substantial extent — in fact, they may convert the organic forms of these substances into mineral form, making them more usable by plant life. When an excess of these nutrients overstimulates the growth of water plants, the result causes unsightly conditions, interferes with drinking water treatment processes, and causes unpleasant and disagreeable tastes and odors in drinking water. The release of large amounts of nutrients, primarily phosphorus but occasionally nitrogen, causes nutrient enrichment which results in excessive growth of algae. Uncontrolled algae growth blocks out sunlight and chokes aquatic plants and animals by depleting dissolved oxygen in the water at night. The release of nutrients in quantities that exceed the affected waterbody’s ability to assimilate them results in a condition called eutrophication or cultural enrichment.
“the ability to provide a sufficient supply of clean water continues to be a major national concern”
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Insider Comment: This is the reason (SWMP) Storm Water Mitigation Plans SU Sumps are required on properties of new construction. Tanks are buried underground and all surface water from roofs, gutters, driveways, (surface run-off) are collected in the tanks where the grit and gravel settle out before being discharged to the storm system. . .
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Removing the grit and gravel that washes off streets or land during storms is very important, especially in cities with combined sewer systems. Large amounts of grit and sand entering a treatment plant can cause serious operating problems, such as excessive wear of pumps and other equipment, clogging of aeration devices, or taking up capacity in tanks that is needed for treatment. In some plants, another finer screen is placed after the grit chamber to remove any additional material that might damage equipment or interfere with later processes. The grit and screenings removed by these processes must be periodically collected and trucked to a landfill for disposal or are incinerated.
In the waste water treatment process, the bacteria use oxygen from the air and consume most of the organic matter in the wastewater as food. As the wastewater passes down through the media, oxygen-demanding substances are consumed by the biomass and the water leaving the media is much cleaner.
Land treatment is the controlled application of wastewater to the soil where physical, chemical, and biological processes treat the wastewater as it passes across or through the soil. The principal types of land treatment are slow rate, overland flow, and rapid infiltration. In the arid western states, pretreated municipal wastewater has been used for many years to irrigate crops. In more recent years, land treatment has spread to all sections of the country. Land treatment of many types of industrial wastewater is also common.
Extensive research has been conducted at land treatment sites to determine treatment performance and study
the numerous treatment processes involved, as well as potential impacts
on the environment, e.g. groundwater, surface water, and any crop that may be grown.
In the case of slow rate infiltration, the wastewater is applied to the land and moves through the soil
where the natural filtering action of the soil along with microbial activity and plant uptake removes most contaminants. Part of the water evaporates or is used by plants.
Chlorine
Chlorine kills micro- organisms by destroying cellular material. any free (uncombined) chlorine remaining in the water, even at low concentrations, is highly toxic to beneficial aquatic life. Therefore, removal of even trace amounts of free chlorine by dechlorination is often needed to protect fish and aquatic life. Due to emergency response and potential safety concerns,
Ozone
Ozone is produced from oxygen exposed to a high voltage current. Ozone is very effective at destroying viruses and bacteria and decomposes back to oxygen rapidly without leaving harmful by products. Ozone is not very economical due to high energy costs.
Pretreatment
The National Pretreatment Program, a cooperative effort of Federal, state, POTWs and their industrial dischargers, requires industry to control the amount of pollutants discharged into municipal sewer systems. Pretreatment protects the wastewater treatment facilities and its workers from pollutants that may create hazards or interfere with the operation and performance of the POTW, including contamination of sewage sludge, and reduces the likelihood that untreated pollutants are introduced into the receiving waters.
Advanced treatment technologies can be extensions of conventional secondary biological treatment to further
stabilize oxygen-demanding substances in the wastewater, or to remove nitrogen and phosphorus. Advanced treatment may also involve physical-chemical separation techniques such as adsorption, flocculation/precipitation, membranes for advanced filtration, ion exchange, and reverse osmosis. In various combinations, these processes can achieve any degree of pollution control desired. As wastewater is purified to higher and higher degrees by such advanced treatment processes, the treated effluents can be reused for urban, landscape, and agricultural irrigation, industrial cooling and processing, recreational uses and water recharge, and even indirect augmentation of drinking water supplies.
Nitrogen Control
Nitrogen in one form or another is present in municipal wastewater and is usually not removed by secondary treatment. If discharged into lakes and streams or estuary waters, nitrogen in the form of ammonia can exert a direct demand on oxygenor stimulate the excessive growth of algae. Ammonia in wastewater effluent can be toxic to aquatic life in certain instances.
By providing additional biological treatment beyond the secondary stage, nitrifying bacteria present in wastewater treatment can biologically convert ammonia to the non-toxic nitrate through a process known as nitrification. The nitrification process is normally sufficient to remove the toxicity associated with ammonia in the effluent. Since nitrate is also a nutrient, excess amounts can contribute to the uncontrolled growth of algae.
Like nitrogen, phosphorus is also a necessary nutrient for the growth of algae. Phosphorus reduction is often needed to prevent excessive algal growth before discharging the treated sewer water, effluent, into lakes, reservoirs and estuaries.
Advanced Methods of Wastewater Treatment
As our country and the demand for clean water have grown, it has become more important to produce cleaner wastewater effluents, yet some contaminants are more difficult to remove than others. The demand for cleaner discharges has been met through better and more complete methods of removing pollutants at wastewater treatment plants, in addition to pretreatment and pollution prevention which helps limit types of wastes discharged to the sanitary sewer system.
Effluent
with nitrogen in the form of nitrate is placed into a tank devoid of oxygen, where carbon-containing chemicals, such as methanol, are added or a small stream of raw wastewater is mixed in with the nitrified effluent. In this oxygen free environment, bacteria use the oxygen attached to the nitrogen in the nitrate form releasing nitrogen gas. Because nitrogen comprises almost 80 percent of the air in the earth’s atmosphere, the release of nitrogen into the atmosphere does not cause any environmental harm.
Biosolids are processed wastewater solids (“sewage sludge”) that meet rigorous standards allowing safe reuse for beneficial purposes. Currently, more than halfof the biosolids produced by municipal wastewater treatment systems is applied to land as a soil conditioner or fertilizer and the remaining solids are incinerated or landfilled.
Prior to utilization or disposal, biosolids are stabilized to control odors and reduce the number of disease-causing organisms. Sewage solids, or sludge, when separated from the wastewater, still contain around 98 percent water. They are usually thickened and may be dewatered to reduce the volume to be transported for final processing, disposal, or beneficial use.
