Wastewater

Wastewater refers to any water that has been used by humans. It includes water that has been contaminated by domestic, industrial, agricultural, and commercial use, and from stormwater runoff and infiltration.¹ After use, water must be treated to reduce pollutants before being released into the environment. Wastewater is collected in sewer systems, transported to treatment facilities, and after processing is either released into bodies of water or reused. The collection and treatment of wastewater is essential to public health and the environment. In the United States, the Environmental Protection Agency (EPA) is responsible for regulating industrial wastewater. It issues industry-specific effluent guidelines, standards for municipal sewage treatment plants and for wastewater that is released to surface water. The Clean Water Act (CWA) sets standards for wastewater treatment and discharge, and all facilities that treat or discharge wastewater must receive permits through the National Pollution Discharge Elimination System (NPDES). These facilities must adhere to discharge limits and specific monitoring and reporting requirements.

Improper wastewater treatment can have long-lasting, irreversible effects on the environment and human health. Wastewater can contain a variety of different substances and organisms, both organic and inorganic. Those substances that can be treated by normal wastewater treatment plants are designated as “conventional” pollutants by the CWA, while others are classified as “toxic pollutants.” All other pollutants are designated “nonconventional.”² Domestic wastewater comes from sinks, showers, bathtubs, toilets, washing machines, and dishwashers. It can be contaminated with substances such as food, oils, soaps, chemicals, and human waste. Storm runoff from roads, parking lots, and rooftops can also pick up harmful substances on its way to the sewers.³ Industrial activity can pose a particular challenge to wastewater treatment plants. For example, mining activities release wastewater, affecting surface and ground water quality and drinking supplies, even after operations shut down. Processes like shale gas extraction produce large volumes of wastewater that often contains unsafe concentrations of dissolved solids, radionuclides, and metals.⁴

Organic, or carbon-based materials come from plants, animals, or synthetic organic compounds. Some of these bacteria and microorganisms are actually helpful in treatment processes, as they can break down organic pollutants. In fact, most treatment systems depend on these kinds of biological processes. But wastewater can also contain dangerous pathogens — viruses, parasites, and bacteria – which may come from domestic water use, hospitals, schools, food processing plants, or farms.  When decaying organic materials accumulate in bodies of water, they use up the dissolved oxygen, causing fish to die and the water quality to degrade. ⁵ Many synthetic organic compounds, like pesticides and other substances developed for industrial use, pose a challenge to wastewater treatment facilities because they are difficult to break down and are highly toxic to humans, fish, and aquatic plants. When they are improperly disposed of, they can enter and cause damage to treatment plants. If discharged into surface waters, they can kill or contaminate fish and other aquatic wildlife.

Inorganic substances can also enter wastewater from industrial and commercial sources, stormwater, and infiltration from cracked pipes. Substances commonly found in wastewater include inorganic minerals, metals, and compounds such as sodium, potassium, calcium, magnesium, cadmium, copper, lead, nickel, and zinc. Because most inorganic substances are difficult to break down, they can accumulate in the environment and in the body. Some are toxic, and can contaminate soil and water, or poison humans and animals. Often extra treatment is needed to remove these substances from water. For example, heavy metals released from industrial sources require more advanced treatment. Chronic ingestion of small amounts of heavy metals and other inorganic substances in drinking water can lead to long-term health effects.⁶ Wastewater also tends to contain high concentrations of nitrogen and phosphorus from human waste, food, and some soaps and detergents.⁷ These can cause eutrophication, or the over-fertilization of surface waters, prompting algae to grow quickly and depleting oxygen in the water, killing fish and other aquatic life.⁸ Nitrogen in drinking water can also be harmful to human health, particularly for infants. Wastewater also may contain gases that produce odors, are toxic, or otherwise affect treatment. Methane gas, for example, can be formed as a byproduct of wastewater treatment, and is toxic and combustible.⁹

Wastewater treatment aims to accelerate natural processes of water purification. The primary treatment removes around 60% of suspended solids and oxygenates the water. Secondary treatment uses biological and physical processes to remove over 90% of suspended solids, using specially designed filter systems.¹⁰ After secondary treatment, wastewater may go through advanced treatment methods to control the concentrations of nutrients, toxic substances, or dissolved materials, and undergo a final disinfection to remove any remaining pathogens.¹¹ The NPDES establishes discharge limits and specific conditions for industrial and commercial wastewater, which may contain high volumes of pollutants that can overload treatment processes in municipal facilities.¹² While wastewater treatment reduces environmental impacts in water systems, the chemicals and biological processes used often produce greenhouse gas emissions.¹³

1. Purdue University
2. Environmental Protection Agency
3. United States Geological Survey
4. Environmental Protection Agency
5. Purdue University
6. Purdue University
7. Environmental Protection Agency
8. United States Geological Survey
9. Purdue University
10. Environmental Protection Agency
11. Purdue University
12. Environmental Protection Agency
13. University of Michigan