Wastewater management

Definition: Treatment and discharge of water supplies that have been already used for consumption, bathing, washing, industrial processes, and irrigation

Not enough fresh water is available on the planet to meet human water needs without the reuse of wastewater. Because human uses of water almost always result in its contamination, managing the treatment and discharge of wastewater is crucial for protecting this vital resource.

Water is arguably the single most important resource on the planet, and one for which there is no alternative. All life depends on water. Drinking water is a basic human necessity, yet in 2022, more than two billion people did not have access to safe drinking water. Humans use about half the freshwater that is available for agricultural (about 70 percent), industrial (about 20 percent), and municipal (about 10 percent) purposes. Because these uses add contaminants such as nutrients, chemicals, and pathogens to water, managing the collection, use, treatment, and distribution of wastewater is of utmost importance. Additionally, healthy ecosystems depend on adequate supplies of clean water.

Wastewater management involves the rules, best practices, and technology of collecting, treating, and distributing wastewater. Wastewater is a major source of nutrient pollution, which leads to the eutrophication of aquatic ecosystems, harmful algal blooms, fish kills, and dead zones. Contaminants such as pesticides and hormones can interfere with the endocrine systems of animals, resulting in reproductive, developmental, and immunological problems observed in fish and amphibians. Some health experts are concerned that these contaminants are affecting humans, too.

Municipal Wastewater

Municipal wastewater contains a mixture of contaminants added to the water by every residence and business in the city. Common contaminants in municipal wastewater include detergents, urine, feces, pharmaceuticals, pesticides, food waste, fats, and chemicals from several kinds of personal care products. Greywater—wastewater from showers or laundry that does not contain human waste—is often combined with black water (water that comes from toilets containing urine and feces) and sent to a common sewage system or septic tank. In the twenty-first century, the reuse of greywater for applications that do not require potable water, such as irrigation and toilet flushing, became popular.

In settlements of low population density, domestic wastewater can be treated with septic systems which accounted for 20 percent of water use in American homes in 2023. Household wastewater is collected in a tank, where anaerobicbacteria digest solid waste and the liquid component is released into a leach field, where water percolates into the soil or is transpired by plants. Water released from septic systems contains high concentrations of nitrogen and phosphorus, which contribute to eutrophication of aquatic ecosystems. Septic tanks must be periodically pumped to remove solids that are not decomposed by bacterial digestion.

In higher-density communities, domestic wastewater is collected in networks of pipes and transported to central wastewater treatment plants. Some collection systems combine stormwater with sewage for treatment. (Stormwater by itself is not considered wastewater, because it has not been used by people.) Municipal sewage comprises a complex mixture of contaminants, but the main contaminants that wastewater treatment plants remove include solids, wastes that impose a high biochemical oxygen demand (BOD), and nitrogenous waste. Municipal wastewater treatment includes physical separation (primary treatment) of large solids, such as rags, condoms, and tampon applicators, using a bar screen; the removal of grit; and gravity separation of smaller solids that sink and fats and oils that float. Biological removal (secondary treatment) of BOD wastes, ammonia, nitrates, and other nutrients takes place in aeration basins, trickle filters, and other kinds of bioreactors.

Although different secondary treatment processes are used, they all engage both aerobicbacteria, to metabolize organic material and carry out nitrification to reduce BOD and ammonia, and anaerobic denitrifying bacteria, to convert nitrates to harmless nitrogen gas and reduce BOD. Protozoans, fungi, and small animals such as rotifers and insects are found in these biological systems, further reducing the amount of waste solids in this bacteria-based food web. Solids and liquids are again separated by gravity in a clarifier. The solids are treated with aerobic or anaerobic digestion, and the resulting biosolid is incinerated, stored in a landfill, or used as a soil amendment. Anaerobic digestion produces methane gas, which can be used for power generation. The effluent is “polished” (a tertiary treatment) through processing that removes additional contaminants. Finally, disinfection is carried out before the treated water is discharged back into the environment. Many communities are increasingly using such “reclaimed water” for irrigation.

Municipal wastewater treatment plants do not remove all contaminants, and many of the chemical contaminants that remain in water discharged from wastewater treatment plants have negative environmental consequences. For example, endocrine-disrupting compounds such as disinfection byproducts, fluorinated compounds, bisphenol A, phthalates, pesticides, and estrogens, found in water create reproductive problems in fishes. Caffeine is another compound which conventional water waste management facilities tend to treat and remove from water at a rate between 64 and 100 percent, making it a commonly used tracer compound for waters affected by human excrement.

Agricultural Wastewater

In the United States, raising animals such as cattle, pigs, chickens, and fish for human consumption creates more than three times as much urine and feces as are produced by humans. This type of agricultural wastewater is often retained in ponds, which allows for some settling of solids and microbial degradation of waste before it is released into the environment. During rainstorms, however, large amounts of this minimally treated wastewater can escape into the larger environment. The solids that are retained are typically composted and used as soil amendments. Although it is possible to treat this wastewater using the processes described above for municipal wastewater, such treatment rarely occurs.

Waste irrigation water is the largest source of wastewater. This wastewater is contaminated by whatever materials are applied to the land, such as fertilizers, insecticides, and herbicides. Irrigation wastewater has been contaminated by the high use of pesticides globally; in 2021, more than 4 metric tons were used worldwide. One of the most effective “best practices” to protect aquatic ecosystems is to maintain a riparian buffer between crop fields and streams. Riparian vegetation slows the flow of runoff, trapping some of the sediment and contaminants in the irrigation wastewater. Nutrient farming is a practice popularized in the early twenty-first century of treating irrigation wastewater using the ecosystem service of nutrient removal. With this practice, coupled nitrification/denitrification is enhanced, removing nitrogen pollution from the water and lessening the problem of eutrophication.

Industrial Wastewater

Industrial processes contaminate water with acids, bases, metals, solvents, oil, grease, organics, and BOD. Urban industries work closely with municipal wastewater treatment plants to devise plans to pretreat wastewater before it is discharged into sewage treatment systems designed to remove wastes, such as metals, that municipal treatment plants do not treat well.

Larger-scale industries, notably the mining industry, produce large amounts of metal-contaminated wastewater that cannot be combined with municipal wastewater. This wastewater is generally held in settling ponds before it is discharged into surface waters. Inadequate treatment can often lead to problems with acid mine drainage and therefore significant environmental impairment. Many Superfund sites (sites of abandoned or uncontrolled hazardous wastes that have been identified by the Environmental Protection Agency and placed on its National Priorities List for eventual cleanup) have had to be established because of inadequate treatment of mining wastes. In February 2023, the EPA's National Priorities List and Superfund Alternative Approach Sites were composed of 1,889 sites across the United States.

Mining operations produce hundreds of thousands of waste in the United States. Metal waste alone was estimated at 1,368.7 million pounds of waste in 2020. Active treatment plants can remove metals from mining wastewater by adjusting the pH level of the waste to around 10, precipitating the metal hydroxides with polymers, separating the precipitates in a clarifier, filtering the water, and adjusting the pH back to neutral (about 7) for discharge into the environment. The metal hydroxides are dewatered and discarded. Passive treatment of mining and other wastewaters can be achieved with constructed wetlands, which carry out the ecosystem services that improve water quality. Passive treatment is cheaper than active treatment, making it more applicable for most contaminated sites in remote locations.

Emerging Technologies

The diversity of materials that contaminate water presents a huge challenge for effective treatment. Removal of endocrine disrupters from municipal wastewater is being achieved using activated carbon, wetlands, reverse osmosis, and even aquaponic systems. Microbial fuel cells are being tested to convert organic waste in water directly to electricity. Some dairy farmers are generating methane from manure and using it to heat buildings or produce electricity. Wastewater effluent flows downgradient to surface waters in nearly all wastewater treatment plants. This hydraulic head can be used to drive turbines for electrical generation. When wastewater is viewed as a resource instead of an environmental hazard, wastewater treatment plants can become power plants.

In 2022, several technological advancements were utilized to improve the efficiency of wastewater management. Thermal hydrolysis simplifies the separation and processing using a three-step process: biogas generation, minimizing waste side production, and water treatment. The internet of things sensors can detect hazardous chemicals, failures in equipment, needed maintenance, and water quality and report these datasets back to the company. Microbial fuel cell technology can be used to treat wastewater, generate clean power, and store energy. These technological advancements aid in managing wastewater, a critical task in the twenty-first century.

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