Over 5,000 people die worldwide everyday from drinking or bathing in water containing the same contaminants that are removed at wastewater treatment plants (Figure 1). Wastewater effluent is the final product of all earlier treatment processes, and it can be discharged to a stream, river, bay, lagoon or wetland. Sometimes effluent is used to irrigate a golf course, green belt or park, or to recharge groundwater (Davis, 2004).
In 1972, the U.S. federal government passed the Clean Water Act (CWA), which "prohibits the discharge of pollutants except in compliance with the effluent limitations and other provisions of the Act" (Clean Water Act, 05/05/08). The Clean Water Act also required states to adopt standards for water quality, and report to the Environmental Protection Agency (EPA) every two years.
Wastewater Effluent must be in such a state of purity that the receiving waters into which it is released are not adversely affected, and these ecosystems are not harmed. There are many constituents in wastewater that can harm the receiving waters (Table 1) and are described below.
|Pollutant||Typical concentrations before treatment||Typical concentrations in effluent|
|Suspended Solids||125 mg/L||30 mg/L|
|BOD5||200-300 mg/L||30 mg/L|
|Nitrogen & Phosphorous||10-40 mg/L||depends on receiving waters|
|pH||6.5 to 8.5||7.0 to 7.2|
|Pharmaceuticals & Pathogens||Not fully known||No current regulations|
These are solids that stay in suspension instead of settling out of wastewater. These solids give the effluent turbidity. Turbidity is problematic in receiving waters of wastewater effluent. It raises the temperature of the water and blocks the sun from oxygen-producing plants and algae, thus reducing the dissolved oxygen of the receiving water.
BOD stands for biochemical oxygen demand. BOD is an indirect measurement of the organic matter present in the water. The test is done by measuring the amount of dissolved oxygen used by microorganisms like bacteria to consume organic matter in the water. The BOD5 is the biochemical oxygen demand after five days, the standard time frame used to measure the dissolved oxygen in effluent. Proper levels of dissolved oxygen are necessary to keep fish and other aquatic organisms alive and healthy.
Nitrogen is present in wastewater effluent from many sources, including human feces and fertilizer. Nitrogen can be a limiting nutrient to algae growth. Hence, nitrogen must be removed before discharge to prevent algal blooms which are disruptive to the ecological balance in the receiving waters. Some wastewater treatment plants use algae to consume the nitrogen, and then kill the algae before discharge.
Phosphorous can also be a limiting nutrient to algae. The most common process used to remove phosphorus is chemical precipitation. However, this can lead to changes in pH.
pH refers to the acidity of the effluent. Domestic wastewater before treatment typically has a pH of 6.5 to 8.5, but a final effluent of 7.0-7.2.
These wastewater constituents include caffeine, steroids, antibiotics and hormones from urine and other sources. Levels in effluent have only recently been studied for their effect on plants and animals. The most concerning are those meant to "stimulate a physiological response in humans, plants, or animals" (Kolpin, 2002). These can cause reproductive system mutation, increase the number of cases of cancer, or lead to the development of antibiotic resistant strains of bacteria.
The other product of wastewater treatment is sludge. This dense liquid contains all the chemicals and pathogens removed in either primary or secondary treatment, so it must be handled appropriately. Sludge must be disposed in accordance with permit issued by the EPA or approved state program.