Wastewater (or waste water) is any water that has been negatively impacted by human use. Wastewater is a byproduct of domestic, industrial, commercial or agricultural activities. In other words, wastewater is "used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff or stormwater, and any sewer inflow or sewer infiltration". Wastewater contains a large number of physical, chemical and biological pollutants at varying concentration levels. Wastewater characteristics vary, depending on the source, namely whether it is domestic wastewater from households, municipal wastewater from communities (also called sewage) or industrial wastewater from industrial activities.
Households may produce wastewater by using flush toilets, sinks, dishwashers, washing machines, bath tubs, and showers. Wastewater produced by households in urban communities with sewers is called municipal wastewater or sewage. Households that use dry toilets produce significantly less wastewater than households that use flush toilets.
Wastewater may be conveyed in a sanitary sewer (which conveys only sewage) or a combined sewer (which includes stormwater runoff and may also include industrial wastewater). After treatment at a wastewater treatment plant, the treated wastewater (also called effluent) is discharged to a receiving water body. Treated wastewater can also be reused for various purposes - an approach called "wastewater reuse" or "water reclamation".
In developing countries and in rural areas with low population densities, wastewater is often not collected in centralized sewer systems but rather dealt with by various on-site sanitation systems. These can include septic tanks connected to drain fields, on-site sewage systems (OSS), vermifilter systems and many more.
The overarching term sanitation includes the management of wastewater, human excreta, solid waste and storm water. The term sewerage refers to the physical infrastructure required to transport and treat wastewater.
Sources of wastewater include the following domestic or household activities:
Activities producing industrial wastewater:
Other activities or events:
Wastewater can be diluted or mixed with other types of water by the following mechanisms:
The composition of wastewater varies widely. This is a partial list of pollutants that may be contained in wastewater:
Since all natural waterways contain bacteria and nutrients, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what is measured in the laboratory as the biochemical oxygen demand (BOD). Such chemicals are also liable to be broken down using strong oxidizing agents and these chemical reactions create what is measured in the laboratory as the chemical oxygen demand (COD). Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of oxidizable pollutants.
Any oxidizable material present in an aerobic natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased.
There are numerous processes that can be used to clean up wastewaters depending on the type and extent of contamination. Wastewater can be treated in wastewater treatment plants which include physical, chemical and biological treatment processes. Municipal wastewater is treated in sewage treatment plants (which may also be referred to as wastewater treatment plants). Agricultural wastewater may be treated in agricultural wastewater treatment processes, whereas industrial wastewater is treated in industrial wastewater treatment processes.
One type of aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed of micro-organisms able to absorb and adsorb the organic matter carried in the wastewater. Anaerobic wastewater treatment processes (UASB, EGSB) are also widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. Constructed wetlands are also being used.
In some urban areas, municipal wastewater is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to either of these systems is typically through a manhole.
During high precipitation periods a combined sewer system may experience a combined sewer overflow event, which forces untreated sewage to flow directly to receiving waters. This can pose a serious threat to public health and the surrounding environment.
Sewage may drain directly into major watersheds with minimal or no treatment but this usually has serious impacts on the quality of an environment and on the health of people. Pathogens can cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.
Wastewater may be pumped underground through an injection well.
Treated wastewater can be reused in industry (for example in cooling towers), in artificial recharge of aquifers, in agriculture and in the rehabilitation of natural ecosystems (for example in wetlands). In rarer cases it is also used to augment drinking water supplies. There are several technologies used to treat wastewater for reuse. A combination of these technologies can meet strict treatment standards and make sure that the processed water is hygienically safe, meaning free from bacteria and viruses. The following are some of the typical technologies: Ozonation, ultrafiltration, aerobic treatment (membrane bioreactor), forward osmosis, reverse osmosis, advanced oxidation.
Some water demanding activities do not require high grade water. In this case, wastewater can be reused with little or no treatment. One example of this scenario is in the domestic environment where toilets can be flushed using greywater from baths and showers with little or no treatment.
Irrigation with recycled wastewater can also serve to fertilize plants if it contains nutrients, such as nitrogen, phosphorus and potassium. In developing countries, agriculture is using untreated wastewater for irrigation - often in an unsafe manner. There can be significant health hazards related to using untreated wastewater in agriculture. The World Health Organization developed guidelines for safe use of wastewater in 2006.
As part of the Environmental Protection Act 1994, the Environmental Protection (Water) Policy 2009 is responsible for the water management of Queensland, Australia.
In Nigeria, the Water Resources Act of 1993 is the law responsible for all kinds of water management.
In the Philippines, Republic Act 9275, otherwise known as the Philippine Clean Water Act of 2004, is the governing law on wastewater management. It states that it is the country's policy to protect, preserve and revive the quality of its fresh, brackish and marine waters, for which wastewater management plays a particular role.