Wastewater
Wastewater   is any water that has been adversely affected in quality by anthropogenic influence.  It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations. In the most common usage, it refers to the municipal wastewater that contains a broad spectrum of contaminants resulting from the mixing of wastewaters from different sources.
Sewage is correctly the subset of wastewater that is contaminated with faeces or urine, but is often used to mean any waste water. "Sewage" includes domestic, municipal, or industrial liquid waste products disposed of, usually via a pipe or sewer or similar structure, sometimes in a cesspool emptier.
The physical infrastructure, including pipes, pumps, screens, channels etc. used to convey sewage from its origin to the point of eventual treatment or disposal is termed sewerage.

# Wastewater origin
Wastewater or sewage can come from (text in brackets indicates likely inclusions or contaminants) :-
- Human waste, usually from lavatories: (fces, used toilet paper, wipes, urine, other bodily fluids) also known as black water
- Cesspit leakage
- Septic tank discharge
- Sewage treatment plant discharge
- Washing water (personal, clothes, floors, dishes, etc.) also known as greywater or sullage
- Rainfall collected on roofs, yards, hard-standings, etc. (traces of oils and fuel but generally clean)
- Groundwater infiltrated into sewerage.
- Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.)
- Urban rainfall run-off from roads, car-parks, roofs, side-walks or pavements (contains oils, animal faeces, litter, fuel residues, rubber residues, metals from vehicle exhausts etc)
- Seawater ingress (salt, micro-biota, high volumes)
- Direct ingress of river water (micro-biota, high volumes)
- Direct ingress of man-made liquids (illegal disposal of pesticides, used oils, etc.)
- Highway drainage (oil, de-icing agents, rubber residues)
- Storm drains (almost anything including cars, shopping trolleys, trees, cattle etc.)
- Black water - surface water contaminated by sewage
- Industrial waste:-
- industrial site drainage (silt, sand, alkali, oil, chemical)
Industrial cooling waters (biocides, heat, slimes, silt)
Industrial process waters
Organic - bio-degradable - includes waste from abattoirs and creameries and ice-cream manufacture.
Organic - non bio-degradable or difficult to treat - for example Pharmaceutical or Pesticide manufacturing
Inorganic - for example from the metalworking industry
extreme pH - from acid/alkali manufacturing, metal plating
Toxic - e.g. from metal plating, cyanide production, pesticide manufacturing
Solids and Emulsions - e.g. Paper manufacturing, food stuffs, lubricating and hydraulic oil manufacture
agricultural drainage - direct and diffuse
- Industrial cooling waters (biocides, heat, slimes, silt)
- Industrial process waters
- Organic - bio-degradable - includes waste from abattoirs and creameries and ice-cream manufacture.
- Organic - non bio-degradable or difficult to treat - for example Pharmaceutical or Pesticide manufacturing
- Inorganic - for example from the metalworking industry
- extreme pH - from acid/alkali manufacturing, metal plating
- Toxic - e.g. from metal plating, cyanide production, pesticide manufacturing
- Solids and Emulsions - e.g. Paper manufacturing, food stuffs, lubricating and hydraulic oil manufacture
- agricultural drainage - direct and diffuse

# Wastewater constituents
The composition of wastewater varies widely. This is a partial list of what it may contain:
- Water ( > 95%) which is often added during flushing to carry the waste down a drain
- Pathogens such as bacteria, viruses, prions and  parasitic worms.
- Non-pathogenic bacteria (> 100,000 / ml for sewage)
- Organic particles such as faeces, hairs, food, vomit, paper fibers, plant material, humus, etc.
- Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.
- Inorganic particles such as sand, grit, metal particles, ceramics, etc.
- Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.
- Animals such as protozoa, insects, arthropods, small fish, etc.
- Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead pets, body parts, etc.
- Gases such as hydrogen sulfide, carbon dioxide, methane, etc.
- Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.
- Toxins such as pesticides, poisons, herbicides, etc.

# Wastewater quality indicators
Any oxidizable material present in a 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.  Basically, the reaction for biochemical oxidation may be written as:
Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:
Since all natural waterways contain bacteria and nutrient, 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).
Oxidizable chemicals (such as reducing chemicals) introduced into a natural water will similarly initiate chemical reactions (such as shown above).  Those 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 biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.
The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period.  The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD.  The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.
There are also many different COD tests.  Perhaps, the most common is the 4-hour COD.
It should be emphasized that there is no generalized correlation between the 5-day BOD and the Ultimate BOD.  Likewise, there is no generalized correlation between BOD and COD.  It is possible to develop such correlations for a specific waste contaminant in a specific wastewater stream ... but such correlations cannot be generalized for use with any other waste contaminants or wastewater streams.
The laboratory test procedures for the determining the above oxygen demands are detailed in the following sections of the "Standard Methods For the Examination Of Water and Wastewater" available at www.standardmethods.org:
- 5-day BOD and Ultimate BOD: Sections 5210B and 5210C
- COD: Section 5220

# Sewage disposal
In some urban areas, sewage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to either of these is typically through a manhole. During high precipitation periods a sanitary sewer overflow can occur, causing potential public health and ecological damage.
Sewage may drain directly into major watersheds with minimal or no treatment. When untreated, sewage can have 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.

# Treatment
There are numerous processes that can be used to clean up waste waters depending on the type and extent of contamination.  Most wastewater is treated in industrial-scale wastewater treatment plants (WWTPs) which may include physical, chemical and biological treatment processes. However, the use of septic tanks and other On-Site Sewage Facilities (OSSF) is widespread in rural areas, serving up to one quarter of the homes in the U.S. The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by micro-organisms able to absorb and adsorb the organic matter carried in the wastewater. Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some waste waters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes. After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality. Nitrates can be removed from wastewater by microbial denitrification, for which a small amount of methanol is typically added to provide the bacteria with a source of carbon. Ozone Waste Water Treatment is also growing in popularity, and requires the use of an ozone generator, which decontaminates the water as Ozone bubbles percolate through the tank.
Disposal of wastewaters from an industrial plant is a difficult and costly problem. Most petroleum refineries, chemical and petrochemical plants have onsite facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the local and/or national regulations regarding disposal of wastewaters into community treatment plants or into rivers, lakes or oceans.

# Reuse
Treated wastewater can be reused as drinking water (Singapore), in industry (cooling towers), in artificial recharge of aquifers, in agriculture (70% of Israel's irrigated agriculture is based on highly purified wastewater) and in the rehabilitation of natural ecosystems (Florida's Everglades).

# Etymology
The words "sewage" and "sewer"  came from Old French essouier = "to drain", which came from Latin exaquare. Their formal Latin antecedents are exaquaticum and exaquarium.