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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. In the past the word "sewage" also meant what is now called "sewerage". Possibly because of that, the word "sewerage" is often mistakenly used to mean "sewage".
Wastewater origin
Wastewater or sewage can come from (text in brackets indicates likely inclusions or contaminants) :-
- Human waste, usually from lavatories: (faeces, 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)
- Ground water 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)
- Sea water 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
Wastewater types
the composition of wastewater varies widely. It may contain these (this list is incomplete):-
- Water ( > 95%) (often added during flushing to carry the waste down a drain)
- Non-pathogenic bacteria (> 100,000 / ml for sewage)
- Pathogens - (Bacteria, viruses, prions, parasitic worms).
- Organic particles (faeces, hairs, food, vomit, paper fibres, plant material, humus etc.)
- Soluble organic material (urea, fruit sugars, soluble proteins, drugs, pharmaceuticals etc.)
- Inorganic particles (sand, grit, metal particles, ceramics, etc)
- Soluble inorganic material (ammonia, road-salt, sea-salt, cyanide, hydrogen sulphide, thiocyanates, thiosulphates)
- Animals (protozoa, insects, arthropods, small fish, etc.)
- Macro-solids (sanitary towels, nappies/ diapers, condoms, needles, children's toys, dead pets, body parts, etc.)
- Gases (hydrogen sulphide, carbon dioxide, methane)
- Emulsions (oils in emulsion, paints, adhesives, mayonnaise, hair colourants, etc)
- Toxins (pesticides, poisons, herbicides )
Wastewater quality indicators
- Main article: 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:
- Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganics such as NO3 or SO4
Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:
- S-- + 2 O2 → SO4--
- NO2- + ½ O2 → NO3-
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.
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 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 degrade 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 (See: http://www.biozone.com/ozone_generators.html ), 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 exaquāre. Their formal Latin antecedents are exaquāticum and exaquārium.
References
External links
- Industrial Pollution Information from the Coastal Ocean Institute, Woods Hole Oceanographic Institution
- Domestic Pollution and Sewage Information from the Coastal Ocean Institute, Woods Hole Oceanographic Institution
See also
- Industrial water treatment
- Sewer
- Sewage treatment
- Sewerage
- Water pollution
- Water resources
- Water treatment
Sewerage | Environmental engineering | Environmental soil science | Waste | Water pollution | Waste management
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"Wastewater"