Tuesday, May 3, 2011


Assistant Professor
Department of Architecture and Planning
NED University of Engineering and Technology

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 feces 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 or sewage can come from (text in brackets indicates likely inclusions or contaminants):

  1. Human waste (fæces, used toilet paper or wipes, urine, or other bodily fluids), also known as blackwater, usually from lavatories;
  2. Cesspit leakage;
  3. Septic tank discharge;
  4. Sewage treatment plant discharge;
  5. Washing water (personal, clothes, floors, dishes, etc.), also known as greywater or sullage;
  6. Rainfall collected on roofs, yards, hard-standings, etc. (generally clean with traces of oils and fuel);
  7. Groundwater infiltrated into sewage;
  8. Surplus manufactured liquids from domestic sources (drinks, cooking oil, pesticides, lubricating oil, paint, cleaning liquids, etc.);
  9. Urban rainfall runoff from roads, carparks, roofs, sidewalks, or pavements (contains oils, animal fæces, litter, fuel or rubber residues, metals from vehicle exhausts, etc.);
  10. Seawater ingress (high volumes of salt and micro-biota);
  11. Direct ingress of river water (high volumes of micro-biota);
  12. Direct ingress of manmade liquids (illegal disposal of pesticides, used oils, etc.);
  13. Highway drainage (oil, de-icing agents, rubber residues);
  14. Storm drains (almost anything, including cars, shopping trolleys, trees, cattle, etc.);
  15. Blackwater (surface water contaminated by sewage);
  16. Industrial waste
  17. industrial site drainage (silt, sand, alkali, oil, chemical residues);
    1. Industrial cooling waters (biocides, heat, slimes, silt);
    2. Industrial process waters;
    3. Organic or bio-degradable waste, including waste from abattoirs, creameries, and ice cream manufacture;
    4. Organic or non bio-degradable/difficult-to-treat waste (pharmaceutical or pesticide manufacturing);
    5. extreme pH waste (from acid/alkali manufacturing, metal plating);
    6. Toxic waste (metal plating, cyanide production, pesticide manufacturing, etc.);
    7. Solids and Emulsions (paper manufacturing, foodstuffs, lubricating and hydraulic oil manufacturing, etc.);
    8. Agricultural drainage, direct and diffuse.

The composition of wastewater varies widely. This is a partial list of what it may contain:

  1. Water ( > 95%) which is often added during flushing to carry waste down a drain;
  2. Pathogens such as bacteria, viruses, prions and parasitic worms;
  3. Non-pathogenic bacteria;
  4. Organic particles such as feces, hairs, food, vomit, paper fibers, plant material, humus, etc.;
  5. Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.;
  6. Inorganic particles such as sand, grit, metal particles, ceramics, etc.;
  7. Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.;
  8. Animals such as protozoa, insects, arthropods, small fish, etc.;
  9. Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead animals or plants, etc.;
  10. Gases such as hydrogen sulfide, carbon dioxide, methane, etc.;
  11. Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.;
  12. Toxins such as pesticides, poisons, herbicides, etc.
  13. Pharmaceuticals and other hormones.

Sewage is water-carried wastes, in either solution or suspension, that is intended to flow away from a community. Also known as wastewater flows, sewage is the used water supply of the community. It is more than 99.9% pure water and is characterized by its volume or rate of flow, its physical condition, its chemical constituents, and the bacteriological organisms that it contains. Depending on their origin, wastewater can be classed as sanitary, commercial, industrial, agricultural or surface runoff.

The spent water from residences and institutions, carrying body wastes, washing water, food preparation wastes, laundry wastes, and other waste products of normal living, are classed as domestic or sanitary sewage. Liquid-carried wastes from stores and service establishments serving the immediate community, termed commercial wastes, are included in the sanitary or domestic sewage category if their characteristics are similar to household flows. Wastes that result from an industrial process or the production or manufacture of goods are classed as industrial wastes. Their flows and strengths are usually more varied, intense, and concentrated than those of sanitary sewage. Surface runoff, also known as storm flow or overland flow, is that portion of precipitation that runs rapidly over the ground surface to a defined channel. Precipitation absorbs gases and particulates from the atmosphere, dissolves and leaches materials from vegetation and soil, suspends matter from the land, washes spills and debris from urban streets and highways, and carries all these pollutants as wastes in its flow to a collection point.

Wastewater from all of these sources may carry pathogenic organisms that can transmit disease to humans and other animals; contain organic matter that can cause odor and nuisance problems; hold nutrients that may cause eutrophication of receiving water bodies; and can lead to ecotoxicity. Proper collection and safe, nuisance-free disposal of the liquid wastes of a community are legally recognized as a necessity in an urbanized, industrialized society.

"Sewage" and "Sewerage" may be used interchangeably in the USA but elsewhere they retain separate and different meanings - sewage being the liquid material and sewerage being the pipes, pumps and infrastructure through which sewage flows.

Sewage pumping is normally done by a submersible pump.

This became popular in the early 1960s, when a guide-rail system was developed to lift the submersible pump out of the pump station for repair, and ended the dirty and sometimes dangerous task of sending people into the sewage or wet pit. Growth of the submersible pump for sewage pumping since has been dramatic, as an increasing number of specifiers and developers learned of their advantages.

Three classes of submersible pumps exist:

Smaller submersible pumps, used in domestic and light commercial applications, normally handle up to 55mm spherical solids and range from 0.75kW to 2.2kW.
Larger submersible pumps, handle 65mm and larger solids and normally have a minimum of 80mm discharge. They are generally used in municipal and industrial applications for pumping sewage and all types of industrial wastewater.
Submersible chopper pumps, which are used to handle larger concentrations of solids and/or tougher solids that conventional sewage pumps cannot handle. Chopper pumps are generally used in municipal and industrial wastewater applications and provide clog-free operation by macerating those solids that might clog other types of submersible pumps.
Submersible pumps are normally used in a packaged pump station where drainage by gravity is not possible. Vertical type sewage pumps have also been used for many years. They have the motor above the floor so work on the motor can be done without entering the sewage pit.

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, forcing untreated sewage to flow back into the environment. 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. 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.

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. Other Industrial processes that produce a lot of waste-waters such as paper and pulp production has created environmental concern leading to development of processes to recycle water use within plants before they have to be cleaned and disposed of.

Treated wastewater can be reused as drinking water, 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.

A Soak Pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground. Pre-settled effluent from a Collection and Storage/Treatment or (Semi-) Centralized Treatment technology is discharged to the underground chamber from where it infiltrates into the surrounding soil.

The Soak Pit can be left empty and lined with a porous material (to provide support and prevent collapse), or left unlined and filled with coarse rocks and gravel. The rocks and gravel will prevent the walls from collapsing, but will still provide adequate space for the wastewater. In both cases, a layer of sand and fine gravel should be spread across the bottom to help disperse the flow. The soak pit should be between 1.5 and 4m deep, but never less than 1.5m above the ground water table. As wastewater (pre-treated greywater or blackwater) percolates through the soil from the Soak Pit, small particles are filtered out by the soil matrix and organics are digested by micro-organisms. Thus, Soak Pits are best suited to soils with good absorptive properties; clay, hard packed or rocky soils are not appropriate.

Soak pits are a good way to eliminate stagnant run-off water from latrines and they are a good alternative to large or small cess pools.


PVC PIPING: Usually around a meter per soak pit.  It depends on the distance from the middle of your pit to the outlet.  1m~ 1500CFA

COVER: We used sheets of plastic 1.5m*1m per soak pit.  They are sold in 2m*1m sheets at ~300CFA/meter.  Rice sacks and straw can also be used.

CEMENT:  A small amount is needed to crepe the inside of the latrine wall and floor around the pipe.  1 bag 5750-6000CFA, we used less than two bags for 50 soak pits.  We used the mortar mix of 4:1 sand to cement ration.

ROCKS: enough to fill a 1m*1m hole

SAND: enough to make cement mortar

TOOLS: Shovels, dabas, buckets (something to transport cement in maybe a wheelbarrow or another bucket), a sieve to clean sand if dirty, pick axes

In my village, we had a group of men who were my soak pit team.  Concessions agreed to pay 500CFA for each soak pit they got, dig their own hole, and collect for rocks.  Then, when the holes were ready, my team went around and filled the hole, stuck the piping in, covered the hole and cemented the area around the pipe inside the latrine.  We could do 10 in one day if the holes were dug and rocks were ready.

1.      Pick latrines with wastewater coming out of it and determine if a hole can be dug there
2.      Dig the hole.  We tried shooting for 1m*1m*1m holes, but often we hit rock at around half a meter, so then we just made the hole wider or longer.
3.     Fill the hole in with rocks of various sizes.  The rocks should be big enough so that there is space between them for water to go.  The Hippo handbook recommends putting a thin layer of sand and or gravel at the bottom of the pit.  We did not do this, but I think it is recommended for soak pits with larger volumes of water to help with absorption.
4.      Place in the pipe.  The pipe should exit the latrine and end in the middle of your pit.  The end should be sitting on top of a flat rock, when the rock hits a flat surface it can spread out easily from there instead of constantly hitting and thus eroding a small, pointy rock.
5.       Place rocks around the pipe where it enter the pit so that it will be protected when covered.
6.    Cover the pit with plastic/rice sacks/straw and then dirt.  Make sure there is enough dirt and it is packed down well enough so that things will not get damaged when walked over.  We also covered the pipe, because my villagers were worried that donkeys or cows would walk across the pipe and crunch it.
7.      Cement the pipe into place in the outlet, covering the area around the wall and floor where the water exits the latrine. So that water will exit through the pipe and nowhere else.

A total of 175 soak pits were constructed since the conception of my soak pit project which began back in September or so.  All and all the project has gone very well.  At the beginning of the project, there was a work crew of ten workers.  They were supposed to work together in building all of the soak pits.  But six of the ten quit during the project stating that it was too much work.  In a way, this was a good sign because it proved to me that the remaining four workers are the men who are extremely motivated to do sanitation work. As for the soak pits themselves, I have run into very few problems.  Some of the soak pits were constructed poorly and because of this, the cavity that was next to the pipe which allowed water to openly flow has caved in.  This is going to be fixed wherever the problem has occurred. I have discovered a problem with the soak pits that were dug.  The pipes have become clogged with dirt on a very regular basis (this has also happened with some soak pits too but it is a problem that really can’t be avoided with soak pits if your latrine is made of mud.  The problem can be slowed down though if the latrine is swept regularly.)  The pipes are too long for one to unclog with a metal bar or other instrument.  So I have decided that having a covered drain leading to the pump soak pit is impractical.  Therefore, I will rip up the concrete covering and enclosing the top of the pipe.  This will allow the water to flow openly and if there is dirt or other debris in the drainage trench, it will be easily accessible and therefore easy to move.  I will have a screen put at the end of the trench where it meets the soak pit to keep dirt and trash from entering the soak pit, thus allowing the soak pit to work appropriately for a longer period of time.  But my homologue and I also need to meet with the chief of the village who can help us form pump committees who could then oversee the maintenance and upkeep of the pump areas on a daily basis.  It is only with this last detail will the community be able to keep the pump areas sanitized.   

A septic tank is a key component of the septic system, a small-scale sewage treatment system common in areas with no connection to main sewage pipes provided by local governments or private corporations. (Other components, typically mandated and/or restricted by local governments, optionally include pumps, alarms, sand filters, and clarified liquid effluent disposal means such as a septic drain field, ponds, natural stone fiber filter plants or peat moss beds.) Septic systems are a type of On-Site Sewage Facility (OSSF). In North America, approximately 25% of the population relies on septic tanks; this can include suburbs and small towns as well as rural areas (Indianapolis is an example of a large city where many of the city's neighborhoods are still on separate septic systems). In Europe, they are generally limited to rural areas only.

The term "septic" refers to the anaerobic bacterial environment that develops in the tank and which decomposes or mineralizes the waste discharged into the tank. Septic tanks can be coupled with other on-site wastewater treatment units such as biofilters or aerobic systems involving artificial forced aeration.[1]

Periodic preventive maintenance is required to remove the irreducible solids which settle and gradually fill the tank, reducing its efficiency. In most jurisdictions this maintenance is required by law, yet often not enforced. Those who ignore the requirement will eventually be faced with extremely costly repairs when solids escape the tank and destroy the clarified liquid effluent disposal means. A properly maintained system, on the other hand, can last for decades or possibly even a lifetime.

A septic tank generally consists of a tank (or sometimes more than one tank) of between 4000 - 7500 litres (1,000 and 2,000 gallons) in size connected to an inlet wastewater pipe at one end and a septic drain field at the other. These pipe connections are generally made via a T pipe which allows liquid entry and exit without disturbing any crust on the surface. Today, the design of the tank usually incorporates two chambers (each of which is equipped with a manhole cover) which are separated by means of a dividing wall which has openings located about midway between the floor and roof of the tank.

Wastewater enters the first chamber of the tank, allowing solids to settle and scum to float. The settled solids are anaerobically digested, reducing the volume of solids. The liquid component flows through the dividing wall into the second chamber where further settlement takes place, with the excess liquid then draining in a relatively clear condition from the outlet into the leach field, also referred to as a drain field or seepage field, depending upon locality.

The remaining impurities are trapped and eliminated in the soil, with the excess water eliminated through percolation into the soil (eventually returning to the groundwater), through evaporation, and by uptake through the root system of plants and eventual transpiration. A piping network, often laid in a stone filled trench (see weeping tile), distributes the wastewater throughout the field with multiple drainage holes in the network. The size of the leach field is proportional to the volume of wastewater and inversely proportional to the porosity of the drainage field. The entire septic system can operate by gravity alone, or where topographic considerations require, with inclusion of a lift pump. Certain septic tank designs include siphons or other methods of increasing the volume and velocity of outflow to the drainage field. This helps to load all portions of the drainage pipe more evenly and extends the drainage field life by preventing premature clogging.

An Imhoff tank is a two-stage septic system where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage. Also, some septic tank designs have a second stage where the effluent from the anaerobic first stage is aerated before it drains into the seepage field.

Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank, or else the septic tank fills up and undecomposed wastewater discharges directly to the drainage field. Not only is this bad for the environment, but if the sludge overflows the septic tank into the leach field, it may clog the leach field piping or decrease the soil porosity itself, requiring expensive repairs.

How often the septic tank has to be emptied depends on the volume of the tank relative to the input of solids, the amount of indigestible solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures). The required frequency varies greatly depending on jurisdiction, usage, and system characteristics. Some health authorities require tanks to be emptied at prescribed intervals, while others leave it up to the determination of the inspector. Some systems require pumping every few years or sooner, while others may be able to go 10–20 years between pumpings. Contrary to what many believe, there is no "rule of thumb" for how often tanks should be emptied. An older system with an undersized tank that is being used by a large family will require much more frequent pumping than a new system used by only a few people. Anaerobic decomposition is rapidly re-started when the tank re-fills.

A properly designed and normally operating septic system is odor free and, besides periodic inspection and pumping of the septic tank, should last for decades with no maintenance.

A well designed and maintained concrete, fibreglass or plastic tank should last about 50 years.

A manhole (alternatively utility hole, cable chamber, maintenance hole, inspection chamber, access chamber or confined space) is the top opening to an underground utility vault used to house an access point for making connections or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains and gas. It is protected by a manhole cover, also known as a 'biscuit', a plug designed to prevent accidental or unauthorized access to the manhole. Those plugs are usually made of metal or constructed from precast concrete (especially in Europe). Manholes are usually outfitted with metal or polypropylene steps installed in the inner side of the wall to allow easy descent into the manhole.

Manholes are generally found in urban areas, in streets and occasionally under sidewalks. They are usually in circular shape to prevent accidental fall of the cover in the hole.

In rural and undeveloped areas, services such as telephone and electricity may be carried on pylons rather than underground.

[1] Wastewater; From http://en.wikipedia.org/wiki/Wastewater (Retrieved May 6, 2011)
[2] Sewage; From: http://en.wikipedia.org/wiki/Sewage (Retrieved May 6, 2011)
[3] Sewage_treatment; From: http://en.wikipedia.org/wiki/Sewage_treatment (Retrieved May 6, 2011)
[4] Soak Pits; From: http://www.akvo.org/wiki/index.php/Soak_Pit (Retrieved May 6, 2011)
[5] Soak Pits; From: http://kalanke.web.officelive.com/Soak_Pits.aspx (Retrieved May 6, 2011)
[6] Septic Tank; From: http://en.wikipedia.org/wiki/Septic_tank (Retrieved May 6, 2011)
[7] Manhole; From: http://en.wikipedia.org/wiki/Manhole (Retrieved May 6, 2011)

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