C.4 Conventional Gravity Sewer

Conventional Gravity Sewers are networks of underground pipes that convey blackwater, greywater and, in many cases, stormwater from individual households to a (semi-) centralised treatment facility, using gravity and pumps where necessary.Mixture of urine, faeces and flushwater along with anal cleansing water (if water is used for cleansing) and/or dry cleansing materials. Blackwater contains the pathogens, nutrients and organic matter of faeces and the nutrients of urine that are diluted in the flushwater.Refers to (semi-solid) excrement that is not mixed with urine or water. Depending on diet, each person produces approximately 50–150 L per year of faecal matter of which about 80 % is water and the remaining solid fraction is mostly composed of organic material. Of the total essential plant nutrients excreted by the human body, faeces contain around 39 % of the phosphorus (P), 26 % of the potassium (K) and 12 % of the nitrogen (N). Faeces also contain the vast majority of the pathogens excreted by the body, as well as energy and carbon rich, fibrous material.Total volume of water generated from washing food, clothes and dishware, as well as from bathing, but not from toilets (see blackwater). It may also contain traces of excreta (e.g. from washing diapers) and, therefore, some pathogens. Greywater accounts for approximately 65 % of the wastewater produced in households with flush toilets.General term for rainfall runoff collected from roofs, roads and other surfaces. Very often the term is used to refer to rainwater that enters a sewerage system. It is the portion of rainfall that does not infiltrate into the soil.The liquid produced by the body to rid itself of nitrogen in the form of urea and other waste products. In this context, the urine product refers to pure urine that is not mixed with faeces or water. Depending on diet, human urine collected from one person during one year (approx. 300 to 550 L) contains 2 to 4 kg of nitrogen. The urine of healthy individuals is sterile when it leaves the body but is often immediately contaminated by coming into contact with faeces.The liquid that has passed through a filter.
Any substance that is used for growth. Nitrogen (N), phosphorus (P) and potassium (K) are the main nutrients contained in agricultural fertilisers. N and P are also primarily responsible for the eutrophication of water bodies.
An organism or other agent that causes disease.An open channel or closed pipe used to convey sewage. See C.3 and C.4
The physical sewer infrastructure (sometimes used interchangeably with sewage).
User interface used for urination and defecation. The organic molecule (NH2)2CO that is excreted in urine and that contains the nutrient nitrogen. Over time, urea breaks down into carbon dioxide and ammonium, which is readily used by organisms in soil. It can also be used for on-site faecal sludge treatment. See. S.18Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.
The liquid that has passed through a filter.
Waste matter that is transported through the sewer.
The physical sewer infrastructure (sometimes used interchangeably with sewage).

The Conventional Gravity Sewer system is designed with many branches. Typically, the network is subdivided into primary (main sewer lines along main roads), secondary and tertiary networks (networks at the neighborhood and household level).

An open channel or closed pipe used to convey sewage. See C.3 and C.4
Waste matter that is transported through the sewer.

Design Considerations

Conventional Gravity Sewers normally do not require on-site pre-treatment, primary treatment or storage of household wastewater. The sewer must be designed, however, so that it maintains a self-cleansing velocity (i.e., a flow that will not allow particles to accumulate). For typical sewer diameters, a minimum velocity of between 0.6 to 0.7 m/s during peak dry weather conditions should be adopted. This requires a daily water consumption rate of more than 100 L per person per day. A constant downhill gradient must be guaranteed long the sewer length to maintain self-cleansing flows, which can require deep excavations. When a gradient cannot be maintained, a pumping station must be installed. Primary sewers are laid beneath roads, at depths between 1.5 to 3 m to avoid damages caused by traffic loads. The depth also depends on the groundwater table, the lowest point to be served (e.g. a basement) and the topography. The selection of the pipe diameter depends on projected average and peak flows. Access manholes are placed at set intervals above the sewer, at pipe intersections and at changes in pipeline direction (vertically and horizontally). Manholes should be designed to ensure that they do not become a source of stormwater inflow or groundwater in filtration. In the case that connected users discharge highly polluted wastewater (e.g. from industry or restaurants), on-site pre- and primary treatment may be required before discharge into the sewer system to reduce the risk of clogging and the load of wastewater to the treatment plant. When the sewer carries stormwater (known then as a combined sewer), overflows are required to avoid hydraulic surcharge of treatment plants during rain events. However, combined sewers are no longer be considered state of the art. Rather, local retention and in filtration of stormwater or a separate drainage system for rainwater is recommended. The wastewater treatment system then requires smaller dimensions and is, therefore, cheaper to build, and has a higher treatment efficiency for less diluted wastewater.

General term for rainfall runoff collected from roofs, roads and other surfaces. Very often the term is used to refer to rainwater that enters a sewerage system. It is the portion of rainfall that does not infiltrate into the soil.The liquid that has passed through a filter.
A mechanical separation process using a porous medium (e.g., cloth, paper, sand bed, or mixed media bed) that captures particulate material and permits the liquid or gaseous fraction to pass through. The size of the pores of the medium determines what is captured and what passes through.Water that is located beneath the earth’s surface.
The first major stage in wastewater treatment that removes solids and organic matter mostly by the process of sedimentation or flotation.
Gravity settling of particles in a liquid such that they accumulate. An open channel or closed pipe used to convey sewage. See C.3 and C.4
The physical sewer infrastructure (sometimes used interchangeably with sewage).
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.
The liquid that has passed through a filter.
Waste matter that is transported through the sewer.
The physical sewer infrastructure (sometimes used interchangeably with sewage).

Materials

Commonly used materials are concrete, PVC, vitrified clay and ductile or cast-iron pipes. Excavation requires an excavator or numerous workers with shovels, depending on soil properties.

Applicability

Sewers in the humanitarian context are usually applicable where sewers are already existing and can be rehabilitated, for example in host communities. Furthermore, the construction of a new sewer line can be part of recovery actions. As they can be designed to carry large volumes, Conventional Gravity Sewers are very appropriate to transport wastewater to a (semi-) centralized treatment facility. Planning, construction, operation and maintenance requires expert knowledge. Construction of conventional sewer systems in dense, urban areas is complicated as it disrupts urban activities and traffic. Conventional Gravity Sewers are expensive to build and, because the installation of a sewer line is disruptive and requires extensive coordination between authorities, construction companies and property owners, a professional management system must be in place. Ground shifting may cause cracks in manhole walls or pipe joints, which may become a source of groundwater in filtration or wastewater ex filtration, and compromise the performance of the sewer. Conventional Gravity Sewers can be constructed in cold climates as they are dug deep into the ground and the large and constant water flow resists freezing.

A mechanical separation process using a porous medium (e.g., cloth, paper, sand bed, or mixed media bed) that captures particulate material and permits the liquid or gaseous fraction to pass through. The size of the pores of the medium determines what is captured and what passes through.Water that is located beneath the earth’s surface.
An open channel or closed pipe used to convey sewage. See C.3 and C.4
Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.
Waste matter that is transported through the sewer.

Operation and Maintenance

Manholes are used for routine inspection and sewer cleaning. Debris (e.g. grit, sticks or rags) may accumulate in manholes and block the lines. To avoid clogging caused by grease, it is important to inform users about proper oil and grease disposal. Common cleaning methods for Conventional Gravity Sewers include rodding, flushing, jetting and bailing. Sewers can be dangerous because of toxic gases and should be maintained only by professionals, although, in well-organised communities, maintenance of tertiary networks might be handed over to a well-trained group of community members. Proper personal protective equipment should always be used when entering a sewer.

An open channel or closed pipe used to convey sewage. See C.3 and C.4
Waste matter that is transported through the sewer.

Costs

Conventional Gravity Sewers have very high capital as well as operation and maintenance (O & M) costs. Conventional Gravity Sewer O & M is constant and labor intensive. The costs of household sewer connections must be included in the total cost calculations.

An open channel or closed pipe used to convey sewage. See C.3 and C.4
Waste matter that is transported through the sewer.

Social Considerations

If well-constructed and maintained, Conventional Gravity Sewers are a safe and hygienic means of transporting wastewater. This technology provides a high level of hygiene and comfort for the user. However, because the waste is conveyed to an offsite location for treatment, the ultimate health and environmental impacts are determined by the treatment provided by the downstream facility.

Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

Fact Sheet Overview

Inputs

Blackwater
Greywater
Stormwater

Outputs

Blackwater
Greywater
Stormwater

Response Phase

Stabilisation +
Recovery + +

Challenging Ground Conditions

Unsuitable

Application Level

Neighbourhood +
City + +

Water-based or Dry Technology

Water-Based

Management Level

Public + +

Technical Complexity

High

Functional Group

Emptying and Transport

Required Space

Medium

Objectives & Key Features

• Conveyance of wastewater and stormwater

Strength & Weakness

  • Greywater and possibly stormwater can be managed concurrently
  • Can handle grit and other solids, as well as large volumes of flow
  • Very high capital costs; high O & M costs
  • A minimum velocity must be maintained to prevent the deposition of solids in the sewer
  • Difficult and costly to extend as a community changes and grows
  • Requires expert design, construction and maintenance

Selected References

Technical aspects and standard designs

Bizier, P. (2007): Gravity Sanitary Sewer Design and Construction. American Society of Civil Engineers (ASCE), New York, US

Tchobanoglous, G. (1981): Wastewater Engineering: Collection and Pumping of Wastewater. McGraw-Hill, New York, US

EPA (0): Collection Systems Technology Fact Sheet – Sewers, Conventional Gravity. United States Environmental Protection Agency (EPA)

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