Conventional Gravity Sewer

Conventional gravity sewers are large networks of underground pipes that convey blackwater, greywater and, in many cases, stormwater from individual households to a (Semi-) Centralized Treatment facility, using gravity (and pumps when necessary).

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 neighbourhood and household level).


Design Considerations

Conventional gravity sewers normally do not require onsite pre-treatment, primary treatment or storage of the household wastewater before it is discharged. The sewer must be designed, however, so that it maintains self-cleansing velocity (i.e., a flow that will not allow particles to accumulate). For typical sewer diameters, a minimum velocity of 0.6 to 0.7 m/s during peak dry weather conditions should be adopted. A constant downhill gradient must be guaranteed along the length of the sewer to maintain self-cleansing flows, which can require deep excavations. When a downhill grade cannot be maintained, a pumping station must be installed. Primary sewers are laid beneath roads, at depths of 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 the projected average and peak flows. Commonly used materials are concrete, PVC, and ductile or cast iron pipes.

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 such that they do not become a source of stormwater inflow or groundwater infiltration. 

In the case that connected users discharge highly polluted wastewater (e.g., industry or restaurants), onsite pre- and primary treatment may be required before discharge into the sewer system to reduce the risk of clogging and the load of the wastewater treatment plant.

When the sewer also carries stormwater (known as a combined sewer), sewer overflows are required to avoid hydraulic surcharge of treatment plants during rain events. However, combined sewers should no longer be considered state of the art. Rather, local retention and infiltration of stormwater or a separate drainage system for rainwater are recommended. The wastewater treatment system then requires smaller dimensions and is, therefore, cheaper to build, and there is a higher treatment efficiency for less diluted wastewater.

Appropriateness

Because 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 require expert knowledge. Construction of conventional sewer systems in dense, urban areas is complicated because 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 infiltration or wastewater exfiltration, 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. 

Health Aspects/Acceptance

If well constructed and maintained, 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.

Operation & Maintenance 

Manholes are used for routine inspection and sewer cleaning. Debris (e.g., grit, sticks or rags) may accumulate in the manholes and block the lines. To avoid clogging caused by grease, it is important to inform the 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, the maintenance of tertiary networks might be handed over to a well-trained group of community members. Proper protection should always be used when entering a sewer.

References

Further Readings

  • Cover image of a reference book or miscellany.

    TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; SCHERTENLEIB, R.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies (Arabic). 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). PDF

    This is the Arabic version of the Compendium of Sanitation Systems and Technologies. The Compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

  • Cover image of a reference book or miscellany.

    U.S. EPA (Editor) (n.y.): Sewers, Conventional Gravity. United States Environmental Protection Agency (EPA). URL [Accessed: 10.08.2010]. PDF

    This factsheet deals with technical aspects of gravity sewers: applicability, advantages and disadvantages, design criteria, performance, O&M, costs, etc.

  • Cover image of a reference book or miscellany.

    EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Lecture Notes . Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). PDF

    Lecture notes on technical and non-technical aspects of sanitation systems in developing countries.

  • Cover image of a reference book or miscellany.

    TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies. 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). URL [Accessed: 28.07.2014]. PDF

    This compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

Case Studies

  • Cover image of a reference chapter of a book/miscellany.

    ENGINEERING CONCEPTS INC. (Editor) (2010): Chapter 6. Wastewater Collection System Alternatives. In: Engineering Concepts Inc.; Kapoho Beach Lots (Editor); Farm Lots (Editor); Vacationland Estates Wastewater Feasibility Report (Editor) (2010): Chapter 6 - Wastewater Collection System Alternatives. Hilo. URL [Accessed: 17.08.2010]. PDF

    Preliminary study presenting different alternative systems for the wastewater collection in Kapoho, Hawaii. Different system are presented and discussed with their respective requirements and technical characteristics.

  • Cover image of a reference book or miscellany.

    WSP (Editor) (2010): Water and Sewerage Services in Karachi. Citizen Report Card: Sustainable Service Delivery Improvements. Washington: Water and Sanitation Program. URL [Accessed: 03.10.2011]. PDF

    This report discusses the key findings and recommendations emerging from a pilot Citizen Report Card (CRC) on water, sanitation, and sewerage services in Karachi. The CRC, pioneered by the Public Affairs Center (PAC), Bengaluru, provides public agencies with systematic feedback from users of public services. CRC gains such feedback through sample surveys on aspects of service quality that users know best, and enable public agencies to identify strengths and weaknesses in their work.

Awareness Material

  • Cover image of a reference book or miscellany.

    PEARCE-OROZ, G. (Editor) (2011): Rural Water Supply and Sanitation Challenges in Latin America for the Next Decade. Lima: Water and Sanitation Program (WSP). URL [Accessed: 14.05.2012]. PDF

    Based on market research, this new WSP technical paper analyzes the main features of the fecal sludge collection businesses in each city, including the marketing mix, potential demand, supply capacity, and legal frameworks. In addition, the paper spotlights major challenges and opportunities in fecal sludge management, describing the current and potential market for fecal sludge removal, collection, and disposal in peri-urban areas—which typically struggle with high population density, limited land planning, high citizen insecurity, and low coverage of basic services.

Training Material

  • Cover image of a reference chapter of a book/miscellany.

    HHPSD (Editor) (2000): Section IX: Gravity Sanitary Sewer System. Design Guidelines. In: HHPSD (Editor) (2000): Administrative Procedures and Guidelines for Water and Sanitary Sewer System Desig. URL [Accessed: 10.08.2010]. PDF

    Technical characteristics and dimensions for designing a gravity sewer system (based on the example of the Hilton Head Island).

  • Cover image of a reference chapter of a book/miscellany.

    HHPSD (Editor) (2000): Section X: Gravity Sanitary Sewer System. Materials for Construction. In: HHPSD (Editor) (2000): Administrative Procedures and Guidelines for Water and Sanitary Sewer System Desig. URL [Accessed: 10.08.2010]. PDF

    Materials used for construction of sewer systems (based on the example of the Hilton Head Island).

  • Cover image of a reference chapter of a book/miscellany.

    HHPSD (Editor) (2000): Section XI: Gravity Sanitary Sewer System. Construction Procedures. In: HHPSD (Editor) (2000): Administrative Procedures and Guidelines for Water and Sanitary Sewer System Desig. URL [Accessed: 10.08.2010]. PDF

    This document contains construction procedures for sewer systems and contains indications for each step and element (based on the example of Hilton Head Island).

  • Cover image of a reference book or miscellany.

    USAID (Editor) (1982): Designing Sewer Systems. Washington: United States Agency for International Development. URL [Accessed: 17.08.2010]. PDF

    This technical note describes the main technical elements involved in designing a sewer system.

  • Cover image of a reference book or miscellany.

    USAID (Editor) (1982): Operating and Maintaining Sewer Systems. Washington: United States Agency for International Development. URL [Accessed: 17.08.2010]. PDF

    This technical note describes the main technical elements involved in maintaining a sewer system.

  • Cover image of a reference book or miscellany.

    USAID (Editor) (1982): Constructing Sewer Systems. Washington: United States Agency for International Development. URL [Accessed: 17.08.2010]. PDF

    This technical note describes the main technical elements involved in constructing a sewer system.

  • Cover image of a reference book or miscellany.

    USAID (Editor) (1982): Methods of Combined Washwater and Excreta Disposal. Washington: United States Agency for International Development. URL [Accessed: 17.08.2010]. PDF

    This technical note describes the basic features of four methods of combined disposal: cesspool, septic tank with subsurface disposal system, septic tank with non-conventional disposal system and sewer with stabilisation ponds.

  • Cover image of a reference book or miscellany.

    USAID (Editor) (1982): Estimating Sewage or Washwater Flows. Washington: United States Agency for International Development. URL [Accessed: 17.08.2010]. PDF

    This technical note describes five methods of estimating sewage or washwater flows: on-site measurement, on-site estimating, water metering estimates, community water use data and survey estimating.

Important Weblinks

  • http://www.akvo.org/ [Accessed: 10.08.2010]

    This webpage summarises several sewer system construction elements and contains a description on gravity sewer system characteristics.