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Conventional Gravity Sewer

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

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The Conventional Gravity Sewer 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 (network 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.

Conventional Gravity Sewers do not require onsite pretreatment or storage of the wastewater. Because the waste is not treated before it is discharged, the sewer must be designed to maintain self-cleansing velocity (i.e. a flow that will not allow particles to accumulate). A self-cleansing velocity is generally 0.6–0.75m/s. A constant downhill gradient must be guaranteed along the length of the sewer to maintain self-cleaning flows. When a downhill grade cannot be maintained, a pump pumping station must be installed. Primary sewers are laid beneath roads, and must be laid at depths of 1.5 to 3m 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 ofthe 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 along above the sewer, at pipe intersections and at changes in pipeline direction (vertically and horizontally). The primary network requires rigorous engineering design to ensure Manholes should be designed such that they do not become a selfsource ofstormwater inflow or groundwater infiltration. In the case that connected users discharge highly pollutedwastewater (e.g., industry or restaurants), onsite pre-cleansing velocity is maintained, that manholes are placed as and primary treatment may be required before dischargeinto the sewer system to reduce the risk of clogging and that the load of the wastewater treatment plant. When the sewer line can support 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 traffic weightart. As wellRather, extensive construction 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 required , therefore, cheaper to remove build, and replace the road above.there is a higher treatment efficiency for less diluted wastewater

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{{procontable | pro=- Stormwater Less maintenance compared to Simplified and greywater Solids-Free Sewers <br>- Greywater and possibly stormwater can be managed at the same time. concurrently <br> - Construction can provide short-term employment to local labourers. Can handle grit and other solids, as well as large volumes of flow <br>| con=- Very high capital costs; high operation and maintenance costs <br>- A long time required minimum velocity must be maintained to connect all homes. prevent the deposition of solids in the sewer <br> - Not all parts and materials may be available locally. Requires deep excavations <br> - Difficult and costly to extend as a community changes and grows. <br> - Requires expert design , construction and construction supervision. maintenance <br> - Effluent Leakages pose a risk of wastewater exfiltration and sludge (from interceptors) requires secondary treatment groundwater infiltration and/or appropriate discharge. <br> - High capital and moderate operation cost. are difficult to identify

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===AdequacyAppropriateness=== Because they can be designed to carry so much volumelarge volumes, Conventional Gravity conventional gravity sewers are only very appropriate when there is a centralized treatment facility that is able to receive the transport wastewater to a (i.e. smaller, decentralized facilities could easily be overwhelmedSemi-)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 gravity sewers are expensive to build and, because the installation of a sewer line is disruptive and requires extensive coordination between the authorities, construction companies and the property owners, a professional management system must be in place. When stormwater is also carried by the sewer (called a Combined Sewer), sewer overflows are required. Sewer overflows are needed to avoid hydraulic surcharge of treatment plants during rain events. Infiltration into the sewer in areas where there is a high water table may compromise the performance of the Conventional Gravity 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===

This technology provides Ground shifting may cause cracks in manhole walls or pipe joints, which may become a high level source of hygiene groundwaterinfiltration or wastewater exfiltration, and comfort for compromise the user at performance of the point of usesewer. However, because Conventional gravity sewers can be constructed in cold climates as they are dug deep into the waste is conveyed to an offsite location for treatment, ground and the ultimate health large and environmental impacts are determined by the treatment provided by the downstream facilityconstant water flow resists freezing.

===MaintenanceHealth 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 installed wherever there is a change of grade or alignment and 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 cleaningmethods for conventional gravity sewers include rodding, flushing, jetting and bailing. Sewers can be dangerous because of toxic gases and should only 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===

* Bizier, P. (Ed.) (2007). Gravity Sanitary Sewer Design and Construction. Second Edition. ASCE Manuals and Reports on Engineering Practice No. 60, WEF MOP No. FD-5. American Society of Civil Engineers, New York, US.

(A standard design text used in North America, although local codes and standards should be assessed before

choosing a design manual)

* ASCE (1992). Gravity Sanitary Sewer Design and Construction, ASCE Manuals and Reports on Engineering Practice No. 60, WPCF MOP No. FD-5. American Society of Civil Engineers, New York. (A standard design text used in North America although local codes and standards should be assessed before choosing a design manual.) * Tchobanoglous, G. (1981). Wastewater Engineering: Collection and Pumping of Wastewater. McGraw-Hill, New York, US.

* TchobanoglousU.S. EPA (2002). Collection Systems Technology Fact Sheet. Sewers, GConventional Gravity.832-F-02-007. U.S. Environmental Protection Agency, BurtonWashington, FLD.C. and Stensel, HDUS. (2003Good description of the technology, including more detailed design criteria and information on operation and maintenance):Available at: [http://www. Wastewater Engineering: Treatment and Reuse, 4th Editionepa. Metcalf & Eddy, New Yorkgov epa.gov]

===Acknowledgements===

{{:Acknowledgements Sanitation}}

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