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 '''A Solidssolids-Free Sewer free sewer is a network of small -diameter pipes that transports pre-treated and solids-free or pre-treated wastewater (such as [[Septic Tank]] or settling tank effluent) to . It can be installed at a shallow depth and does not require a treatment facility for further treatment minimum wastewater flow or slope to a discharge point. Solids-Free Sewers are also referred to as settled, small-bore, small-diameter, variable-grade gravity, or septic tank effluent gravity sewersfunction.'''

Solids-free sewers are also referred to as settled, smallbore, variable-grade gravity, or septic tank effluent gravity sewers. A precondition for Solidssolids-Free Sewer networks free sewers is efficient pre-primary treatment at the household level. The An interceptor, septic or settling tank removes typically a single-chamber [[Septic Tank]] (S.9), captures settleable particles that could clog small pipes. A grease trap should The solids interceptor also be addedfunctions to attenuate peak discharges. Because there is little risk of depositions and clogging, the solids-free sewers do not have to be self-cleaning (cleansing, i.e. , no minimum flow velocity) and can therefore be laid at shallow depths, can have fewer or tractive tension is needed. They require few inspection points (manholes), can follow the topography more closely and have inflective gradients (i.e. , negative slopeslopes)and follow the topography. When the sewer roughly follows the ground contours, the flow in the sewer is allowed to vary between open channel flow and pressure (full-bore) flow. However ===Design Considerations=== If the interceptors are correctly designed and operated, care should be taken with negative this type of sewer does not require self-cleansing velocities or minimum slopes . Even inflective gradients are possible, as long as they may lead to surging above the ground downstream end of the sewer is lower than theupstream end. In sections where there is pressure flow, the water level during peak flowsin any interceptor tank must be higher than the hydraulic head within the sewer, otherwise the liquid will flow back into the tank. Inspection At high points should in sections with pressure flow, the pipes must be provided at major connection points or when the size of the pipe changesventilated.

Despite the possibility that some portions of the pipe can rise higher when going downstreamSolids-free sewers do not have to be installed on a uniform gradient with a straight alignment between inspection points. The alignment may curve to avoid obstacles, allowing for the total pipe the downstream end greater construction tolerance. A minimum diameter of the sewer must be lower than the upstream end75 mm is required to facilitate cleaning. Expensive manholes are not needed because access for mechanical cleaning equipment is not necessary. When choosing a pipe diameter (Cleanouts or flushing points are sufficient and are installed at least 75mm)upstream ends, high points, intersections, the depth of water or major changes in the direction or pipe during peak flow within each section size. Compared to manholes, cleanouts can be more tightly sealed to prevent stormwater from entering. Stormwater must be less than the diameter of the excluded as it could exceed pipecapacity and lead to blockages due to grit depositions. In sections where Ideally, there is pressure flowshould not be any storm- and groundwater in the sewers, but, in practice, the invert some imperfectly sealed pipe joints must be expected. Estimates of any interceptor tank outlet groundwater infiltration and stormwater inflow must higher than , therefore, be made when designing the hydraulic head within the sewer just prior to system. The use of PVC pipes can minimize the point risk of connection otherwise the liquid will backflow into the tank. If this condition is not met, then either select the next larger pipe diameter for the sewer or increase the depth at which the sewer is laidleakages.

{{procontable | pro=- Greywater can be managed at the same time. Does not require a minimum gradient or flow velocity <br> - Can be built and repaired with locally available materials. <br> - Construction can provide short-term employment to local labourers. used where water supply is limited <br> - Capital Lower capital costs are less than [[Conventional Gravity Sewer|Conventional Gravity Sewers]]conventional gravity sewers; low operating costs. <br> - Can be extended as a community changes and grows. <br>- Greywater can be managed concurrently| con=- Space for interceptors is required <br>- Interceptors require regular desludging to prevent clogging <br>- Requires expert design training and construction supervision. acceptance to be used correctly <br> - Requires repairs and removals of blockages more frequently than a Conventional Gravity Sewer. conventional gravity sewer <br> - Requires education expert design and acceptance to be used correctly. construction <br> - Effluent Leakages pose a risk of wastewater exfiltration and sludge (from interceptors) requires secondary treatment groundwater infiltration and/or appropriate discharge.<br>-High water consumption for excreta removal. are difficult to identify

===AdequacyAppropriateness===This type of sewer is best suited to medium-density (peri-)urban areas and less appropriate in low-density or rural settings. It is most appropriate where there is no space for a [[Leach Field]] (D.8), or where effluents cannot otherwise be disposed of onsite (e.g., due to low infiltration capacity or high groundwater). It is also suitable where there is undulating terrain or rocky soil. A solids-free sewer can be connected to existing Septic Tanks where infiltration is no longer appropriate (e.g., due to increased housing density and/or water use).

Solids-Free Sewers are appropriate for both full and partially filled flowsAs opposed to a [[Simplified Sewer]] (C. Although 4) a constant supply of solids-free sewer can also be used where domestic water consumptionis required, less water limited. This technology is needed compared to a flexible option that can be easily extended as the population grows. Because of shallowexcavations and the Simple use of fewer materials, it can be built at considerably lower cost than a [[Conventional Gravity Sewer because self-cleansing velocities are not required| Conventional Sewer]] (C.6).

[[Septic Tank|Septic Tanks]] ===Health Aspects/Acceptance=== If well constructed and Solids-Free Sewers can be built for new areasmaintained, or sewers are a Solids-Free Sewer can be connected to an existing primary treatment technology where local infiltration is inappropriatesafe and hygienic means of transporting wastewater. A Solids-Free Sewer can Users must be built for 20% to 50% less than [[Conventional Gravity Sewer|Conventional Gravity Sewerage]]well trained regarding the health risks associated with removing blockages and maintaining interceptor tanks.

This technology must be connected to an appropriate [[(Semi-) Centralized Treatment System|(Semi-) Centralized Treatment technology]] that can receive the wastewater. It is appropriate for densely populated areas where there is no space for a [[Soak Pit]] or [[Leach Field]]. This type of sewer is best suited to urban and less appropriate in low-density or rural areas. ===Health Aspects/AcceptanceOperation & Maintenance=== This technology requires regular maintenance on Trained and responsible users are essential to avoid clogging by trash and other solids. Regular desludging of the part Septic Tanks is critical to ensure optimal performance of the users and is therefore, not as passive as [[Conventional Gravity Sewer|Conventional Gravity Sewers]]sewer. Users must assume some level Periodic flushing of responsibility for the technology and accept pipes is recommended to insure against blockages. Special precautions should be taken to prevent illegal connections, since it is likely that some potentially unpleasant maintenance may interceptors would not be requiredinstalled and solids would enter the system. AlsoThe sewerage authority, a private contractor or users committee should be aware that, because responsible for the management of the system is community based, they may have particularly, to work with ensure that the interceptorsare regularly de-sludged and/or coordinate maintenance activities with other users. The system will provide a high level of service and may offer a significant improvement to non-functioning [[Leach Field|Leach Fields]]prevent illegal connections.

 Solids-Free Sewers are good upgrading options for [[Leach Field|Leach Fields]] that have become clogged and/or saturated with time as well as for rapidly growing areas that would not accommodate more [[Septic Tank|Septic Tanks]] with Leach Fields. ===Maintenance=== The septic/interceptor tank must be regularly maintained and desludged to insure optimal performance of the Solids-Free Sewer network. If the pre-treatment is efficient, the risk of clogging in the pipes is low, but some maintenance will be required periodically. The sewers should be flushed once a year as part of the regular maintenance regardless of their performance.

* Azevedo Netto, MM. and Reid, R. (1992). Innovative and Low Cost Technologies Utilized in Sewerage. Environmental Health Program, Technical Series No. 29. Pan American Health Organization, Washington DC. (A Short summary and component diagrams-Chapter 5.) * Crites, R. and Tchobanoglous, G. (1998). Small and Decentralized Wastewater Management Systems. WCB and /McGraw-Hill, New York, USAUS. pp 355–364. 355-364. (Book; A short summary of design and construction considerations.)

* Mara, DDD. D. (19961996a). [https://www.susana.org/en/knowledge-hub/resources-and-publications/library/details/2972 Low-Cost Sewerage]. Wiley, ChicheserChichester, UK. (Assessment of different low-cost systems and case studies.)

* Mara, DDD. D. (19961996b). Low-Cost Urban Sanitation. Wiley, Chichester, UK. pp 93–108. 93-108. (Book; Comprehensive summary including design examples.)

* Otis, RJR. J. and Mara, DDD. D. (1985). [https://www.ircwash.org/resources/design-small-bore-sewer-systems The Design of Small Bore Sewer Systems (]. UNDP Interreg. Interregional Project INT/81/047). TAG Technical Note No.14. United Nations Development Programme + , The World Bankand UNDP, Washington, D. Available: wwwC.wds, US.worldbank.org (Comprehensive summary of design, installation and maintenance.)