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[[Image:Icon_solids_free_sewer.png |right|95px80px]]'''A Solids-Free Sewer is a network of small diameter pipes that transports solids-free or pre-treated wastewater (such as [[Septic Tank]] or settling tank effluent) to a treatment facility for further treatment or 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 sewers.''' A precondition for Solids-Free Sewer networks is efficient pre-treatment at the household level. The interceptor, septic or settling tank removes settleable particles that could clog small pipes. A grease trap should also be added. Because there is little risk of clogging, the sewers do not have to be self-cleaning (i.e. no minimum flow velocity) and can therefore be laid at shallow depths, can have fewer inspection points (manholes), can follow the topography more closely and have inflective gradients (i.e. negative slope). 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, care should be taken with negative slopes as they may lead to surging above the ground level during peak flows. Inspection points should be provided at major connection points or when the size of the pipe changes.

Despite the possibility that some portions of the pipe can rise higher when going downstream, for the total pipe the downstream end of the '''A solids-free sewer must be lower than the upstream end. When choosing is a pipe network of small-diameter pipes that transports pre-treated and solids-free wastewater (such as Septic Tank effluent). It can be installed at least 75mm), the a shallow depth of water in the pipe during peak and does not require a minimum wastewater flow within each section must be less than the diameter of the pipe. In sections where there is pressure flow, the invert of any interceptor tank outlet must higher than the hydraulic head within the sewer just prior or slope to the point 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 laidfunction.'''

{{procontable | pro=- Greywater can be managed at the same time. <br> Solids- Can be built and repaired with locally available materialsfree sewers are also referred to as settled, smallbore, variable-grade gravity, or septic tank effluent gravity sewers. <br> A precondition for solids- Construction can provide short-term employment to local labourersfree sewers is efficient primary treatment at the household level. <br> An interceptor, typically a single- Capital costs are less than chamber [[Conventional Gravity Sewer|Conventional Gravity SewersSeptic Tank]]; low operating costs(S. <br> - Can be extended as a community changes and grows9), captures settleable particles that could clog small pipes. | con=- Requires expert design and construction supervisionThe solids interceptor also functions to attenuate peak discharges. <br> - Requires repairs Because there is little risk of depositions and removals of blockages more frequently than a Conventional Gravity Sewer. <br> clogging, solids- Requires education and acceptance free sewers do not have to be used correctlyself-cleansing, i.e., no minimum flow velocity or tractive tension is needed. <br> - Effluent and sludge They require few inspection points, can have inflective gradients (from interceptorsi.e., negative slopes) requires secondary treatment and/or appropriate dischargefollow the topography.<br>When the sewer roughly follows the ground contours, the flow is allowed to vary between open channel and pressure (full-High water consumption for excreta removalbore) flow. }}==Adequacy==

Solids-Free Sewers ===Design Considerations=== If the interceptors are appropriate for both full correctly designed and partially filled flowsoperated, this type of sewer does not require self-cleansing velocities or minimum slopes. Although a constant supply Even inflective gradients are possible, as long as the downstream end of water the sewer is lower than theupstream end. In sections where there is requiredpressure flow, less the water is needed compared to level in any interceptor tank must be higher than the hydraulic head within the sewer, otherwise the liquid will flow back into the tank. At high points in sections with pressure flow, the Simple Sewer because self-cleansing velocities are not requiredpipes must be ventilated.

[[Septic Tank|Septic Tanks]] and Solids-Free Sewers can free sewers do not have to be built installed on a uniform gradient with a straight alignment between inspection points. The alignment may curve to avoid obstacles, allowing for new areasgreater construction tolerance. A minimum diameter of 75 mm is required to facilitate cleaning. Expensive manholes are not needed because access for mechanical cleaning equipment is not necessary. Cleanouts or flushing points are sufficient and are installed at upstream ends, high points, intersections, or a Solids-Free Sewer can be connected major changes in direction or pipe size. Compared to an existing primary treatment technology where local infiltration is inappropriate. A Solids-Free Sewer manholes, cleanouts can be built for 20% more tightly sealed to 50% less than [[Conventional Gravity Sewer|Conventional Gravity Sewerage]]prevent stormwater from entering.

This technology Stormwater must be connected excluded as it could exceed pipe capacity and lead to blockages due to an appropriate [[(Semigrit depositions. Ideally, there should not be any storm-) Centralized Treatment System|(Semi-) Centralized Treatment technology]] that can receive and groundwater in the wastewatersewers, but, in practice, some imperfectly sealed pipe joints must be expected. It is appropriate for densely populated areas where there is no space for a [[Soak Pit]] or [[Leach Field]]Estimates of groundwater infiltration and stormwater inflow must, therefore, be made when designing the system. This type The use of PVC pipes can minimize the risk of sewer is best suited to urban and less appropriate in low-density or rural areasleakages.

==Health Aspects/Acceptance=<br>{{procontable | pro=- Does not require a minimum gradient or flow velocity <br>This technology requires regular maintenance on the part of the users and - Can be used where water supply is therefore, not limited <br>- Lower capital costs than conventional gravity sewers; low operating costs <br>- Can be extended as passive as [[Conventional Gravity Sewera community grows <br>- Greywater can be managed concurrently|Conventional Gravity Sewers]]. Users must assume some level of responsibility con=- Space for the technology interceptors is required <br>- Interceptors require regular desludging to prevent clogging <br>- Requires training and accept that some potentially unpleasant maintenance may acceptance to be required. Also, users should be aware that, because the system is community based, they may have to work with used correctly <br>- Requires repairs and removals of blockages more frequently than a conventional gravity sewer <br>- Requires expert design and/or coordinate maintenance activities with other users. The system will provide construction <br>- Leakages pose a high level risk of service wastewater exfiltration and groundwater infiltration and may offer a significant improvement are difficult to non-functioning [[Leach Field|Leach Fields]].identify}}

==Upgrading=Appropriateness=== 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 good upgrading options for As opposed to a [[Leach Field|Leach FieldsSimplified Sewer]] (C.4) a solids-free sewer can also be used where domestic water consumptionis limited. This technology is a flexible option that have become clogged can be easily extended as the population grows. Because of shallowexcavations and/or saturated with time as well as for rapidly growing areas that would not accommodate more the use of fewer materials, it can be built at considerably lower cost than a [[Septic TankConventional Gravity Sewer |Septic TanksConventional Sewer]] with Leach Fields(C.6).

==Maintenance=Health Aspects/Acceptance=== If well constructed and maintained, sewers are a safe and hygienic means of transporting wastewater. Users must be well trained regarding the health risks associated with removing blockages and maintaining interceptor tanks.

The septic/interceptor tank must be regularly maintained ===Operation & Maintenance=== Trained and desludged responsible users are essential to insure avoid clogging by trash and other solids. Regular desludging of the Septic Tanks is critical to ensure optimal performance of the Solids-Free Sewer networksewer. If the pre-treatment is efficient, the risk Periodic flushing of clogging in the pipes is lowrecommended to insure against blockages. Special precautions should be taken to prevent illegal connections, but some maintenance will since it is likely that interceptors would not be required periodicallyinstalled and solids would enter the system. The sewers sewerage authority, a private contractor or users committee should be flushed once a year as part responsible for the management of the regular maintenance regardless of their performancesystem, particularly, to ensure that the interceptorsare regularly de-sludged and to prevent illegal connections.

==References=Upgrading===Solids-Free Sewers are good upgrading options for Leach Fields that have become clogged and/or saturated with time as well as for rapidly growing areas that would not accommodate more Septic Tanks with Leach Fields.

===References===* Azevedo Netto, MMJ. and Reid, R. (1992). [https://www.ircwash.org/sites/default/files/332-92IN-9897.pdf Innovative and Low - Cost Technologies Utilized in Sewerage]. Environmental Health Program, Technical Series No. 29. , Environmental Health Program, Pan American Health Organization, Washington DC, D.C., US. (A Short 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.)

===Acknowledgements===