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'''A precondition for Solidssolids-Free Sewer networks free sewer is efficient a network of small-diameter pipes that transports 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 selftreated and solids-cleaning free wastewater (isuch as Septic Tank effluent).e. no minimum flow velocity) and It can therefore be laid installed at a shallow depths, can have fewer inspection points (manholes), can follow the topography more closely depth and have inflective gradients (i.e. negative does not require a minimum wastewater flow or 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 changesfunction.'''

Despite the possibility that some portions of the pipe can rise higher when going downstream<br>Solids-free sewers are also referred to as settled, smallbore, variable-grade gravity, or septic tank effluent gravity sewers. A precondition for solids-free sewers is efficient primary treatment at the total pipe the downstream end of the sewer must be lower than the upstream endhousehold level. When choosing An interceptor, typically a pipe diameter single-chamber [[Septic Tank]] (at least 75mmS.9), the depth of water in the pipe during captures settleable particles that could clog small pipes. The solids interceptor also functions to attenuate peak flow within each section must be less than the diameter of the pipedischarges. In sections where Because there is pressure flowlittle risk of depositions and clogging, the invert of any interceptor tank outlet must higher than the hydraulic head within the sewer just prior solids-free sewers do not have to the point of connection otherwise the liquid will backflow into the tankbe self-cleansing, i.e. If this condition , no minimum flow velocity or tractive tension is not metneeded. They require few inspection points, can have inflective gradients (i.e., then either select negative slopes) and follow the next larger pipe diameter for topography. When the sewer or increase roughly follows the depth at which ground contours, the sewer flow is laidallowed to vary between open channel and pressure (full-bore) flow.

{{procontable | pro===Design Considerations===- Greywater can be managed at If the same time. <br> - Can be built interceptors are correctly designed and repaired with locally available materials. <br> operated, this type of sewer does not require self- Construction can provide short-term employment to local labourerscleansing velocities or minimum slopes. <br> - Capital costs Even inflective gradients are less possible, as long as the downstream end of the sewer is lower than [[Conventional Gravity Sewer|Conventional Gravity Sewers]]; low operating coststheupstream end. <br> - Can In sections where there is pressure flow, the water level in any interceptor tank must be extended as a community changes and grows. | con=- Requires expert design and construction supervision. <br> - Requires repairs and removals of blockages more frequently higher than a Conventional Gravity Sewerthe hydraulic head within the sewer, otherwise the liquid will flow back into the tank. <br> - Requires education and acceptance to At high points in sections with pressure flow, the pipes must be used correctly. <br> - Effluent and sludge (from interceptors) requires secondary treatment and/or appropriate dischargeventilated.<br>-High water consumption for excreta removal. }}

SolidsStormwater must be excluded as it could exceed pipe capacity and lead to blockages due to grit depositions. Ideally, there should not be any storm-Free Sewers are appropriate for both full and partially filled flowsgroundwater in the sewers, but, in practice, some imperfectly sealed pipe joints must be expected. Although a constant supply Estimates of water is requiredgroundwater infiltration and stormwater inflow must, less water is needed compared to therefore, be made when designing the system. The use of PVC pipes can minimize the Simple Sewer because self-cleansing velocities are not requiredrisk of leakages.

[[Septic Tank<br>{{procontable |Septic Tanks]] and Solidspro=-Free Sewers can be built for new areas, Does not require a minimum gradient or a Solidsflow velocity <br>-Free Sewer can Can be connected to an existing primary treatment technology used where local infiltration water supply is inappropriate. A Solidslimited <br>-Free Sewer Lower capital costs than conventional gravity sewers; low operating costs <br>- Can be extended as a community grows <br>- Greywater can be built managed concurrently| con=- Space for 20% interceptors is required <br>- Interceptors require regular desludging to 50% less prevent clogging <br>- Requires training and acceptance to be used correctly <br>- Requires repairs and removals of blockages more frequently than [[Conventional Gravity Sewer|Conventional Gravity Sewerage]].a conventional gravity sewer <br>- Requires expert design and construction <br>- Leakages pose a risk of wastewater exfiltration and groundwater infiltration and are difficult to identify}}

===Appropriateness=== This technology must be connected type of sewer is best suited to an appropriate [[medium-density (Semiperi-) Centralized Treatment System|(Semiurban areas and less appropriate in low-) Centralized Treatment technology]] that can receive the wastewaterdensity or rural settings. It is most appropriate for densely populated areas where there is no space for a [[Soak Pit]] or [[Leach Field]](D. This type 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 best suited no longer appropriate (e.g., due to urban increased housing density and less appropriate in low-density /or rural areaswater use).

This technology requires regular maintenance on the part ===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. ===Operation & Maintenance=== Trained and responsible users are essential to avoid clogging by trash and other solids. Regular desludging of the Septic Tanks is therefore, not as passive as [[Conventional Gravity Sewer|Conventional Gravity Sewers]]critical to ensure optimal performance of the 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 and/or coordinate maintenance activities with other users. The system will provide a high level of service ensure that the interceptorsare regularly de-sludged 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.)