What are they? Sewered interceptor tank systems (SITS) are sewer systems that include interceptor tanks on house connections, designed to settle solids and allow solids-free sewage to continue on to the sewers. Because the interceptor tanks reduce the peak flows, sewers can be smaller than for conventional systems. Because they do not have to carry solids, they can be laid to flatter gradients than conventional sewers. SITS are also commonly referred to as “settled sewerage”.
When to use them? Consider their use in the following circumstances:
Existing household septic tanks can be used as interceptor tanks.
The area is very flat so that the reduction in fall relative to conventional sewers reduces the need for sewage pumping.
There is a high water table so that the reduced fall reduces or removes the need to lay sewers below the water table.
Advantages: SITS can be cheaper than conventional sewers, particularly in the situations identified in the “When to use them?” section above. They also tend to concentrate maintenance requirements at the interceptor tanks. Even when poorly maintained, they may catch large solids and so reduce the incidence of blockage in sewers.
Disadvantages: These systems will require periodic desludging of the interceptor tanks to avoid overflows of sewage. Apart from this expense, the institutional arrangements for fecal sludge management will have to be strong and effective from the outset to avoid system failure. Government authorities may not recognize and accept the system. In areas with good falls, it may be more expensive than conventional sewerage. While interceptor tanks will reduce oxygen demand by a factor of perhaps 30%, there will still be a need for treatment.
Technical requirements: Many interceptor tanks are rectangular in plan and designed with two compartments like a conventional septic tank. A circular plan will be cheaper and only marginally less efficient than a conventional tank. Some designs provide capacity equivalent to that of a septic tank (See septic tank note). Others provide small tanks, which are likely to fill quickly with sludge and thereafter operate mainly as a trap for large objects. The tank should have a T pipe on the outlet. SITS systems have operated successfully with sewer diameters as low as 38mm but a more normal minimum diameter would be 50mm or 75mm. These low diameters should only be used if regular desludging of interceptor tanks can be guaranteed.
What is it? Sewers are closed conduits, usually circular in cross-section section which carry wastewater flowing by gravity. Sewerage refers to systems of sewers, and includes pump stations, overflows and other associated infrastructure. Most sewers are designed to convey either sewage or storm water but many are “combined sewers” and carry both, in practice if not in theory. Some sewers, mainly in low-income areas with on-plot sanitation, have been designed to carry only sullage and storm water. ‘Conventional’ sewerage (designed in accordance with the building norms, practices and traditions of the industrialized world) is normally assumed to be unaffordable to low-income households and communities and thus inappropriate for use in the areas where they live. In fact, many people living in low-income areas do use simple forms of sewerage to manage their liquid wastes. The cost of these simplified systems is often comparable with that of other sanitation systems.
When to use? Consider sewerage when water use is greater than at least 60 liters per person per day and housing densities make it difficult to dispose of wastewater on or near plots. (This will typically be the case at population densities greater than about 200 people per hectare). Explore other options, including sewered interceptor tank system (SITS), in flat areas in which pumping is necessary and particularly where the groundwater table is high. If pumping is unavoidable, make sure that the financial and management arrangements required to support it are in place.
Advantages: Sewerage is an attractive option for users because it removes problems from their doorsteps, at least as long as it is operating satisfactorily. It deals with both fecal wastes and sullage water and can also be used to deal with storm water.
Disadvantages: There will usually be a need for treatment to protect the environment. Sewers in low-income areas often require high levels of maintenance, particularly in areas where solid waste collection is deficient or non-existent.
Technical requirements: The focus here is on the technical requirements to reduce costs and improve operation, in accordance with the principles of “condominial” and “simplified” sewerage:
Limit the sewer depth where possible. Do this by routing sewers through gardens and yards, beneath sidewalks and/or in narrow lanes, thus avoiding heavy traffic.
For shallow sewers, use small inspection chambers rather than large manholes. The purpose of manholes and chambers is to gain access to the sewer and this can be done from ground level if the sewer depth is less than about 1,25 meters). Provide benching up to the crown level of the pipe in manholes and chambers.
Use appropriate locally available materials. Spun concrete pipes can be appropriate in some circumstances but may suffer corrosion if there are blockages and/or insufficient slope to prevent hydrogen sulfide generation.
Pay particular attention to the design of manhole covers and ensure that covers can be replaced if they break, in order to minimize entry of garbage and silt to the sewer.