Solids-free Sewer

Solids-free sewer systems are similar to conventional sewer systems, with the difference that the wastewater is pre-settled and solids removed. 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 sewers is efficient primary treatment at the household level.

An interceptor, typically a single-chamber Septic Tank , biogas settler or anaerobic baffled reactor) , captures settleable particles that could clog small pipes. The solids interceptor also functions to attenuate peak discharges. Solids-free sewers bring the pretreated wastewater to a further treatment (e.g. free-surface or horizontal and vertical subsurface flow wetlands, waste stabilisation ponds, etc.) or to a discharge point connected to another sewer system.

Because there is little risk of depositions and clogging, solids-free sewers do not have to be self-cleansing, i.e., no minimum flow velocity or tractive tension (see also vacuum sewers or pressurised sewers) is needed. They require few inspection points, can have inflective gradients (i.e., negative slopes) and follow the topography. Due to the simplified design, solids-free sewers can be built cheaper. Nevertheless, expert design and construction supervision is essential and repairs and removal of blockages may be required more frequently than for a conventional gravity sewer.

Moreover, effluent and sludge (from interceptors) require secondary treatment and/or appropriate discharge (e.g. settling and thickening, drying and mineralization, non-planted filters, mechanical dewatering, composting, further anaerobic digestion at large scale). Small-bore sewers also require a certain level of responsibility of users, because maintenance is high due to the high risk of clogging in case of bad operation and maintenance (e.g. clandestine discharge of greywater etc. that has not been pre-settled).

When the sewer roughly follows the ground contours, the flow is allowed to vary between open channel and pressure (full-bore) flow.

A precondition for solids-free sewer networks is an efficient pre-treatment at the household level. If the interceptors (e.g. septic tank, biogas settler or anaerobic baffled rector) are correctly designed and operated, this type of sewer does not require self-cleansing velocities or minimum slopes. Even inflective gradients are possible, as long as the downstream end of the sewer is lower than the upstream end. Solids-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 greater construction tolerance.

When the sewer roughly follows the ground contours, the flow in the sewer is allowed to vary between open channel flow and pressure flow. However, care should be taken with negative slopes as they may lead to surging above the ground level during peak flows. Inspection points (see picture) should be provided at major connection points or when the size of the pipe changes. At high points in sections with pressure flow, the pipes must be ventilated. A minimum diameter of 75 mm is required to facilitate cleaning. When choosing a pipe diameter (at least 75 mm), the depth of water in the pipe during peak 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 be higher than the hydraulic head within the sewer just prior 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 laid. 

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 major changes in direction or pipe size. Compared to manholes, cleanouts can be more tightly sealed to prevent stormwater from entering. Stormwater must be excluded as it could exceed pipe capacity and lead to blockages due to grit depositions. Ideally, there should not be any storm- and groundwater in the sewers, but, in practice, some imperfectly sealed pipe joints must be expected. Estimates of groundwater infiltration and stormwater inflow must, therefore, be made when designing the system. The use of PVC pipes can minimize the risk of leakages.

Pumping may be necessary where the elevation differences do not permit gravity flow. The operation of a pressure sewer, however, relies on a reliable source of electricity.

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.

Due to the simplified design, solids-free sewers can be built for 20% to 50% less costs than conventional sewerage. However, expert design and construction supervision is essential. Moreover, repairs and removal of blockages may be required more frequently than for a conventional gravity sewer. Also the costs for emptying the pre-settling unit (e.g. septic tank, biogas settler) must be considered.

Trained and responsible users are essential to avoid clogging by trash and other solids. Regular desludging and emptying (see human powered or motorised emptying and transport) of the pre-settling units such as septic tanks , biogas settler , is critical to ensure optimal performance of the sewer. Periodic flushing of the pipes is recommended to insure against blockages.

The risk of pipe clogging is low if the sewers are well operated and maintained, however, some maintenance is required periodically. Regardless of performance, the sewers should be flushed once a year.

Special precautions should be taken to prevent illegal connections, since it is likely that interceptors would not be installed and solids would enter the system.

The sewerage authority, a private contractor or users committee should be responsible for the management of the system, particularly, to ensure that the interceptors are regularly desludged and to prevent illegal connections.