Conventional Sewers (Combined Sewers)

The conventional gravity sewer system is designed with many branches. Typically, the network is subdivided into primary (main sewer lines along main roads), secondary and tertiary networks (networks at the neighbourhood and household level). This system is mostly found in urban areas.

There are basically three types of sewer lines for conventional gravity sewers: primary, secondary and tertiary networks. The main line runs through the centre of the system, and all lines empty into it. It then carries wastewater to a semi-centralised or centralised treatment facility. Main lines generally have a diameter of 200 to 300 mm, and branch lines extend from them much like branches from the trunk of a tree. The sewage from one or more buildings are house laterals, usually of 100 mm in diameter, and empties into a branch line which usually also has a diameter of 100 mm. All sewer pipes are laid out in straight lines whenever possible and generally meet at right angles although the connection may be curved to ease the flow. 

The collection system is the link between the user interface (which is water based in the present case, such as pour-flush toilets or cistern flush toilets) and the gravity sewer. There is no need for on-site storage or pre-treatment with conventional sewers: the product collected in conventional sewers is blackwater mixed with storm- and greywater. Treatment systems are semi-centralised or centralised (e.g. waste stabilisation ponds, aerated ponds or activated sludge systems).

Conventional gravity sewers normally do not require onsite pre-treatment, primary treatment or storage of the household wastewater before it is discharged. The sewer must be designed, however, so that it maintains self-cleansing velocity (i.e., a flow that will not allow particles to accumulate). For typical sewer diameters, a minimum velocity of 0.6 to 0.7 m/s during peak dry weather conditions should be adopted. A constant downhill gradient must be guaranteed along the length of the sewer to maintain self-cleansing flows, which can require deep excavations. When a downhill grade cannot be maintained, a pumping station must be installed. Primary sewers are laid beneath roads, at depths of 1.5 to 3 m to avoid damages caused by traffic loads. The depth also depends on the groundwater table, the lowest point to be served (e.g., a basement) and the topography. The selection of the pipe diameter depends on the projected average and peak flows. Commonly used materials are concrete, PVC, and ductile or cast iron pipes.

Access manholes are placed at set intervals above the sewer, at pipe intersections and at changes in pipeline direction (vertically and horizontally). Manholes should be designed such that they do not become a source of stormwater inflow or groundwater infiltration. 

In the case that connected users discharge highly polluted wastewater (e.g., industry or restaurants), onsite pre- and primary treatment may be required before discharge into the sewer system to reduce the risk of clogging and the load of the wastewater treatment plant.

When the sewer also carries stormwater (known as a combined sewer), sewer overflows are required to avoid hydraulic surcharge of treatment plants during rain events. However, combined sewers should no longer be considered state of the art. Rather, local retention and infiltration of stormwater or a separate drainage system for rainwater are recommended. The wastewater treatment system then requires smaller dimensions and is, therefore, cheaper to build, and there is a higher treatment efficiency for less diluted wastewater.

The primary network requires robust engineering design to ensure that a self-cleansing velocity is maintained, that manholes are placed as required and that the sewer line can support the traffic weight. Furthermore, extensive construction is required to remove and replace the road above.

If well constructed and maintained, sewers are a safe and hygienic means of transporting wastewater. This technology provides a high level of hygiene and comfort for the user. However, because the waste is conveyed to an offsite location for treatment, the ultimate health and environmental impacts are determined by the treatment provided by the downstream facility.

The initial cost of such systems is high (50 to 80% more than simplified sewer systems according to TILLEY et al. 2008) because they need excavation and refilling of trenches to lay the pipes, especially below roads that have to be rebuilt afterwards. The system design is also costly, as it requires specialised engineers. Also the maintenance costs are high compared to decentralised systems and comprise mainly inspection from time to time and eventual unblocking and repair. These can be kept rather low with a careful building design, but they still require specialised operators. The extension of the system can be difficult and costly because entire parts of the system may have to be redesigned due to the flow requirements and dimensioning.

Manholes are used for routine inspection and sewer cleaning. Debris (e.g., grit, sticks or rags) may accumulate in the manholes and block the lines. To avoid clogging caused by grease, it is important to inform the users about proper oil and grease disposal. Common cleaning methods for conventional gravity sewers include rodding, flushing, jetting and bailing. Sewers can be dangerous because of toxic gases and should be maintained only by professionals, although, in well-organised communities, the maintenance of tertiary networks might be handed over to a well-trained group of community members. Proper protection should always be used when entering a sewer. The maintenance is to be systematically planned and carefully implemented. When stormwater is also carried by the sewer (which is then called a combined sewer, as opposed to a separate sewer), sewer overflows are required to avoid hydraulic surcharge of treatment plants during heavy rain events. Local and temporary pollution may occur due to the untreated excess water.