Simplified and Condominal Sewers

The high rate of urbanisation creates high-density low-income areas in many developing countries. In this context, simplified sewerage is technically and institutionally feasible, economically appropriate and financially an affordable sanitation option (MARA 1996). Wastewater is collected, pre-settled and then transported to the main network or directly to a semi-centralised secondary treatment system such as constructed wetlands (free-surface, horizontal or vertical), or waste stabilisation ponds). The sludge from interceptor tanks (and other pre-settling units) needs also secondary treatment (see also settling and thickening, drying beds, non-planted filters, mechanical dewatering, composting, further anaerobic digestion at large scale), after emptying (see human powered or motorised emptying and transport). 

Because simplified sewers are laid on or around the property of the users, higher connection rates can be achieved, fewer and shorter pipes can be used and less excavation is required as the pipes will not be subject to heavy traffic loads. However, this type of conveyance technology requires careful negotiation between stakeholders (see stakeholder analysis), since design and maintenance must be jointly coordinated.

Conceptually, simplified sewerage is the same as Conventional Gravity Sewerage, but without unnecessarily conservative design standards and with design features that are better adapted to the local situation. The pipes are usually laid within the property boundaries, through either the back or front yards, rather than beneath the central road, allowing for fewer and shorter pipes. Because simplified sewers are typically installed within the condominium, they are often referred to as condominial sewers. The pipes can also be routed in access ways, which are too narrow for heavy traffic, or underneath pavements (sidewalks). Since simplified sewers are installed where they are not subjected to heavy traffic loads, they can be laid at a shallow depth and little excavation is required.

In contrast to conventional sewers that are designed to ensure a minimum self-cleansing velocity, the design of simplified sewers is based on a minimum tractive tension of 1 N/m2 (1 Pa) at peak flow. The minimum peak flow should be 1.5 L/s and a minimum sewer diameter of 100 mm is required. A gradient of 0.5% is usually sufficient. For example, a 100 mm sewer laid at a gradient of 1 m in 200 m will serve around 2,800 users with a wastewater flow of 60 L/person/day.

PVC pipes are recommended to use. The depth at which they should be laid depends mainly on the amount of traffic. Below sidewalks, covers of 40 to 65 cm are typical. The simplified design can also be applied to sewer mains; they can also be laid at a shallow depth, provided that they are placed away from traffic.

Expensive manholes are normally not needed. At each junction or change in direction, simple inspection chambers (or cleanouts) which can also take the function of interceptor tanks are sufficient to prevent solids to enter the main system . Inspection boxes are also used at each house connection. Where kitchen greywater contains an appreciable amount of oil and grease, the installation of grease traps (see Pre-Treatment Technologies) or a settling unit (e.g. imhoff tank, septic tank, biogas settler is recommended to prevent clogging.

Greywater should be discharged into the sewer to ensure adequate hydraulic loading, but stormwater connections should be discouraged in order to prevent overload during heavy rains . However, in practice it is difficult to exclude all stormwater flows, especially where there is no alternative for storm drainage. The design of the sewers (and treatment plant) should, therefore, take into account the extra flow that may result from stormwater inflow.

Blocks of community-based simplified sewers are connected to an existing conventional gravity sewer or routed to a simplified sewer main constructed with pipes of a larger diameter. Oftentimes, the community will purchase, and connect to, a single legal connection to the main sewer; the combined effluent of the condominal sewer network then flows into the main sewer line or directly to a semi-centralised secondary treatment system.

(Adapted from MARA 1996)

The costs of simplified sewerage systems are low (50 to 80 % less expensive than conventional gravity sewerage), sometimes even lower than those of onsite sanitation. This is because simplified sewerage (especially condominal sewerage), in common with settled sewerage (see also solids-free sewers), uses shallow excavation depths, small diameter pipe and simple inspection units (in place of large manholes). Additionally, the sewer gradients of simplified sewerage are much flatter than those of conventional systems requiring smaller excavation for laying the network. Also, the costs for emptying interceptors and inspection chambers or, if existing, the grease traps or pre-settling must be considered.

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 inspection chambers.

Trained and responsible users are essential to ensure that the flow is undisturbed and to avoid clogging by trash and other solids. Occasional flushing of the pipes is recommended to insure against blockages. Blockages can usually be removed by opening the cleanouts and forcing a rigid wire through the pipe. Inspection chambers and interceptor tanks must be periodically emptied to prevent grit overflowing into the system. The system should also be checked by the authority responsible for the main sewer network, because the system will be at risk if solids can enter. The sludge in the interceptor tank and pre-settling untis also needs to be removed regularly and appropriately treated (see above). The operation of the system depends on clearly defined responsibilities between the sewerage authority and the community. Ideally, households will be responsible for the maintenance of pre-treatment units and the condominial part of the sewer. However, in practice this may not be feasible because users may not detect problems before they become severe and costly to repair. Alternatively, a private contractor or users committee can be hired to do the maintenance.

Besides checking the interceptor tanks and inspection chambers, the main organisational activity of the authority is to provide desludging services. The principal problems related to desludging revolve around responsibility. Normally, this lies with the property owners, since the interceptor tanks are on their property. But residents who are not owners have no incentive to desludge the tanks regularly, since this costs money and is inconvenient, and the overflowing sludge in the sewerage system does not directly affect them (even though it will affect the communal sewerage system downstream). If the sewer system is to work effectively, responsibility for tank desludging must fall on the organisation responsible for communal sewer maintenance. This organisation should also bear responsibility for treating effluents from the sewers.