Simplified Sewerage - Investment Cost Comparison

Investment Cost Comparison

The cost of sewerage - conventional or simplified - are always site-specific, and estimates are subject to controversies.

Construction costs of simplified sewerage are up to half the costs of conventional sewerage. Investment cost savings come from various design features that may or may not be present in a particular simplified sewerage system.

Cost-saving features of any simplified sewerage system are a smaller diameter of pipes, smaller and shallower trenches and simplified manholes. The two latter features are estimated to account for most of the cost savings.

Other features that could further reduce costs may only be present in some systems, such as:

• shorter networks;
• avoidance of the need to damage pavements and sidewalks (if they already exist and if pipes are laid in front or back yards);
• decentralized, small-scale wastewater treatment, and consequently elimination of main collectors and sewage pumping stations.

An element that may slightly increase costs compared to conventional sewerage is the introduction of grease traps or of interceptor chambers for the settlement of sludge. The latter are more common in South Asia and are not used in the condominial model.

A recent study of four countries shows cost savings of 31-57% from the use of simplified sewerage compared to conventional sewerage with unit costs varying from US$ 119 per connection in a neighborhood in Bolivia and to US$ 759 per connection in a small town in Paraguay. A detailed estimate gives the costs of simplified sewerage in Lima as at least US$ 700 per household (US$120–140 per person), including in-house sanitary facilities (US$ 100 per household) and including design, supervision and social intermediation costs (US$ 126 per household, which are common costs shared with water infrastructure), but excluding taxes.

In general, at higher population densities sewer systems are cheaper than on-site sanitation (such as septic tanks). The switching value at which sewerage becomes less costly is largely determined by the type of sewerage, conventional or simplified. A 1983 study in Natal showed that the investment costs for simplified sewerage were lower than for on-site systems at the quite low population density of about 160 people per hectare. Conventional sewerage, however, was cheaper only at densities above 400 people per hectare.