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Sewerage Systems

Conventional approaches
The collection and disposal of waste and wastewater is essential in order to control the transmission of waterborne diseases and to prevent degradation of the environment, including groundwater and surface waters. The standard 'Western' service level of water supply comprises high quality piped water with multiple connections per household.

This concept results in high water consumption and produces large volumes of rather dilute wastewater that needs to be collected via an extensive sewer system and is finally treated in centralised treatment works.

The trend in the industrialised world is towards further development and improvement of these conventional systems. This requires highly skilled labour, large amounts of capital and steady socio-economic conditions with regard to finance and chemical supplies, etc. Although this conventional approach is the standard in the industrialised countries, its application as standard solution for developing countries is not always realistic.

Developing countries
There is at present only limited infrastructure for the effective treatment of sewage in developing countries. Municipal sewerage and the extent of domestic and industrial wastewater treatment are inadequate in most urban situations. When there is a municipal sewerage network in place, the coverage is usually incomplete and the treatment level is insufficient. Even when treatment facilities exist, poor maintenance and operation often results in failing treatment processes, causing pollution of the effluent receiving surface waters. The risk of water borne diseases may actually increase in developing countries as a result of the introduction of a conventional sewerage scheme, where it is not accompanied by effective end-of-pipe treatment.

Beyond the latrine
The diversity of current provision patterns cautions against universal policy prescriptions.
Much of Sub-Saharan Africa has low coverage by sewerage networks, with less than 10% of the urban population connected. The same holds for countries at higher average incomes. Cities such as Jakarta and Manila have lower levels of sewerage coverage (8%–10%) than West African cities such as Dakar and Abidjan. Where coverage levels are low but cities have extensive trunk sewer systems, the costs of connecting households through feeder systems may not be prohibitive. Costs rise rapidly, however, where household connections would require large investments in trunk sewerage provision.

Moving from open defecation at one extreme to the safe collection, storage and disposal of human excreta and the treatment or recycling of sewage effluents poses different challenges in different contexts. In rural areas sewerage networks are often not available. Improved sanitation usually means passing through a hierarchy of pit latrines, with pour-flush latrines and septic tank latrines the plausible options. In urban areas the picture is more mixed. For high-density urban areas sewerage systems have obvious advantages. Connections to feeder sewers and trunk sewers are the safest way to separate people and drinking water from human waste: an age-old human development challenge. But where the reach of the sewerage network is limited and the unserved population is large, the capital costs of developing a sewerage system capable of connecting all households can be prohibitive. Under these conditions onsite sanitation or public facilities may be the most viable short- to medium-run option.

Source: Human development report 2006 chapter 3