Simplified sewerage

Schematic of a simplified sewer: Smaller diameter pipes are laid at a shallower depth and at a flatter gradient than for conventional sewers.[1]

Simplified sewerage, also called small-bore sewerage, is a sewer system that collects all household wastewater (blackwater and greywater) in small-diameter pipes laid at fairly flat gradients. Simplified sewers are laid in the front yard or under the pavement (sidewalk) or - if feasible - inside the back yard, rather than in the centre of the road as with conventional sewerage. It is suitable for existing unplanned low-income areas, as well as new housing estates with a regular layout.[2] It allows for a more flexible design.[1] With simplified sewerage it is crucial to have management arrangements in place to remove blockages, which are more frequent than with conventional sewers. It has been estimated that simplified sewerage reduces investment costs by up to 50% compared to conventional sewerage.

Simplified sewerage is sometimes also referred to as conventional sewerage with appropriate standards, implying that most conventional sewers are overdesigned.

The concept of simplified sewerage emerged in parallel in Natal, Brazil and Karachi, Pakistan in the early 1980s without any interaction or communication. In both cases particular emphasis was given to community mobilization, an essential element for the success of simplified sewerage. In Latin America, and particularly in Brazil, simplified sewerage is also known as condominial sewerage, a term that undercores the importance of community participation in planning and maintenance at the level of a housing block (known as condominio in the Spanish and Portuguese use of the term).

Background

In developing countries, connection to sewer systems is often costly for poor households, despite typically low monthly sewer tariffs. This apparent paradox is explained by the high costs of in-plot and in-house sanitary installations that have to be paid entirely by the user, by sometimes high sewer connection fees levied by utilities, and by a lack of community consultation. As a result, in many cities in developing countries conventional sewers are laid at high costs under a street, while many users on that street do not connect to them. In Brazil, in some cities connection rates in the early 1990s were less than 40% of the intended beneficiary population.[3]

Application

Simplified sewerage is most widely used in Brazil. It is estimated that in Brazil some 5 million people in over 200 towns and cities are served with simplified sewerage - or condominial sewerage.[4] This corresponds to about 3% of the population of Brazil and about 6% of the population connected to sewers. They serve poor and rich alike.

Simplified sewerage has also been used in

In Pakistan, beginning with the Orangi Pilot Project in Karachi, a variation of simplified sewerage using larger diameter pipes has been used.

Community participation

Community participation in the planning of any sewer system is a fundamental requirement to achieve higher household connection rates and to increase the likelihood of proper maintenance of in-block sewers. In addition, it can motivate users to assume parts of the costs of the sewer system that they are able to assume, such as contribution of labor for construction and/or maintenance.

Typically, in the planning process for a simplified sewerage system, meetings are carried out at the housing block (condominio) level for information, discussions and clarifications required for a joint group decision on network design, community contributions during construction and maintenance responsibilities. Users might finance and implement in-house sanitary installations and household connections and would agree on a suitable type of condominial branch. They are asked to comply with agreements established for construction and operation of the condominial branch, as well as payment of tariffs. In turn, the service provider agrees to fulfill his responsibilities as established in the “Terms of Connection ” between the parties.[4] The community participation process also provides a good opportunity for complementary actions like hygiene promotion, which can have a significant impact on public health at a relatively limited cost.

Design and construction

Simplified sewers are usually laid in the front yard or under the pavement (sidewalk). In some rare cases it is possible to lay them in the back yard.[10] Sidewalk branches are usually preferred in regular urbanizations, while the front and back yard branches are particularly suited to neighborhoods with challenging topography or urbanization patterns. However, in some cases neither of these options is possible. For example, in South Asia, in many cities there is no sidewalk or front yard, so pipes have to be laid in the middle of the street as with conventional sewers.[11]

In Latin America typical simplified sewer diameters are 100 mm, laid at a gradient of 1 in 200 (0.5 percent). Such a sewer will serve around 200 households of 5 people with a wastewater flow of 80 litres per person per day. In Pakistan, however, there are no rigorous standards for sewer diameters. In a small pilot as part of the Orangi Pilot Project pipes with a diameter of 150mm were used.[12]

Laying small diameter pipes at fairly flat gradients requires careful construction techniques. Plastic pipes are best used as they are more easily jointed correctly. This reduces wastewater leakage from the sewer and groundwater infiltration into it. With simplified sewerage there is no need to have the large expensive manholes of the type used for conventional sewerage — simple brick or plastic junction chambers are used instead.[2]

Construction can be carried out by contractors or by trained and properly supervised community members. Training and proper supervision are actually needed in both cases, since contractors in many cities are not familiar with simplified sewerage.

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:

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 2006 study of four countries showed 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.[13] 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.[14]

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.[2]

Operation and maintenance

Good operation and maintenance (O&M) is essential for the long-term sustainability of any sewerage system, but particularly for simplified sewerage, since the low diameter of pipes and low gradients make the system more vulnerable to clogging.

The original concept of householders being responsible for O&M of in-block condominial sewers has not worked well in the long term.[15] A study of simplified sewerage systems in Brazil has shown that effective maintenance of sewers by utilities has often been the result of community pressure by neighborhood associations. Without such pressure maintenance by utilities has often been inadequate, and community maintenance has not come about either.[16]

Therefore, alternative management systems had to be developed and a few examples are provided below:

Concerning maintenance costs, available information indicates similar costs and requirements for the simplified and the conventional system under the same conditions. Simplified systems typically require more interventions, but the cost per intervention is lower. Comparative analytical studies are not yet available, however.

Constraints for application

According to Jose Carlos Melo, who is considered to be the "father" of condominial sewers in Brazil, some important constraints for the application of simplified sewerage are:[4]

Over the last years, countries like Bolivia and Peru reviewed and modernized technical standards according to methods and criteria established and accepted in Brazil in the 1980s, thus overcoming the latter constraint.

See also

References

  1. 1 2 Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C.: Compendium of Sanitation Systems and Technologies - (2nd Revised Edition). Swiss Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf, Switzerland, 2014
  2. 1 2 3 4 Simplified Sewerage Design Manual, Duncan Mara, Leeds University
  3. ↑
    • Gabrielle Watson: Good Sewers Cheap? Agency-Customer Interactions in Low-Cost Urban Sanitation in Brazil, The World Bank, 1995, p. 7
  4. 1 2 3 Jose Carlos Melo: La ciudad y el saneamiento, 2007
  5. ↑ WSP: IRC/WSP 2004: The El Alto Condominial Sewer Pilot Project in Bolivia, 2004
  6. ↑ Building Partnerships for Development in Water and Sanitation,:The El Alto Condominial Sewer Pilot Project in Bolivia, 2000
  7. ↑ 10 UNICEF/Agua para todos 2005
  8. ↑ UNICEF:The construction of low-cost sewerage systems in Tegucigalpa: A feasible solution for the urban poor?, 1997
  9. ↑ Condominial Sewer Pilot Project in Johannesburg, South Africa, 2002
  10. ↑ In Brazil this has apparently been done in 10% of the cases, according to a communication by Klaus Neder
  11. ↑ Communication from Kevin Tayler
  12. ↑ Communication by Kevin Tayler
  13. ↑ Miguel Vargas-Ramírez and Teresa Cristina Lampoglia: Scaling-up using condominial technology, waterlines, Vol. 25, No. 2, October 2006, p. 11-13
  14. ↑ Teresa Lampoglia of the World Bank-funded PAC project in Lima; communication by Martin Gauss, WSP
  15. ↑ Roszler, MIT: Reasons for the failure of community participation in the maintenance of Brazilian condominial sewers
  16. ↑ Gabrielle Watson: Good Sewers Cheap? Agency-Customer Interactions in Low-Cost Urban Sanitation in Brazil, The World Bank, 1995, p. 43-44
  17. ↑ Neder and Nazareth:Condominial sewer systems for the Federal District of Brazil, 1983

External links

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