Non-conventional sewerage system : a Latin American case

FAQ sheet on simplified sewerage design, prepared by Cinara, Colombia

Introduction

Access to water and sanitation is considered a basic need and a right of all communities. Its favourable impact on public health and on the economy is a well-known fact. In 2000 Latin America and the Caribbean had a population of about 519 million (391 urban and 128 rural). The urban population in LAC is expected to double by 2025. This situation is a great challenge to water and sanitation, especially in rural settlements and peri-urban areas in big cities, where the most serious problems related to these services are concentrated. It is estimated that 50% of the rural population and 15% of the urban population lack adequate sanitation. This situation is associated with the high cost of conventional technologies.

Non-conventional systems emerge as a technical, social and economic option that contributes towards the improvement of sanitation for population groups that do not have access to sanitation services. In Latin America, simplified sewer designs -or shallow sewerage- and solids-free sewerage (settled sewerage), are two collection options that have indicated substantial potential for further development.

Simplified Sewerage Design

Simplified sewerage is a collective sanitation scheme used to remove excreta and household wastewater. This technology was first developed in Brazil—where it is also known as condominial sewerage—in the early eighties by CAERN, the water and sewerage company of the northeastern State of Rio Grande do Norte.

Conceptually the system is similar to conventional sewerage, but it eliminates conservative design features and tries to match design standards to the local situation.

Various approaches to the design of simplified sewerage have been developed independently of each other but the scheme most widely used is the design developed in Brazil. The key characteristics of this approach are:

Layout

In order to reduce costs, the system is designed to be laid in sidewalks, back or front yards. The idea is to consider this design for use in blocks or housing estates, according to the spatial and architectonic layout of the houses. Figure 1 shows a layout for groups of houses. The designer, together with the users, determine the final layout, which will obviously be subject to the topography and urban development of the area.

Diameter and depth

Simplified sewers are laid at shallow depths, with minimum covers of 40 to 50 cm, since they are not exposed to heavy loads. As we mentioned before, sewers are routed through either sidewalks, back or front yards. The minimum initial sewer diameter is 10 cm rather than the 20 cm diameter that is normally required for conventional sewerage. The relatively shallow depth and diameter of the sewers allow the use of small access chambers from 40x40 cm onwards.

Figure 1. Simplified sewerage layout for groups of houses in planned and unplanned areas. Source: Mara, 2000.

The proposed layouts further reduce costs by laying sewers through back yards at shallow depths away from heavy traffic loads.

Figure 2 shows that as the population density increases, simplified sewerage can become cheaper than on-site sanitation systems. In Natal, Brazil, this occurred at a population density of 160 persons per hectare.

Figure 2. Total annual cost per household in US$ versus Population density in persons per ha. It shows the costs of conventional and simplified (in-block layout) sewerage, and individual sanitation systems in Natal in Northeast Brazil. Prices in 1983. Source: Sinnatamby (1983).

Hydraulic design

Conceptually the hydraulic design of simplified sewerage is not different from the design of conventional sewerage. The first factor is to determine the expected maximum flow of wastewater to be transported. This depends on the population’s water consumption. However, for short distances of simplified sewers this flow can be calculated as 1.5 l/s when the real value is less than the said number. In certain cases, the minimum value allows rainwater accumulated on household roofs to be drained into simplified sewers but only over short distances. For larger projects, a solution for handling rainwater should be found.

There are two methods of calculating the minimum gradient. One is based on the water velocity for self-cleansing that is adopted from a minimum value of 0.5 m/s. This is less than in conventional systems, where water velocity fluctuates between 0.6 and 0.7 m/s and is based on the principle that this velocity exceeds the threshold for movement of the particles transported by wastewater. The other method is based on tractive force, which is the tangential force stressed by the flow of water, by the unit of the wet area. Based on the developments in Brazil, the value of the tractive force of 1 Pascal (Pa) is adequate and guarantees the flow of particles in wastewater. Greater values have a strong influence on the gradient and thus contribute to increased costs, as they require steeper slopes and more excavation.

If households produce an appreciable amount of fat and grease, it is important that it is removed in grease traps before the water is discharged into the sewer. In certain cases, this grease trap can also be used to attenuate the flow of rainwater, especially when there is no separate possibility for draining household water from yards and roofs.

PVC and vitrified clay pipes are recommended. Where possible concrete should be avoided because hydrogen sulphide (H2S) is expected to occur, and the presence of sulphuric acid (H2SO4) can corrode the tubes. Besides being resistant to corrosion, PVC pipes are easy to handle and thus allow for rapid installation and minimise infiltration.

To guarantee successful operation and maintenance (O&M) of a simplified sewerage scheme, there must be an effective partnership between the sewerage authority and the community served. This will enable the correct use of the system and good O&M. It is important that both parties are clear about their duties and responsibilities before the system is put into operation.

Bibliography and additional reading

ABNT (1986). Projeto de Redes Coletoras de esgotos Sanitario. Brazilian Standard No 9649. Associacao Brasileira de Normas Técnicas, Rio de Janeiro, Brasil.

Alcantarillados Condominiales : experiences and aspects of simplified sewerage (Spanish only). Website providing experiences and technical, social and economic aspects of simplified sewerage globally, but mostly in Latin America. (in Spanish only). URL: http://www.col.ops-oms.org/saludambiente/acueductos/default.htm

Cinara/Univalle (2000). Memorias del Seminario-Taller Internacional Saneamiento en asentamientos formales e informales con énfasis en alcantarillados condominiales. Cinara/Universidad del Valle, Cali, Colombia.

Cinara-EMCALI (1997). Proyectos de Aprendizaje en equipo para la zona rural del municipio de Cali. Informe final del proyecto de la comunidad de Altos de Menga. Cinara/Universidad del Valle, Cali, Colombia.

Cinara-EMCALI (1997). Proyectos de Aprendizaje en equipo para la zona rural del municipio de Cali. Informe final del proyecto de la comunidad de la Vorágine. Cinara/Universidad del Valle, Cali, Colombia.

De Melo, J.C.(1985). Uma Solucao eficaz e de baixo custo para o esgotamento sanitario urbano. Engenharia Sanitária, Rio de Janeiro 24 (2), 239-241.

De Melo, J.C.(1994). Sistema condominial de esgotos: Razoes, Teoria e prática. Brasília: Caixa Economica Federal.

Lobo, L. (2000). Guía de proyectos de sistema de alcantarillado Condominial de acuerdo a los criterios utilizados en el Alto, Bolivia. Seminario-Taller Internacional Saneamiento en asentamientos formales e informales con énfasis en alcantarillados condominiales. Cinara/Universidad del Valle, Cali, Colombia.

Mara, D. (2000). Condominial Sewerage: Planning, Design, Construction, Operation and Maintenance. Seminario-Taller Internacional Saneamiento en asentamientos formales e informales con énfasis en alcantarillados condominiales. Cinara/Universidad del Valle, Cali, Colombia.

Mara, D. (1998). Low-Cost Sewerage. In: Simpson Hebert, M.; Wood, S. (eds.). Sanitation Promotion, WSSCC Working Group on Promotion of Sanitation, pp. 249 - 262.WHO, Geneva. WHO/EOS/98.5. URL: http://www.efm.leeds.ac.uk/CIVE/Sewerage/articles/WHO_EOS_98.5_pp249-262.pdf [0.7 MB]

Mara, D. (1996). Low-Cost Sewerage. Chichester: John Wiley & Sons, United Kingdom.

Mara, D.; Sleigh, A. ; Tayler, K (2001). PC- Simplified Sewer Design. Leeds University, United Kingdom. URL: http://www.efm.leeds.ac.uk/CIVE/Sewerage/sewerage_index.html

See also: Support page of the manual PC-based simplified sewerage design, and links to other related sources. Website of the School of Civil Engineering of the University of Leeds, giving support for the publication PC-Based Simplified Sewerage Design and it’s accompanying Windows based design program. Links are also given to publications on this and other low cost sewerage systems. (available in English, Spanish, Portuguese). URL: http://www.efm.leeds.ac.uk/CIVE/Sewerage/

Sinnatamby, G.S. (1983). Low-cost Sanitation systems for Urban Peripheral Areas in Northeast Brazil. PhD Thesis. Leeds: University of Leeds, Department of Civil Engineering, United Kingdom.

Sinnatamby, G.S. (1986). The design of Shallow Sewer Systems. Nairobi: United Nations Centre for Human Settlements, Kenya.

Sistemas condominiales de alcantarillado sanitario : manual de diseno y construccion. Manual on design and construction for condominial water supply, and sanitation networks. This manual helps to replicate the condominial system, as validated within the Bolivian context, through the El Alto Pilot Project. The Alto project has been a very important experience on simplified sewerage systems. (in Spanish only). URL: http://www-wds.worldbank.org/servlet/WDS_IBank_Servlet?pcont=dehttp://www-wds.worldbank.org/servlet/WDS_IBank_Servlet?pcont=details&eid=000094946_02082704193057tails&eid=000094946_02082704193057

Contact persons

• Prof. David Duncan Mara. School of Civil Engineering, University of Leeds. United Kingdom.
• Ing. Carlos Arturo Madera P. MSc. Colombia.

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Date written: 01 September 2003
Date revised: 27 January 2004 (link update)
Author: Ing. Carlos Arturo Madera P. MSc
Organisation: Universidad del Valle, Instituto Cinara. Cali, Colombia
Reviewed by:Jo Smet, IRC International Water and Sanitation Centre
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