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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

5.2.1 Off-site Sanitation

Conventional Sewerage

Until a couple of years ago, aspiring toward "conventional sewerage" appeared to be the only solution for the sanitation of urban areas in the Region. Even though the History of Sanitation in the Region has shown that only the main core of the most important cities has benefited from conventional sewerage. These small gains in sanitation on a Regional scale are attributed firstly to the diversion of most of the resources generated from sewerage tariffs to improve the financial situation of the existing water systems and secondly because conventional sewerage, at least under the design codes prevailing in almost all the countries of the Region, is in itself too expensive. As a result the sewerage coverage in the Region has been disastrous.

The experience in the Region to date has been to copy, without adaptation, prevailing urban patterns of American sewerage design codes. This has resulted in incredibly over designed sewerage systems with codes covering, for instance, the use of a 200 mm diameter for minimum pipe and final flow as the only design flow. This code may well be correct if we assume stable population areas, as happen to occur in the developed world cities. However, for a typical Region city whose estimated population in the next 25 years is expected to be 4 times present population the adoption of developed world codes for flow slopes relating to the final flow would possibly mean over flat slopes and oversized diameters. Aimed at solving this problem the Brazilian sewerage design code in the year 1975 introduced the ingenious self- cleansing velocity concept as follows: for initial flows the self-cleansing slope, that conducts to the self-cleansing velocity is calculated; then with this slope and the final flow, the pipe diameter is calculated. Later, early last decade Brazilian sewerage design code introduced to the Region (i) the concept already used in the developed world of minimum tractive force (ii) the use of 100mm as the minimum sewerage pipe whenever suitable for the design flow.

Solids-free Sewerage2

Mr. Rizo Pombo, a very competent and ingenious Colombian Sanitary Engineer developed in Colombia the so-called solids free sewerage system (ASAS). It is apparently quite similar to systems that have been used in Australia and consists of interceptors tanks used to sediment solids that are present in sewage. As the sewerage system gets sewage free of solids, it may operate under dramatically reduced slopes, which will translate to significant cost reductions in comparison to conventional sewer lines. But overall ASAS costs must include interceptor tanks maintenance costs (with vacuum tanks' clean out operation and further discharge to an anaerobic digester) in the comparison with conventional sewerage. Even so the bottleneck for a massive implementation of ASAS in the Region has been the mentioned bias against anything other than conventional sewerage.


2 : The ASAS system has been reported as “Case StudyE

Condominium Sewerage3

Condominium Sewerage has been by far the Regions brightest step forward towards the increase of sanitation coverage. The system was invented by Mr. José Carlos Melo, a Brazilian Sanitary Engineer, and used for the first time in 1982 in the sewerage component of the Rocas and Santos Reis Subproject held in the City of Natal, State of Rio Grande do Norte, out of the World Bank-funded Medium-Sized Cities Project. At that time the Consultant, while working for TAG, had to risk his job when he approved the condominium sewerage investment, even though it was an unapproved technology and its concepts were outside the then prevailing sewerage design codes. Presently over 4,000 km of condominium sewerage has been successfully implemented and operated in Brazil with some other countries being encouraged by the World Bank to use such technology. Moreover in the words of Mr.Carl Bartone, World Bank staff, "... the conjunction of low-water volume toilets with condominium sewerage is the most powerful tool to make adequate sanitation feasible even for the (urban) poor".

In short, condominium sewerage consists of the following aspects:

  • To split a typical sanitation basin in small operational sub-basins under the concept of sewage deconcentration;
  • A simplified sewerage system designed so as to minimize its length since it does not consider the property limits as a given for sewerage lay-out;
  • The use of an urban block as the minimum sewage unit, i.e, the condominium, as the connected households represent a typical horizontal condominium, quite similar to what happens in terms of sewage flow in a vertical apartment building;
  • The connection from the condominium outlet to a conventional public sewerage system, being current practice that the condominium outlet won't be connected to the public system unless an agreement at the block level is achieved either through connection(s) to the public sewerage system or even through individual connections, with higher prices in the latter case.

3 : The condominium sewerage has been reported as "Case Study"

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