Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
5.2.1 Off-site Sanitation
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.
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
Condominium Sewerage has been by far the Regions 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