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This graph
has an important message: simplified/condominial sewerage can be
cheaper than on-site sanitation systems. At low population
densities it is more expensive, but as the population density increases
there comes a point at and above which simplified sewerage is cheaper. The
graph is for the low-income periurban areas in the city of
Natal in northeast Brazil where simplified sewerage was
first installed in the early 1980s. In this particular case
simplified
sewerage became cheaper than on-site systems at the relativley low
population density of ~160 persons per ha. See here for a Google Earth view of Rocas and Santos Reis (the part of Natal where simplified sewerage was first installed).
In periurban
areas of Natal served with simplified sewerage the capital cost per
connection (household) in 1983 was US$ 325 [it is much lower now*].
The water & sewerage company borrowed the money for the
project from the then National Housing Bank and it calculated that it
could repay this loan over 30 years by surcharging the water bill by
40% (rather than by 100% which was the surcharge for non-poor
households served with conventional sewerage). The periurban
households had an unmetered yard-tap water supply for which they paid
US$ 3.75 per month, so the charge for simplified sewerage was 40% of
this - i.e., US$ 1.50 per month. There was no connection fee.
So it was a very low-cost pro-poor solution.
For January 2008 charges see here.
Even if on-site sanitation systems are cheaper in capital cost terms,
low-income households may be better off financially being served by simplified
sewerage as the monthly payment may well be less than that to repay a
short-term microcredit loan. Moreover operation and maintenance
is much easier with simplified sewerage as almost all the O&M is
done by the sewerage service provider.
A final point: if a city or town has a sewerage system serving at least
part of its area, then there will be at least some local knowledge
(however imperfect) of sewerage and sewer O&M. In contrast the city or
town Department of Environmental Health may have very little experience
of on-site sanitation systems and the O&M required for, for example,
latrine pit emptying.
*See The
Experience of Condominial Water and Sewerage
Systems in Brazil (WSP, 2005).
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