ENVIRONMENT AND ENERGY
person has been shown to be twice lower than the cost
of conventional sewerage.
COMBINED SEWERAGE
One important issue in relation to wastewater collection
is its source. In old systems, like the one used in P aris,
the original sewers (from 1852) were designed only
for rainwater and grey water; a later decree from 1894
imposed the house owners to put all kinds of wastewater,
including blackwater, in the combined sewers.
Although a variety of users connect to sewer networks,
most systems were designed as so-called ‘combined
systems’, in which stormwater and other types of urban
runoff are discharged to the sewers. This was done,
presumably, in order to limit the costs of purchasing
large diameter drains, but it resulted in dilute sewage
in periods of high rainfall. Although this may have been
acceptable when population densities were low and
the assimilative capacity of the receiving waters was
adequate, recent development and city expansions have
led to a complex and often hazardous combination of
different chemical and biological substances.
insufficient governmental budgets, high poverty,
and expensive infrastructure.
These low-cost systems come in many different types, but
they normally use piped networks with a smaller diameter,
laid at shallower gradients and at shallower depths
underground. The design principles differ from those used
in conventional sewer design and also focus on the concept
that solid-free sewage is conveyed in the system, with
interceptor boxes (similar to small septic tanks) collecting
raw wastewater from a household or group of households.
These systems lend themselves to community
management and are also very well suited to extend and
expand existing systems. One drawback is that they are
not suitable for stormwater drainage. The concept was
first pioneered in Brazil. Low-cost sewerage systems have
become a method of choice for neighbourhoods of all
income levels, as they have all the characteristics required
to be the de facto standard for all sewerage. However,
conservatism among public health authorities and sanitary
engineers has resulted in only sporadic uptake worldwide.
21
Many large cities
in developed and
transitioning
economies
have extensive
sewerage systems,
some of which are
still functioning
effectively some
100 years after
construction.
Combined sewerage should therefore generally not be
considered an effective solution. In an effort to move
away from combined systems, much work has been
undertaken on sustainable urban drainage systems
(SUDS). Sewer systems are suitable for so-called ‘point
sources’ of pollution, but the real challenge is how to
collect diffuse or non-point pollution sources.
Two major sources are the runoff from agricultural land
that has received fertilisers, and the runoff from areas
where intensive livestock is kept, as this often leads to
drugs used for veterinary purposes being present in
the water. Although many intensive agricultural facilities
install collection and treatment systems, this is still not
general practice due to the high costs associated and/or
the lack of regulation or enforcement. PA
Workers constructing a
decentralised wastewater
treatment system in India.
www.plumbingafrica.co.za
At this point, there are not yet many examples of
the technology being evaluated, but the cost data,
particularly from countries like Brazil, clearly show that
it can be financially sustainable. In Brazil, the cost of
simplified sewerage (a type of low-cost sewerage) per
July 2018 Volume 24 I Number 5