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ENVIRONMENT AND ENERGY
<< Continued from page 21
Figure 1:
Percentage of
population served
by different types
of sanitation
systems.
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Large urban centres include megacities, urban
areas with a clear central business district (CBD),
and well-developed suburbs with varying levels of
progressively decreasing population density with
increasing distance from the CBD. The large centre
may be connected (or not) to smaller satellite centres
by transport corridors. These cities often have
extensive sewer networks.
Large urban centres resulting from
conurbations, where two or more distinct urban
centres progressively grow and see their population
density increase, until they more or less merge into
one metropolitan area. These areas have extensive
sewer networks in developed sections of each of
the former city centres, which may have formed in
different ways, and often have separate treatment
facilities and municipal administrations. These types
of urban centres also have large unsewered areas.
Examples include the Accra-Tema conurbation in
Ghana or the conurbation of smaller centres in Metro
Manila.
Smaller urban centres typically are towns that
have a small CBD, possibly some small satellites
and radial linear expansion along the major routes.
These smaller urban centres often have very limited
sewer networks, mostly relying on on-site sanitation.
They may be physically close to other centres, but
they have different municipal administrations, and
therefore separated institutional responsibility.
Large villages and small towns are typically
quite compact but differ from urban centres as they
have little fringe expansion. These could also be
settlements that have developed around industrial
or commercial activities. Examples include college
campuses, airports, and mines.
Rural areas are typically almost entirely served by
on-site systems, without any formal sewer systems.
February 2018 Volume 23 I Number 12
Some urban run-off management may be practised.
The classification of each centre is dependent on the
region. In China, for example, an urban centre with
a population of five million might be considered a
‘small’ city. In addition, each of the categories above
might include slum populations. The proportion of
slums tends to be greater in larger cities, due to the
increased opportunities for work and the need for
low-cost housing, but they also pose a challenge
for smaller urban centres. In the next one or two
decades, the largest rates of urbanisation will occur
in the smaller urban centres of between 500 000
and one million inhabitants. This will greatly impact
wastewater production.
SOURCES OF WASTEWATER IN
MUNICIPAL AND URBAN SYSTEMS
The composition of municipal wastewater can vary
considerably, reflecting the range of contaminants released
by the different combination of domestic, industrial,
commercial, and institutional sources.
The precise urban form and legislative/institutional
environment usually dictates how this wastewater is collected
and treated. However, in most countries, only a proportion
of the wastewater is formally collected. A large proportion,
mainly from low-income settings, is typically disposed of to
the closest surface water drain or informal drainage canal.
In economies that are heavily industrialised or in the
process of development and where legislative environments
are weak, much of the wastewater is mixed together before
treatment and discharge.
Where waterborne sewerage is the norm, so-called
‘combined sewerage’ remains common. This is a perfectly
logical approach if large volumes of water are used for
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