ENVIRONMENT AND ENERGY
flushing, resulting in diluted sewage combined with few other wastewater sources.
It is important to note that, in many cases, large volumes of wastewaters that are
legally discharged to decaying and/or badly operated sewerage networks, both
combined and separate, never actually reach a treatment plant. Much is lost en
route as a result of broken pipes, or ends up in surface water drains, polluting both
groundwater and surface watercourses. There are also many cases of illegal water
reuse by communities who deliberately tamper with trunk sewer systems.
SANITATION AND THE PRODUCTION OF
WASTEWATER IN INFORMAL SETTLEMENTS
Wastewater generation is one of the biggest challenges associated to the growth of
informal settlements (slum s) in the developing world. In sub-Saharan Africa, 62%
of the urban population live in slums. The most alarming statistics can be found in
countries emerging from conflict and in West Asia, where the proportion living in
slums has increased from 67% to 77% and 21% to 25%, respectively.
Slums vary in type, form, and population density. However, most are characterised
by a lack of paved roads, durable housing, water and sanitation infrastructure,
and drainage. In these situations, high levels of faecal matter and solid waste are
disposed of to surface water drainage canals and ditches.
Poor solid waste disposal causes blockages in drainage systems, resulting in
flooding. Uncollected wastewater and urban run-off flows are often equivalent to
sewered wastewater in terms of toxicity and health risks. Although many slums rely
on on-site sanitation, faecal matter is not usually contained and wastewater is still
produced as residents often use latrines as bathrooms for personal ablutions with
so called ‘bucket showers’.
Slum dwellers frequently have to rely on un-sewered communal public toilets, use
open space or dispose of faeces in polythene bags (i.e. flying toilet). Communal
toilets are not widely used, due to lack of water, poor maintenance, and the cost to
the user. A study in the slums of Delhi found that the average low-income family of
five could spend 37% of its income on communal toilet facilities. Finding a suitable
place to go to the toilet is especially problematic for women, causing risks related to
personal security, embarrassment, and hygiene.
COMPOSITION OF MUNICIPAL AND URBAN WASTEWATER
The precise composition of wastewater varies around the world and is governed by
a wide range of factors, including domestic water use and the level of commercial/
industrialisation. In developed regions, the BOD:COD* ratio 4 is likely to be lower
than in the developing world, due to a higher proportion of industrial wastewater.
This will lower the water’s suitability for biological treatment. In some areas, high
levels of inorganic substances, sulphates, and alkalinity for example, can affect the
wastewater’s suitability for post-treatment use. Sulphates tend to result in hydrogen
sulfide production with implications for sewer corrosion. High alkalinities, or water
hardness, are likely to cause limescale deposits and will affect the suitability of the
water for reuse as process water.
WASTEWATER FROM PARTICULARLY HAZARDOUS SOURCES
Wastewater from domestic sources are usually relatively free from hazardous
substances, but there are growing concerns about commonly used medications
that, even at low concentrations, may have long-term impacts. Industries that use
red list substances (substances classed with significant hazard) in their production
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February 2018 Volume 23 I Number 12
*BOD:COD ratio
Biochemical oxygen demand (BOD) is
the amount of dissolved oxygen needed
(i. e. demanded) by aerobic biological
organisms to break down organic
material present in a given water sample
at a certain temperature over a specific
time period.
Chemical oxygen demand (COD)
is the standard method for indirect
measurement of the amount of pollution
(that cannot be oxidised biologically)
in a sample of water. The higher
the chemical oxygen demand, the
higher the amount of pollution (mostly
inorganic) in the test sample.
If the BOD:COD ratio for untreated
wastewater is 0.5 or greater, the waste
is considered to be easily treatable by
biological means. If the ratio is lower
than 0.3 approximately, either the waste
may have some toxic components, or
acclimated microorganisms may be
required for its stabilisation.
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