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ENVIRONMENT AND ENERGY
Composition of municipal and
urban wastewater (Part 2)
In this edition, Plumbing Africa discusses the
composition of municipal and urban waste water.
Extracted from United Nations World Development Report
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 ratio4 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 biochemical oxygen demand
(BOD), which 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 oxidized 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.
High alkalinities, or water hardness, are likely to cause
limescale deposits and will affect the suitability of the
water for reuse as process water, for example.
Wastewater from particularly hazardous sources
Wastewater from domestic sources is 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: some
known endocrine disrupters in particular (Falconer, 2006).
Industries that use ‘Red List’ substances (Table 4) in
their production processes are required to ensure that
Cities are increasingly concerned with the effects of climate change, which include higher risks of flooding
and raised temperatures, combined with increasing demands for safe drinking water supplies.
October 2018 Volume 24 I Number 8
www.plumbingafrica.co.za