Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
2.4 Sustainable versus unsustainable
wastewater and stormwater management
The natural purification processes and biogeochemical cycles
described in section 2 (2.2) provide a basis for determining what is
environmentally sustainable management practices for wastewater and stormwater.
Discharge of wastewater and stormwater into an environment exceeding the natural
purification capacity of that environment will result in the accumulation of
organic materials (carbon), nitrogen, phosphorus or other pollutants that cannot
be absorbed by the ecosystem constituting the receiving environment.
Accumulation of organic materials will result in a high oxygen demand that
cannot be met by oxygen transfer from the atmosphere. Undesirable anaerobic
conditions are a consequence (See section 1 (1) on discharge of wastewater with
a high BOD to a river). Figure 2.8 illustrates an unsustainable practice where
the natural purification capacity of a river into which wastewater is discharged
is exceeded, and where in addition the local biogeochemical cycles are not
Figure 2.8: Unsustainable wastewater management practice from not closing the
local biogeochemical cycles resulting in the natural purification capacity of
the receiving environment to be exceeded (Lange and Otterpohl, 1997).
Nitrogen and phosphorus in wastewater are discharged to a river
resulting in their accumulation in the river. Eutrophication of the river is an
outcome. The nitrogen and phosporus in the wastewater come from food consumed by
people. To grow this food fertilisers containing nitrogen and phosphorus are
required. These are manufactured chemically from atmospheric nitrogen and from
phosphate rock. The flow of materials (N & P) is one way from the atmosphere
for N and from the phosphate rock mine for P into the river. There is depletion
of a resource (mined phosphate rock) and accumulation and pollution in the
river. This practice is unlikely to be sustainable in the long term, because
phosphate rock deposits will be exhausted and pollution of the river by N and P
needs further treatment of the wastewater.
One way of managing the wastewater sustainably is by closing the
material cycles locally (Figure 2.9).
Nutrients in the wastewater are reused to grow food. In this way
there is not the need to to use as much chemical fertilisers and at the same
time there much less discharge of nutrients to the river. The problem of
resource depletion and pollution of the river is overcome by closing the
material cycles. Figure 2.9 also emphasises the need to treat industrial
wastewaters containing toxic substances separately, and not to mix industrial
wastewaters with domestic wastewater. In addition stormwater should be
separately collected and treated and infiltrated locally.
Figure 2.9: Sustainable wastewater management practice by closing the local
biogeochemical cycles (Lange and Otterpohl, 1997).