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United Nations Environment Programme
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

Channelisation of urban streams; recipients of solid and liquid wastes
and stormwater; problems with clogging and flooding

Sewers connected to channelised streams

Uncontrolled urbanisation in the upper parts of streams;
covering of urban streams; building of separate sewer for wastewater

Figure 1.3: Sewerage system to convey wastewater and stormwater away from communities

When the amount of wastes disposed to the environment increases with the increase in settlement population, the capacity of the receiving environment to assimilate the wastes is exceeded and degradation of the environment takes place (Figure 1.1). Communities have responded in different ways to the public health problem and environmental degradation that are created. Even though there have been numerous ways in which the problem has been addressed, we may generalise these in terms of stages depicted in Figures 1.3 - 1.5.

Because of the importance of dealing with health problems caused by wastewater within the community, wastewater is transported away from the community. This is done by improving drainage, while still conveying both wastewater and stormwater through the same drains. Measures to reduce the incidence of flooding are usually applied, by for example, deepening drainage channels, preventing solid wastes from being dumped into drains, and covering of the drains represents the first attempt to provide a sewerage system (Figure 1.3). In this way wastewater and the inherent human pathogens in it are removed from the community as a source of public health threat.

Environmental degradation of the receiving water still continues. If the wastewater is disposed to a river the water will affect people using it for bathing and washing, and downstream communities may withdraw the water for drinking purposes. The amenity value of the river for recreational purposes, for fishing, agriculture and industry is devalued. The classification of rivers is a good illustration of how the quality of a river is determined by its pollution load (Table 1.1).

Table 1.1: River pollution classification
Class Description DO. & BOD* Characteristics
Class I Unpolluted or recovered from pollution BOD < 3 mg/L No toxic or suspended discharges which affect the river
Class II Doubtful quality and needing improvement BOD > 3 mg/L, toxic and reduced DO in dry flow times BToxic and suspended discharges occur but have no major effect on biota
Class III Poor quality, improvement is a matter of some urgency DO < 50% for considerable periods River changed in character, suspected of being actively toxic. Subject to serious complaint
Class IV Grossly polluted rivers BOD > 12 mg/L, completely deoxygenated  Incapable of supporting fish life, grossly offensive
*DO = dissolved oxygen; BOD = biochemical oxygen demand
(based on National Water Council (UK) classification, 1970)

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