 | Urban Wastewater Projects - A Layperson's Guide (EEA, 1998, 124 p.) |
 |  | Chapter 2. An Introduction to Urban and Rural Wastewater Management |
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2.6 Wastewater Treatment
2.6.1 Why is there a need to treat wastewater?
Briefly, the treatment of wastewater is practised to avoid otherwise unacceptable conditions e.g.:
· risks to public health· pollution of natural bodies of water into which effluents are discharged -watercourses, lakes and the sea - to the point where they damage aquatic plant and animal life or prevent their normal economic, social or recreational use through contamination or deoxygenation
· pollution of the general environment by creating offensive odours or sights and the contamination of groundwater.
In addition, the provision of wastewater treatment is good social practice, there being a general public aversion to finding sanitary waste in water bodies of environmental importance.
The European Community’s Urban Wastewater Treatment (UWWT) Directive requires that all Member States pass legislation which ensures that developments having a population above a given figure to treat their wastewater before discharge into watercourses, lakes or the sea. However, the degree of treatment to be provided and the date by which such treatment must be operational, depends partly upon the size of the population served and partly upon the sensitivity of the body of water into which the effluent is discharged. This is further detailed in Chapter 3, Section 3.3.
Wastewater treatment is described in greater detail in Chapters 6 and 7.
Fig. 2.4 Medium-sized wastewater
treatment plant
2.6.2 What are the detrimental effects of discharging untreated wastewater into a recipient and what are the benefits of wastewater treatment?
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Wastewater Constituent |
Detrimental Effects |
Benefit of Wastewater | |
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Large solid material -paper, rags, plastic bags, condoms, etc. |
Unsightly - accumulate as litter on banks of rivers, lakes and beaches |
River banks, lakes and their surroundings and beaches are rendered more pleasant and safer environments for work and for recreation |
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Can constitute a risk to health on contact |
Improved economy where based on recreation and/or tourism |
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Organic matter - food waste, faecal matter and some industrial effluent |
Oxygen levels in receiving waters are reduced by bacteria and higher orders of aquatic life consuming the organic matter - fish and other organisms die and eventually disgusting odours are produced -similar to rotten eggs and rotten cabbage |
Livelihoods dependent upon fishing are protected as is fishing for sport | |
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More pleasant environment for living, working and recreation | |
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Improved economy where based on recreation and/or tourism |
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Oils and greases |
Unsightly and potentially damaging and harmful scum formed on water surfaces Impermeable film on water surface reducing potential for, water to absorb oxygen from atmosphere |
Improved oxygen absorption into the water body from atmosphere assisting aquatic life to survive | |
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More pleasant environment for living, working and recreation | |
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Improved economy where based on recreation and/or tourism |
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Nutrients - nitrogen, phosphorus and trace materials |
Act as fertiliser and stimulates growth of algae, seaweed and other aquatic plants choking watercourses and littering banks of rivers and lakes and beaches with rotting material, eventually becoming organic waste |
Improved and safer conditions for shellfish cultivation and other aquatic organisms | |
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Can stimulate toxic algal blooms which accumulate in shellfish and can infect humans who consume them |
More pleasant environment for living, working and recreation | |
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Improved economy where based on recreation and/or tourism |
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Disease-causing bacteria and viruses - e.g. cholera, typhoid and salmonella |
Contamination of water resources used for drinking or irrigation of crops eaten raw by humans or animals |
Improved public health Improved and safer conditions for shellfish cultivation and other aquatic organisms | |
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Contamination of water used for shellfish cultivation |
Improved economy where based on recreation and/or tourism |
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Contamination of water used for water contact sports | | |
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Toxic substances - generally originating from industrial effluents |
Dependent upon toxicity and concentrations in receiving water can |
Improved conditions for aquatic life | |
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Improved public health | |
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- destroy or damage aquatic life | |
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- accumulate in flesh of fish, shellfish and creatures which feed upon them and eventually affect humans consuming them | |
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