Possible sources of water pollution
Having looked at the various types of pollutant, let us now consider their sources.
Human excreta
Open defecation and poorly constructed pit latrines are obvious sources of human waste and can easily pollute surface and groundwater. Where water-flushed sewerage systems are present, inadequately treated sewage can also be a major source of human waste. (Note the difference between the words ‘sewage’ and ‘sewerage’. Sewage is mixed wastewater that contains human waste from flush toilets, commercial and industrial wastewater, and frequently also surface water run-off. Sewerage is the network of underground pipes – sewers – through which the sewage flows.)
Untreated or partially treated sewage can contribute to high levels of oxygen demand in the water and also introduce toxic substances into the aquatic environment, in addition to pathogenic micro-organisms. Sewage may be treated in waste stabilisation ponds (these will be described in Study Session 11). If not operated properly, these ponds can pollute rivers. In many parts of the world, sewage from large towns and cities is usually treated in large mechanical–biological plants (Figure 4.5) that normally produce good quality effluent but can still be a source of pollution if systems fail.
Figure 4.5 Becton sewage treatment plant in London, the largest plant of its type in Europe, treats the sewage of 3.7 million people.
In low-income countries towns and cities, many households use septic tanks to dispose of their sewage. These are underground tanks into which sewage is piped. The waste remains in the tank for long enough for the solids to settle out and the settled sewage is discharged from the tank, usually into the surrounding soil via a soakaway. If the tank is too small to retain the sewage for long enough, or if many septic tanks are close together, or if they leak or are cracked, this can lead to pollution of groundwater. Figure 4.6 shows the main features of a properly constructed septic tank.
Figure 4.6 Cross-sectional diagram of a septic tank.
Manufacturing and industrial plants
In Study Session 1 you read about some of the ways in which water is used in industry and manufacturing. The range of different uses and processes can produce waste in the form of many different types of organic and inorganic material in suspension or in solution. In many cases, much of the water used can be recycled but there is almost always an effluent discharge that requires treatment.
Food processing generates large volumes of effluent containing natural organic compounds such as carbohydrates, proteins and fats. Factories producing chemicals often generate low volumes of highly toxic waste streams. Toxic effluents can also be produced in the paper, leather and electroplating industries. For example, cyanides and heavy metals may be present in wastewaters from electroplating. These plants can also be the source of highly acidic wastes.
Manufacturing and industrial effluents should be treated at their points of origin, but many production plants in developing countries still do not have proper effluent treatment systems. This results in the discharge of untreated or partially treated effluent into the nearest water body (Figure 4.7).
Figure 4.7 Industrial discharge into a river.
Agriculture and animal rearing
The intensive rearing of animals results in large volumes of organically polluted washwater from cleaning animal houses. This slurry is often stored in lagoons or tanks prior to spreading on land. However, problems occur when these lagoons or tanks leak or overflow, allowing the slurry to flow into watercourses or infiltrate groundwater. Other agricultural pollutants include pesticides and fertilisers.
Cultivation and overgrazing can make soil erosion more likely, resulting in soil particles being washed into rivers and lakes (Figure 4.8).
Figure 4.8 A reservoir.
Domestic and industrial solid waste sites
Domestic and commercial solid waste should be disposed of in a properly designed and constructed landfill site. Many landfill sites, particularly those that are older and less well designed and managed, such as the one shown in Figure 4.9, generate leachate, which is highly polluting. (Leachate is any liquid that has passed through matter and picked up dissolved substances and/or suspended solids as it passed through.) Leachate can contain dissolved organic matter and many different types of inorganic components depending on the type of waste. Where industrial waste has been dumped, a toxic chemical stream may also be produced. These leachates should be collected and treated so that pollution of groundwater and rivers does not arise.
Figure 4.9 A landfill site.
Urban surface water run-off
Rainwater that runs off road surfaces, roofs, parking areas, etc. carries with it a variety of components (Table 4.1). The bulk of the contaminants can be traced to motor vehicles. Surface water run-off can cause damage to streams, rivers and lakes by degrading the water quality and harming aquatic life. The pollutants present can hinder the growth and reproduction of fish and other creatures, and affect photosynthetic activity. Plant nutrients may contribute to eutrophication.
Table 4.1 Pollutants that may be present in rainwater run-off.
| Pollutant | Likely sources |
| Sediment | Construction, road surfaces, emissions from vehicles, industrial sources, vehicle wear |
| Copper | Vehicle brake pads, industrial activities, plumbing and guttering |
| Lead | Industrial activities and residues from historical activities (plumbing, paint, leaded petrol, sprays), tyre-balancing weights, vehicle brake pads |
| Zinc | Vehicle tyres, galvanised building materials, paint, industrial activities |
| Hydrocarbons | Vehicle emissions, lubricating oils |
| Rubber | Tyre wear |
| Detergents | Wash-down areas, domestic discharges (e.g. from car washing), industrial discharges |
| Litter | Discarded material (e.g. plastic bags, cups, cigarette ends), windblown materials, illegal dumping |