17.2.3  Biological pollutants

Biological pollutants are microorganisms (bacteria, viruses, protozoa and helminths) that are harmful to humans and other forms of life. Infectious diseases caused by biological pollutants, such as typhoid and cholera, are the most common and widespread public health risks associated with drinking water.

Microorganisms may get into water with dust from the air as rain falls, and when water passes through soil which is polluted with human and animal wastes. The contamination of water supplies with raw sewage (human and domestic wastes generated from residential areas) is the most common route for biological pollutants to enter water.

When contaminated river water moves downstream it is possible that any pollutant will be diluted as more water flows in and so increases the total volume of water in the river. This dilution may be enough to reduce the contaminants sufficiently to minimise the possible health effects but this process may not work for all pathogens.


Many different types of bacteria are found in fresh water. They are not all pollutants because many are not harmful in any way and play a valuable role in the natural breakdown of organic matter and the cycling of nutrients. Other bacteria, however, as you have learnt in other sessions, are pathogens, and are the cause of many waterborne diseases. The presence of faecal coliform bacteria in drinking water, and E.coli in particular, can indicate a possible presence of harmful, disease-causing organisms.


Enteric (intestinal) viruses are produced by infected persons and excreted in faeces. Viral contamination may come from sewage effluent discharged into a river or from open defecation by an infected person which may be washed by rainwater to a river or stream. Some enteric viruses are resistant to chlorination. The common waterborne viruses are polio, hepatitis A and rotavirus. The presence of any enteric virus in water bodies can be taken as an indication of the possible presence of other harmful viruses.


There are several protozoa that can be discharged into water bodies from infected persons. For example, Cryptosporidium and Giardia are common problems in rural parts of Ethiopia.

  • What type of household water treatment is appropriate for removing protozoa from drinking water?

  • A home sand filter is appropriate for removing protozoa from drinking water. The layers of sand and gravel will trap the protozoa.


Helminths or parasitic worms can also cause ill health in humans. Infection occurs through ingestion of the helminth eggs which may be present in food. For example, helminth eggs may be present in the meat of cattle grazing on land contaminated by faeces.

  • Can you think of an example of a helminth infection that is transmitted by polluted water?

  • Guinea worm (dracunculiasis) is transmitted by drinking water that contains copepods infected by the larvae of the worm.

17.2.4  Chemical pollutants