Canarina Environmental Software

Water pollution · Canarina DESCAR software


Courtesy of Canarina Environmental Software

Natural waters contain micro-organism as well as solutes. There is a distinction between ‘pure’ water, meaning water that is fit to drink and pure water that is a single substance. A water pollutant is a substance that prevents the use of water for a specified purpose. The signs of polluted water are obvious. Poor drinking water tastes and smell bad. Many types of substances which are classified as pollutants are listed.


  1. Pathogens, bacteria and viruses
  2. Dissolved organic compounds and inorganic compounds
  3. Wastes that have a biochemical oxygen demand
  4. Nutrients that cause excessive growth of plants
  5. Thermal pollution



4.1 Biochemical Oxygen Demand


The Oxygen is used for respiration of animals. Fish require the highest concentration of oxygen. If the dissolved oxygen falls below 5ppm (part per million), fish are the first to suffer and tend to die out. Then, the population of bacteria rises to abnormal levels. The imbalances between species is a sign of water pollution. Substances which use up dissolved oxygen and add to the biochemical oxygen demand are pollutants. Such substances come from the human wastes. The amount of dissolved oxygen used up during oxidation by bacteria of the organic matter in a sample of water is called Biochemical Oxygen Demand (BOD). Water is rated as pure if BOD is 1ppm or less, fairly pure with a BOD of 3ppm and suspect when the BOD reaches 5ppm.





Upper limit of BOD5 (European Union) (mg/L)

Pure waters

< 1

Polluted rivers (upper limit in European Union)

> 5

Polluted rivers with salmon (upper limit in European Union)


Polluted rivers with other fish (upper limit in European Union)


Waste waters


Industrial waste waters


      +BOD, not BOD5


BOD is calculated in 20 days. BOD5 is   calculated in 5 days, BOD=5BOD5.





4.2 Eutrophication


In order to grow, plant require about 20 different elements (carbon, hydrogen,…). In normal conditions, water always provides enough elements for plant growth. The rate of growth is limited by the supplies of nitrogen and phosphorous. Lake water may be enriched with nutrients (a process called eutrophication) which encourages plant growth and leads slowly to ageing. Ageing is accelerated when plant nutrients are fed into a lake by human activities (fertilisers, wastewater containing detergent,…). When a lake contains concentrations of nitrates and phosphates higher than the normal, algae flourish and produce a bloom, a green scum which is accompanied by an unpleasant odour in the water. The lowering of oxygen concentration leads to the death of fish. The sources of nitrates and phosphates are sewage and fertilisers. Intensively cultivated land receives generous applications of fertilisers containing nitrates and phosphates.



Nitrates: The World Health Organization recommends that the level of nitrogen in the form of nitrates should not exceed 50ppm. The averaged level in the UK is about 11ppm.




Upper limit (different compounds with nitrogen)

Rivers with water of good quality


Rivers with salmon

1mg NH3-N/L

Drinking water

0.1mg NO2-N/L

Rivers with salmon

0.01mg NO2-N/L

Drinking water

40mg NO3N/L

Rivers with salmon

1mg NO3-N/L



Phosphates: Phosphates enter water from fertilisers and from detergents. Phosphates are added as builders to improve the cleaning power of synthetic detergents. To inhibit the growth of algae, a phosphate level below 0.5mg/L is the aim. The lakes can become eutrophic if the concentration of phosphates overcomes the 30ug/L

(1ug/L=10-6g/dm3). The upper limit in European Union for polluted rivers with salmon is 65ug/L.  



4.3 Dissolved organic compounds


Natural organic compounds have been present in the environment for millions of years. Many synthetic chemicals are harmless to living things, but some interfere with biochemical processes. Some of these synthetic chemicals are purposely introduced into the environment to kill insects. Some organic compounds pollute waterways. They are especially dangerous if they are not biodegradable. Some, like DDT, are concentrated by passage through a food chain.


4.4 Acidic pollutants


A source of acidic inorganic pollutants is the drainage of water from mines. Water flowing from underground mines is usually highly acidic. In lakes and rivers, the acids react with carbonate ions to form carbon dioxide. A raised level of carbon dioxide in the water makes it more difficult for animals to respire. Extreme acid pollution kills aquatic plants and animals.


4.5 Cyanides


Cyanides are widely used in industry for cleaning metals. Cyanides enter waterways in effluents from industries. The toxic effect of cyanide is due to the formation of a complex with iron.


4.6 Aluminium


Aluminium ions are present in the water supply because aluminium sulphate is used in water treatment. There is evidence of a correlation between Alzheimer’s disease and the aluminium content of drinking water. The rate of Alzheimer’s disease in districts where the concentration of aluminium in drinking water exceeds 0.11mg/L (1mg=0.001g, 1L=1dm3) is higher than the rate in districts where the aluminium concentration is less than 0.01mg/L.


4.7 Cadmium


Pollutant cadmium in water may came from industrial discharges. Heavy metals are serious water pollutants. Cadmium is highly toxic, with a recommended upper limit of only 10ppb (part per billion, 1000ppb=1ppm).


4.8 Lead


Leaded petrol is a source of lead in the atmosphere. Particles of soot and lead compounds from vehicle exhausts can fall on land and contaminate. Lead come from lead pipes and solders, from lead glazes on pottery and glasses. The World Health Organisation recommends that the limit for lead in drinking water should be 50 ug/L.


4.9 Mercury


Mercury compounds are found in nature in low concentrations in rocks and soils. However, we find a variety of uses for mercury which add mercury to the environment. Mercury comes from mercury cathode electrolysis cell, from fungicides, paints, coal and disinfectants. It has accumulated in a number of lakes. Mercury is slowly converted into dimethylmercury and methylmercury which are soluble and can be ingested by animals. Sludge from sewage treatment plants contains about 1ppb of mercury.


4.10 Chromium


Chromium compounds are toxic and irritating. They are present in wastes from electroplating plants and tanneries.


4.11 Nickel


Nickel compounds came from electroplating plants and cause damage to brain.


4.12 Copper


Copper compounds are toxic. If the dose is high enough. It can damage brain and eyes.


4.13 Zinc


Zinc enters the water supply from galvanising plants. It is relatively non-toxic, but large doses cause vomiting.


4.14 Arsenic


Contamination by agricultural pesticides has occasionally been a source of arsenic poisoning. It comes from the combustion of fossil fuels and is concentrated in food chains. The level of arsenic in natural water is 2ppb. Ingestion of 100mg of arsenic by an adult is fatal. The recommended upper limit for drinking water is 50 ppb.


4.15 Thermal pollution


The rise in temperature of the water is called thermal pollution. Water is taking from waterways, used for cooling, and returned to waterways. It decreases the solubility of oxygen, increase the metabolic rate of organism,… If the temperature rises sufficiently, fish may die.




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