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DESCAR:

Pollutants I · ouftfall simulation and water quality

Water pollution

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

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.

         +BOD, not BOD5

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

Eutrophication

In order to grow, plant require about 20 different elements (carbon, hydrogen,…). In normal conditions, water always provide 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.

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.  

Pollution map produced by continuous discharge in this region. The Buoyant  Jet model has been used. The blue horizontal line indicates us the position of the water surface plane.  The fucshia square represents a point source (position of the outfall pipe). The red colour represents high pollutant concentrations.

DESCAR