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Organic pollutants

Organic matter includes:

  • Agricultural wastes such as slurry and silage
  • Blood
  • Food and drink
  • Sewage
  • Substances containing organic materials including many fire foams

Although many of these pollutants are not in themselves toxic, they can still have serious indirect consequences. This is because rivers, lakes and other waterways are organic matter processing systems. Adding large quantities of organic matter disrupts the balance of the system.

Microbes process any organic matter spilt into a waterbody and their populations grow exponentially due to the extra food source. As the microbes increase, they consume more dissolved oxygen, reducing oxygen levels in the water. If enough organic pollutant is available, all the dissolved oxygen will eventually be used, and anaerobic conditions will arise. In such conditions most species of aquatic animals will die.

If anaerobic conditions persist, for example due to a continuous discharge of sewage, specialised microbes, called ‘sewage fungus’, will thrive. This appears as a grey filamentous growth in the water. A smell of bad eggs will usually be noticed. Some aquatic organisms are particularly sensitive to any reduction in dissolved oxygen levels and will be affected even if anerobic conditions do not arise, for example, stonefly and mayfly larvae, trout and salmon and will be affected detrimentally by small changes.

Over time, the organic matter is used up and disperses. River water re-oxygenates moving downstream as oxygen dissolves in from the atmosphere and from aquatic plant growth. The temperature and flow rate will influence how quickly this happens. Employing aeration or other pollution mitigation tactics such as the addition of hydrogen peroxide will speed up this process as well as keep aquatic life alive until dissolved oxygen levels in the river recover.


Organic Pollution incidents, such as milk can have serious but usually temporary effects on the local ecosystem

Measuring organic pollution -Biochemical Oxygen Demand (BOD)

The ‘oxygen sag’ is an indirect measure of the amount of organic matter in a liquid. The biochemical oxygen demand (BOD) test is designed to quantify the amount of change imposed on the river by the entry of the particular organic substance. A measure of oxygen requirement will indicate the likely impact of an organic pollutant on the river.

This test provides a standard by which organic pollutants can be compared and it’s used to monitor both river pollution and the effectiveness of treating organic materials before discharge into the water environment.

BOD values for different wastes and effluents

Typical BOD values (mg oxygen/l)
Natural rivers 0.5–5.0
Crude sewage 200–800
Treated sewage 3–50
Poultry waste 24,000–67,000
Silage liquor 60,000
Dairy waste 300–2,000
Skimmed milkand cream 70000-40,0000
Brewery waste 500–1,300
Orange juice 80,000
Paper mill effluent 100–400
Typical firefighting foam concentrate 50,000-600000