Aquatic Pollution and Coastal Zone Managememnt (2+1)

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.4.2.Ecological Damage
Sewage is primarily organic in nature and therefore subject to bacterial decay. As a result of bacterial activity the oxygen concentration in the water is reduced, and sewage is said to have high biological oxygen demand.

C₆H₁₂O₆ + 6O₂ ---------> 6H₂O + 6CO₂

If the oxygen concentration falls below 1.5 ppm, the rate of aerobic oxidation is reduced and anaerobic bacteria start oxidizing organic molecules without oxygen. This can starve aquatic life of the oxygen it needs and also leads to the breakdown of proteins and other nitrogenous compounds, releasing hydrogen sulphide (H₂S) and ammonia (NH₃+), both of which are potentially toxic to aquatic organisms in low concentrations. Solids suspended in sewage may also blanket river and seabeds preventing respiration of the benthic flora and fauna.

Decaying organic matter and nutrients in sewage enhance plant growth. Excessive algal growth and oxygen depletion can lead to alterations in ecosystem structure and to eutrophication.

The dumping of sewage sludge at sea is another cause of ecological damage. Depending on the hydrography, sludge can smother the benthos, increase biomass, decrease species diversity and increase heavy metal concentration.