The problem of oxygen deficit caused by organic wastes concerns aquatic systems only. In a water body organic matter is decomposed to harmless constituents by the activity of series of heterotrophic organisms.
These include bacteria, fungi, protozoans and other small animals which live on dissolved organic material and organic debris. Most of these organisms need oxygen to respire. There are two main sources of dissolved oxygen in an aquatic system:
1. Oxygen absorbed from atmosphere above:
Atmospheric oxygen continuously dissolves into water at air-water interface. In flowing waters, such as streams and rivers, the absorbed oxygen is carried down with currents and is mixed evenly in the entire mass of water. This oxygen forms a large fraction of oxygen available to aquatic life in moving waters.
In stagnant waters, such as those of ponds lakes etc. oxygen availability in sub-surface layers depends upon the degree of mixing of water layers. In absence of proper mixing the upper layers which are in contact with atmospheric air, become saturated with oxygen and its absorption slows down or stops altogether. Lower layers are unable to obtain the oxygen absorbed from the atmospheric air.
2. Oxygen produced by green plants within the system:
Green plants carry on photosynthesis which results in oxygen production. Apart from green plants visible to the naked eye there are a number of tiny, microscopic organisms, the phytoplanktons, distributed throughout the body of water. Photosynthesis is carried on by these organisms also in deeper layers as far as the light can penetrate. Photosynthetically produced oxygen is of great importance in stagnant waters.
When lack of active circulation restricts the bulk of oxygen absorbed from the air to upper layers only, aquatic life in lower takers of the water body has to depend largely on oxygen produced by phytolanktons and other green plants.
Organic matter serves as a source of energy to microbial life and other animals which feed on these microbes. Over-abundance of organic matter causes rapid rise in microbial activity and quick decomposition occurs. This in turn places a heavy demand on oxygen in the aquatic system. Amount of oxygen present in the system is, however, limited. Once effluents containing a heavy load of organic matter, silt and debris has been added, the physico-chemical properties of water change.
Turbidity lowers the rate of photosynthesis and substances produced as a result of microbial activity affect the autotrophic population adversely while the solubility of oxygen in water may also be lowered. Thus in the face of raised demand on oxygen in the system its availability declines. Oxygen already present in dissolved state in water is rapidly consumed and oxygen deficient conditions develop. The consequences of oxygen deficiency in an aquatic system may be summarized as follows;
1. Very low levels of oxygen are injurious to a number of organisms which as a consequence perish or if capable of movement leave the place. Massive fish-kills reported in 1972, 1975 and 1977 in Gomti River near Luck now, U.P., India, were caused mainly due to the development of oxygen deficiency in the river water. There was no toxic or poisonous material present in the distillery effluents which were discharged in the river.
2. Due to oxygen deficiency aerobic activity is replaced by anaerobic activity. It results in the production of a number of undesirable substances. Various organic acids, alcohols, phenols, gases like ammonia, methane, hydrogen sulphide etc. accumulate in the system and harm the aquatic life. The environment becomes congenial to many pathogenic organisms. The water body starts emitting foul smell. Its waters acquire an undesirable taste.
3. Plants also suffer as many products of anaerobic activity are injurious to them. Photosynthesis slows down. The primary production disappears and the system tends to run on energy import or the organic matter inputs from the effluents.
4. Anaerobs and organisms capable of suviving under conditions of low oxygen concentration and heavy organic enrichment survive. The diversity in species composition is lost. Few resistant forms which are often useless or undesirable survive and build up huge populations. The biotic spectrum of the system is changed altogether.