Most of the organic matter present in decomposition of sewage readily through combined biological and chemical processes. Therefore these processes are bio-chemic in nature and converts very high complex organic compound into simpler chemical structure.

The organic matter decomposition, however done by bacteria under biological action is termed as biodegradable organic matter. Thus decomposition of organic matter is cause by different bacteria –

• Aerobic bacteria
• Anaerobic bacteria
• Facultative bacteria

Nitrogenous and Carbonaceous materials in the sewage serve as food for these bacteria.

The biological decomposition is of two types –

• Aerobic decomposition (also called aerobic oxidation)

Aerobic decomposition caused by Aerobic bacteria as well as Facultative bacteria in the presence of oxygen.

End products are – carbon dioxide, nitrates, sulphates etc.

Treatment units which operates aerobically alone are – Aeration tank, Trickling filters, contact beds, Oxidation ponds etc.

• Anaerobic decomposition (also called putrefaction)

Anaerobic decomposition caused by Anaerobic bacteria as well as Facultative bacteria in the absence of oxygen.

End products are – black residue, nitrogen, hydrogen etc.

Treatment units which operates anaerobically alone are – Septic tanks, Imhoff tanks, Sludge digestion tanks etc.

Cycles of Decomposition of the following five elements are of importance

1. Nitrogen cycle
2. Carbon cycle
3. Sulphur cycle
4. Calcium cycle
5. Phosphorous cycle

The first three cycles are related to the decomposition of Nitrogeneous organic matter, Carbonaceous organic matter, Sulphurous organic matter respectively.

1. Nitrogen cycle

1. The nitrogenous organic matter in the form of waste animal and plants matter, consisting of urea, proteins and hydrocarbons; undergoes decomposition either by way of oxidation by aerobic bacteria or by way of putrefaction by anaerobic bacteria, resulting in the formation of ammonia and other gases.
2. By process of nitrification, ammonia is first converted into nitrites by partial oxidation and then finally to nitrates by the action of aerobic bacteria.
3. The products form in step 2 are consume by plant life as food, through photosynthesis.The plant life grows due to which plant tissues, plant protein (seeds) and free oxygen produced.
4. The plant proteins consumed by animals resulting in the production of animal proteins (meat, milk etc.)
5. The animal wastes in the form of urine and other excretions as well as the dead bodies of the animals result in the formation of nitrogeneous organic matter.
6. The death / decay of plant life may directly result in the formation of nitrogenous organic matter.
7. Waste products such as urea and excretion of animals may sometimes decompose directly and form ammonia nitrogen.
8. The nitrates may generally converted or reduced into free nitrogen by anaerobic bacteria, by a process known as denitrification.
9. However free nitrogen may converted into plant proteins by certain type of bacteria present in the roots of plants, through a process known as nitrogen fixation.

2. Carbon cycle

3. Sulphur cycle

4. Calcium cycle

5. Phosphorous cycle