The organic matter in sewage consists of urea from urine, proteins, carbohydrates, fats and oils and soaps. These are normally composed of a combination of carbon, hydrogen and oxygen, together with nitrogen in some cases. Other important elements, such as sulphur, phosphorus, and iron, may also be present.
The organic matter present in sewage is unstable and decomposes readily through chemical and bacterial action. In the process of decomposition which is bio-chemic in nature, highly complex organic matter present in sewage is decomposed into materials or constituents of much simpler chemical structure.
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The organic matter which can be decomposed by bacteria under biological action is called biodegradable organic matter. The decomposition of such organic matter takes place through the agency of different types of bacteria viz., aerobic bacteria, anaerobic bacteria and facultative bacteria.
The nitrogenous and carbonaceous materials present in sewage serve as food for these bacteria. The decomposition of organic matter by bacteria under biological action is termed as biological decomposition.
Depending on the type of bacteria the biological decomposition can be of the following two types:
(i) Aerobic decomposition (also called aerobic oxidation), and
(ii) Anaerobic decomposition (also called putrefaction)
(i) Aerobic Decomposition:
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Aerobic decomposition is caused by both aerobic bacteria as well as facultative bacteria operating aerobically, in the presence of air or oxygen which is available in sewage in the dissolved form. During this process, organic matter is broken up and oxidized to form stable and non-objectionable end products such as carbon dioxide, nitrates, sulphates, etc. Sewage treatment units which work on aerobic decomposition process alone are aeration tanks, trickling filters, contact beds, oxidation ponds, etc.
(ii) Anaerobic Decomposition:
Anaerobic decomposition or putrefaction is caused by anaerobic bacteria as well as facultative bacteria operating anaerobically. Anaerobic bacteria survive by extracting and consuming bounded molecular oxygen present in the oxygen radicals of organic compounds and mineral substances such as nitrites, nitrates and sulphates.
The end products of anaerobic decomposition or putrefaction include gases like hydrogen sulphide, ammonia, methane, etc., and black residue. Sewage treatment units which work on putrefaction alone are septic tanks, Imhoff tanks, sludge digestion tanks, etc.
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The matter of the universe remains constant, but its form changes because of biochemical reactions. The complex organic compounds of biodegradable nature are broken up by biochemical reactions into simple compounds which are consumed as food by plant and animal life and the organic matter is formed again. This cycle thus goes on.
From the point of view of sewage treatment, the cycles of decomposition of the following five elements are of importance:
(1) Nitrogen cycle
(2) Carbon cycle
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(3) Sulphur cycle
(4) Calcium cycle
(5) Phosphorus cycle.
The first three cycles relate to the decomposition of nitrogenous organic matter, carbonaceous organic matter, and sulphurous organic matter respectively.
All the above noted cycles of decomposition are discussed below:
1. Nitrogen Cycle:
The nitrogen cycle, which corresponds to the biochemical degradation or decomposition of nitrogenous organic matter, is an endless chain connecting the processes of life and decay of both animal and plant worlds.
Fig 8.2 shows a nitrogen cycle, which consists of the following steps:
(a) Main Steps:
(1) The nitrogenous organic matter, in the form of waste animal and plant 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 (NH3) and other gases.
(2) By process of nitrification, ammonia is first converted into nitrites (NO2) by partial oxidation and then finally to nitrates (NO3) by the action of aerobic bacteria.
(3) The products formed in step (2) are consumed by plant life as food through photosynthesis. The plant life grows, due to which plant tissues, plant protein (seeds) and free oxygen are produced.
(4) The plant protein is 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 dead bodies of the animals result in the formation of nitrogenous organic matter.
The nitrogen cycle is thus completed and it continues.
However, there may be some short circuits in the above cycle which are shown by dotted lines in Fig. 8.2, and are as described below:
(b) Short Circuit Steps
(6) The death or decay of plant life (as it happens in forests where leaves, flowers, etc. of plants lie in layers on ground surface to decay) may directly result in the formation of nitrogenous organic matter.
(7) Waste products such as urea and excretions of animals may sometimes decompose directly and form ammonia nitrogen.
(8) The nitrates formed in step (2) may be converted into or reduced to free nitrogen (and sometimes to ammonia) by anaerobic bacteria by a process known as de-nitrification.
(9) Free nitrogen produced in step (8) may directly be 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:
The carbon cycle, which corresponds to the biochemical degradation or decomposition of carbonaceous organic matter, is also an endless chain connecting the processes of life and decay of both animal and plant worlds.
Fig. 8.3 shows a carbon cycle, which consists of the following steps:
(a) Main Steps:
(1) The decomposition of carbonaceous organic matter, through oxidation by aerobic bacteria results in the release of carbon dioxide C02, which is its final end product and is the stable form of carbon.
(2) Plants in the presence of sunlight and by the process of photosynthesis consume carbon from carbon dioxide produced in step (1). This results in the formation of plant carbohydrates, fats and proteins (sugars).
(3) The plants are consumed by animals, resulting in the formation of animal fats and proteins.
(4) Waste products as well as death of animals results in the formation of carbonaceous organic matter.
The carbon cycle is thus completed and it continues.
However, there may be some short circuits in the above cycle as shown by dotted lines in Fig. 8.3, and are as described below:
(b) Short Circuit Steps:
(5) (a) Plant life gives off carbon dioxide at night through the process of respiration.
(b) Similarly animal life gives off carbon dioxide during respiration.
(6) Carbonaceous organic matter may also be formed directly by the death/decay of plant life.
3. Sulphur Cycle:
The sulphur cycle, which corresponds to the biochemical degradation or decomposition of sulphurous organic matter, is also an endless chain connecting the processes of life and decay of both animal and plant worlds.
Fig. 8.4 shows a sulphur cycle, which consists of the following steps:
(a) Main steps:
(1) The decomposition of sulphurous organic matter, through the action of anaerobic bacteria, in the absence of oxygen, results in the formation of hydrogen sulphide (H2S).
(2) By process of oxidation, hydrogen sulphide is first converted into sulphur and finally to sulphates.
(3) Sulphates, when consumed by plants, through photosynthesis, change into plant proteins.
(4) Animals consume the plant proteins which results in the formation of animal proteins.
(5) Wastes produced by animals and their dead bodies results in the formation of sulphurous organic matter.
The sulphur cycle is thus completed and it continues.
However, there may be some short circuits in the above cycle as shown by dotted lines in Fig. 8.4, and are as described below:
(b) Short Circuit Steps:
(6) Sulphates in the absence of oxygen, are converted into hydrogen sulphide (H2S), by the process of reduction.
(7) Sulphurous organic matter may directly be produced by the death or decay of plants.
4. Phosphorus Cycle:
The phosphorus cycle relates to the maintenance of level of phosphorus in the soil.
Fig. 8.5 shows a phosphorus cycle, which consists of the following steps:
Steps:
(1) (a) Plants consume phosphorus present in soil through photosynthetic action,
(b) Plants also consume phosphorus supplied from artificial or chemical sources such as manures and fertilizers (phosphates) for their growth.
(2) The plants are consumed by animals, and the phosphorus element is thus transferred to animals.
(3) The wastes produced by animals and their dead bodies result in the transfer of phosphorus back to soil.
(4) The death or decay of plant life also results in the transfer of phosphorus back to soil. The phosphorus cycle is thus completed and it continues.
5. Calcium Cycle:
The calcium cycle relates to the maintenance of level of calcium in the soil.
Fig. 8.6 shows a calcium cycle which consists of the following steps:
Steps:
(1) Calcium present in soil is carried away to water bodies such as rivers, lakes, etc., through surface runoff.
(2) (a) Calcium present in water bodies is consumed by phytoplankton and fish.
(b) Plants consume calcium of water bodies through surface flow (both natural as well as artificial).
(3) Calcium present in soil is also consumed by plants through photosynthetic action for their growth.
(4) (a) plants are consumed by animals, and calcium is thus transferred to animals.
(b) Similarly phytoplankton and fish are consumed by animals resulting in transfer of calcium to animals.
(5) (a) Death of animals results in calcium going back to soil.
(b) Death of plankton and fish also results in the transfer of calcium back to soil,
(c) Similarly death or decay of plants also results in the transfer of calcium back to soil.
The calcium cycle is thus completed and it continues.