Self-Purification of Streams | Sanitary Engineering

In this article we will discuss about:- 1. Introduction to Self-Purification of Streams 2. Factors Affecting Self-Purification of Stream 3. Process.

Introduction to Self-Purification of Streams:

If from the point of disposal of sewage in stream, the stream water is examined towards down streams, it will be observed that the quality of stream water successively changes. Near the place of disposal the water will be polluted and it becomes purified after some travel towards downstream side due to natural forces of purification.

When sewage is discharged into streams, the water becomes polluted. The suspended solids are gradually deposited in the stream bed in layers, depending upon the hydrographic characteristics of the stream. The organic matter of these so formed layers starts decomposition and stabilization. In course of time, the decomposed materials are washed away with the current or converted into simple constituents.

Various organisms present in the sewage start the decomposition of the volatile organic matter and stabilize it. Nitrogen, Sulphur, Carbon and other inorganic elements present in sewage move under their natural cycles as shown in Fig. 10.1 and 10.2. Algae and other microscopic organisms eat mineralized food and supply oxygen to the stream tor maintaining aerobic condition.

The bacteria are eaten away by the protozoa, which further become the food of fish and other aquatic life. Thus, the stream in course of time becomes free of bacteria and protozoa. The natural purification process continues and makes the stream water pure after some time. Therefore in practice, it will be observed that the stream water after some travel from the point of disposal becomes free of sewage pollution and comes in its natural purified state.

Factors Affecting Self-Purification of Stream:

The following conditions affected the self-purification of stream:

(i) Dilution:

When the sewage is mixed up with large volume of water or it is much diluted, the sewage always remains in aerobic condition and anerobic condition never comes because always dissolved oxygen remains present in the water.

(ii) Current:

The self-purification of stream directly depends on the current. When there is no current the sewage matters deposit near the outfall causing formation of sludge bank and foul odours. In heavy current, the sewage is thoroughly mixed up with the stream water preventing all nuisances. In slow current, the sedimentation takes place causing growth of algae resulting in the production of oxygen. Therefore, slow current is better for the self-purification.

(iii) Sedimentation:

With slow current, the heavier solids settle in the stream bed and start anaerobic decomposition. The products of decomposition are again mixed with water by the current. If the dilution is sufficient, the anaerobic condition will not develop and the scouring tendency of streams during flood will wash the deposits.

(iv) Temperature:

As the activities of organisms depend on the temperature, the self- purification will also depend on temperature. At low temperature the organisms activities are slow due to which the rate of decomposition will be slow and in warm temperature this action is reverse. Therefore in summer, the stream will get self-purification is less than in winter. But the quantity of dissolved oxygen is more in cold water than in hot water.

(v) Sunlight:

The pathogens are killed if they are exposed to sunlight, therefore, sunlight helps in self-purification. Algae also grow in sunlight causing production of oxygen. Therefore indirectly sunlight helps in the oxidation of unstable matter.

(vi) Oxidation:

The organic matter immediate after mixing with the stream or large quantity of water starts getting oxidized due to the development of the oxidising organisms present in the water. This process prevails till the complete oxidation of the organic matters. The oxygen demand is satisfied and the stream becomes purified, due to this phenomena. The streams get their oxygen demand by its aeration by wind or by the microscopic organisms.

(vii) Reduction:

The reduction occurs in the streams due to the hydrolysis of the organic matters biologically or chemically, Anaerobic organisms start the splitting complex organic matter present in the sewage. This action produces odours and gases and the stabilization starts.

Process of Self-Purification of Stream:

The self-purification process of streams polluted by sewage can be grouped in the following four zones of divisions:

(a) Degradation Zone:

This zone is situated just near the outfall of sewage. The water is turbid with dark colour. The decomposition of solid matters take place in this zone and the anaerobic decomposition prevail.

(b) Active Decomposition Zone:

This zone is just after the degradation zone. The water is greyish and darker than the previous zone. The objectionable odours of hydrogen sulphide and other sulphur compounds prevail and scum may also be seen on the surface in this zone.

(c) Recovery Zone:

In this zone, the stabilization of organic matters takes place and BOD of water is reduced. The contents of dissolved oxygen start rising up above 40% of the saturation value. The bacterial load decreases as the food supply of bacteria diminishes.

(d) Clear Water Zone:

In this zone, the stream attains normal conditions as were prevailing before sewage was discharged into it.