Test: Self Purification of Natural Streams - Civil Engineering (CE) MCQ

The amount of dissolved Oxygen (DO) in water is one of the most commonly used indicators of a river health. Below 4 or 5 mg/l, the life forms that survive are reduced.

As DO drops below 4 or 5 mg/L the forms of life that can survive begin to be reduced. A minimum of about 2.0 mg/L of dissolved oxygen is required to maintain higher life forms.

Waste water disposal practices are based on the premise that “the solution to pollution is dilution”. In this method relatively small quantities of waste are discharged into large bodies of water.

The transfer of gases into and out of water is an important part of the natural purification process. Gas transfer is accomplished by the replenishment of oxygen lost to bacterial degradation of organic waste.

The speed and completeness of the self purification process depend upon many factors like volume, flow rate, the turbulence of flow, variation in sunlight, etc. Aquatic species are affected by polluted streams.

In a flowing stream, the breakdown of degradable wastes by bacteria depletes dissolved oxygen. This eliminates the populations with high oxygen requirements until the stream is cleansed of wastes.

To control the discharge of pollutants so that the water quality is not degraded to an unacceptable extent below the natural background level is the objective of water quality management.

The impact of pollution depends upon nature of the pollutants and the characteristics of the river like discharge and speed of flowing water, depth of the river, type of bottom, surrounding vegetation, etc.

Self-purification of running stream:
When the sewage effluent is disposed of in a river stream it undergoes the process of self-purification by the following natural mechanism;
Dilution & Dispersion:
In real terms, it is not the mechanism of self-purification it only reduces the potential hazard of the sewage effluent into the river stream.
Sedimentation:
As per the turbulence available in the river stream organic suspended particles get settled in it thereby helping the river water to get self purified.
Sunlight:
In the presence of sunlight, a photosynthesis reaction takes place during which O2 is released which is utilized by the micro-organism for the decomposition of organic matter thereby helping the river water to get self purified.
Oxidation & Reduction:
Decomposition of the organic matter by the action of micro-organism over the surface of the river stream and by the anaerobic micro-organism on the bed of the river collectively helps the river to get self purified.