Thursday, January 19, 2017

Self-purification of water bodies

Self-purification of natural water bodies
Self-purification of natural water bodies is a combination of physical, chemical and biological processes working simultaneously. The amount of dissolved oxygen in water is commonly used to indicate river health. The maximum value of dissolved oxygen in water at 0℃ is 14.6 mg/L and at 25 ℃ is 8.24 mg/L. Oxygen demanding wastes (organic wastes) reduce the dissolved oxygen and increasing temperature also reduces the solubility of oxygen in water.
The factors affecting self purification of water are:
  1. Dilution
  2. Current
  3. Sunlight
  4. Temperature
  5. Rate of oxidation
  6. Reduction
  7. Sedimentation
  8. Hydrography
Dilution: If sufficient water is available in the receiving water body, the dissolved oxygen level might not reach the critical level due to availability of dissolved oxygen before discharge.

Current: The presence of currents in the receiving stream increases the dissolved oxygen in the receiving water body due to turbulence. Strong currents also prevent deposition of solids. Low currents encourage deposition of solids in wastewater resulting in decompoosition and reduction in dissolved oxygen.

Sunlight: Sunlight has a bleaching and stabilizing effect on bacteria. It helps aquatic plants and algae to absorb carbondioxide and release oxygen by a process called photosynthesis. The release of oxygen into water helps in self-purification through oxidation.

Temperature: The quantity of dissolved oxygen available in stream water is higher at colder temperatures. However, the activity of microorganisms is higher at high temperature. Hence, self-purification is carried out quickly in summers than in winters.

Rate of oxidation: Depletion of dissolved oxygen occurs mainly due to oxidation of organic matter. This rate is faster at higher temperature and lower at lower temperature.

Reduction: Reduction occurs due to hydrolysis of organic matter settled at the bottom either chemically or biologically. Anaerobic bacteria helps splitting the composition of sewage into liquids and gases thus paving the way for their ultimate stabilization.

Sedimentation: The settleable solids, if present in effluents settle at the bed of the river, near the outfall, thus helping in self-purification process.

Hydrography: Hydrography affects the velocity and surface expanse of the water body. High velocity and large surface expanse cause turbulence and rapid aeration.

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