Treatment of Sewage Water

Primary treatment: 

This treatment removes suspended solids, odour, colour, gases and oil. The treatment has the following stages.

i) Screening: In this method two metal screens are used. One metal screen has wide openings and other metal screen has small openings. The metal screens are held at 45^o C. The first screen would remove big floating matter. Then the water is passed through the channels of grit chamber. Now, the water passes through the metal screen with small opening. Small floating matters are removed. The metal screens and grit chambers are cleaned using mechanical scrapers.

ii) Sedimentation: Waste Water enters the sedimentation tank. It has V- shaped bottom with an opening at the bottom to remove the sludge. The screened water enters the sedimentation tank from the bottom and slowly rises upwards. The settlable solid particles are pulled down due to gravity. The rate of settling depends upon the size and density of the particles and the temperature. The water is taken out of the chamber while the sludge is removed from the bottom.

iii) Degasification: The waste water is stored in large shallow tanks and exposed to the atmosphere. The dissolved gases escape from the surface.

iv) Oil separation: Air under high pressure is forced in to waste water containing oil and grease. The water is passed into a low pressure chamber. Since the solubility of air in water decreases, surplus air comes out and the bubbles of these carry the oil with them to the top. The oil is removed by skimming.

Secondary treatment: This treatment is also called biological treatment. In the secondary treatment, the dissolved, colloidal and soluble organic matters present in water after primary treatments are removed by micro-organisms. During this process, oxidation of waste results in the formation of species like CO₂, H₂O, nitrates and nitrites. The secondary treatment involves two important process- trickling filters and activated sludge process.

Activated sludge process:

·       

         After the primary treatment, waste water enters into specially designed tanks which are either rectangular or circular shaped.

·         It comes in contact with the gelatinous mass which is heavily laden with micro-organisms. This mass is called activated sludge.

·         Air is introduced into the tank and micro-organisms bring about biological degradation of waste into species like nitrates, sulphates, CO₂ and also synthesis of new bacterial cells. Air is passed for 4-10 hours.

·         The sludge settles down on settling and separated from water. A part of the sludge is retained in the tank for the treatment of fresh batch of waste water and the rest is used as fertilizer.

Advantages:

·         It gives clear effluents.

·         Effluent is free from bad smell.

·         Degree of purity can be varied.

Disadvantages:

·         Skilled supervision and check on return sludge is required.

·         Not suitable for industrial wastes.

·         Large volume of sludge creates disposal problems.