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Read this article to learn about Waste Water Treatment:- 1. Objective of Waste Water Treatment 2. Steps Involved in Waste Water Treatment.
Objective of Waste Water Treatment:
The objective of municipal and industrial waste water treatment is to extract pollutants, remove toxicants, neutralise coarse particles, kill pathogens so that quality of discharged water is improved to reach the permissible level of water to be discharged into water bodies or for agricultural land.
Treatment of water thus aims at reduction of BOD, COD, eutrophication etc. of receiving water bodies and prevention of bio-magnification of toxic substances in food chain.
Steps Involved in Waste Water Treatment:
Various steps involved in treatment of waste water are as follows:
1. Preliminary Treatment.
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Screening:
In this treatment debris, gross solids, grit, oil and grease are removed by passing waste water through screens, grit chambers and skimming tanks.
2. Primary Treatment:
Primary treatment of sewage removes 60% suspended solids, 30% COD, 35% BOD, 10% P and 20% total nitrogen.
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It includes the following processes:
(i) Sedimentation:
About 50% suspended solids can be removed by gravitational settling under quiescent conditions.
(ii) Mechanical Flocculation and Coagulation:
Fine suspended solids and colloidal particles are removed by passing waste water through clariflocculator and using coagulants like alum and poly-electrolytes.
(iii) Neutralisation:
Highly acidic and alkaline waste waters are neutralised by lime slurry or NaOH and H2SO4 or CO2 respectively.
3. Secondary (Biological) Treatment:
The dissolved and colloidal organic matter in waste water/sewage is removed by aerobic or anaerobic processes. The effluent from primary sedimentation tank is first subjected to aerobic oxidation in processes such as aerated lagoons, trickling filters, activated sludge units, oxidation ponds etc.
Then the sludge obtained in these aerobic processes, together with that obtained in the primary sedimentation tank, is subjected to anaerobic digestion in the sludge digester (Fig. 2). Secondary treatment removes about 80% COD, 90% BOD, 30% P, 50% total N and oil, grease, phenol, grit, scum etc.
4. Tertiary Treatment:
Tertiary treatment is the final treatment meant for abolishing the secondary effluents and removal of fine suspended solids, traces of organics and bacteria. The sewage effluent from secondary treatment plant is introduced into a flocculation tank where lime is added to eliminate calcium phosphate.
The solution then enters the NH3 stripping tower. Nitrogen present in waste water exists as NH+4 which is converted to gaseous ammonium ion at high pH(ll). Phosphorus is removed by adding ferric chloride or aluminium sulphate. The remaining organic materials are removed by desalination, ion exchange and finally chlorination is used for disinfection.
The toxic, non-biodegradable chemicals in industrial waste water can be removed by adsorption (on activated charcoal), ion exchange, ultra-filtration, reverse osmosis and electrodialysis.
