Before the disposal of waste water or sewage in river streams or on land, sewage treatment is required to make it safe.
Waste water can be defined as any water or liquid that contains impurities or pollutants in the form of solid, liquid or gases or their combination in such concentration that it is harmful if disposed into the environment. To overcome the harmful effects, sewage treatment is done.
Sewage can be treated in different ways and but mainly the sewage treatment processes are offen classified as
1. Preliminary Treatment
2. Primary Treatment
3. Secondary (or Biological) Treatment
4. Final treatment or Advanced Treatment
1. Preliminary Sewage Treatment
- The prime function of the preliminary treatment is to carry out the removal of suspended solids and heavy organic solids from the waste water.
- This treatment of sewage treatment, removed approx. 10 to 30% of suspended solids present in waste water and satisfies 15 to 30% of BOD associated with it.
- Grit chamber is provided for the removal of the inorganic suspended particles whereas primary settlement tank (PST) is provided for the removal of organic suspended solids and also for greases and oils, skimming tanks are used.
2. Primary Sewage Treatment System
- In this system of treatment, large suspended organic solids are removed and this is usually done by sedimentation in settling basins.
- Sometimes, Preliminary treatment as well as Primary treatment are counted together under Primary Treatment.
- This treatment will not remove the dissolved and colloidal organic content from waste water.
- Large particles or debris are removed by screening process or may be reduced in size by grinding machines.
- This treatment removes approximately 60 to 70% of suspended solids and satisfies 30 to 40% BOD associated with it.
- The removal of organic mattes and inorganic matters should be in two different units because the disposal of inorganic matters is easy as it does not decompose whereas the disposal of organic matters are difficult.
3. Secondary Sewage Treatment System
- Secondary Treatment treat the effluent coming from the primary sedimentation tank.
- Secondary or Biological Treatment is being carried out for the decomposition of dissolved organic matter present in waste water by the action of micro-organisms either in the presence or absence of oxygen.
- To establish the contact between organic matter and micro-organisms, following mechanism are being developed
- Attached Growth System – in this system, a medium is provided for the attachment of biomass and waste water is passed through it.
- Suspended Growth System – In this system, Biomass is suspended in waste water.
- The effluent from the Primary Treatment contains about 60 to 80% of the unstable organic matter originally present in sewage. This organic matter, that was passed the primary clarifiers without settling there, are than removed by further treatment and this treatment of sewage is called secondary or biological treatment. By the help of oxidation or nitrification, character of the organic matters are changed and form a stable version of them (like nitrates, sulphates etc.)
- All the secondary treatment process are designed to work only on aerobic bacterial decomposition method because in aerobic bacterial decomposition, bad smells and gases are not produced whereas bad smell and gases are produced in anaerobic decomposition. Also the aerobic bacterias are more active than the anaerobic bacterias as the rate of doing work of aerobic bacteria is three times the rate of work of anaerobic bacteria at 300 celcius.
- Aerobic Biological Units – The treatment reactors in which the organic matter is decomposed (oxidized) with the help of aerobic bacteria are called aerobic biological units and the units may consist of:
- Aeration tanks with the feed of recycled activated sludge (i.e. the sludge which is settled in secondary sedimentation tank getting effluents from the aeration tank).
- Oxidation Ponds and Aerated Lagoons.
- Various filters used in aerobic decomposition treatment are
- Contact Beds
- Trickling Filters
- Intermittent Sand Filters
- All these aerobic biological units used primary settled sewage, therefore they are classified as secondary unts.
- Anaerobic Biological Units – The treatment reactors in which the organic matter is destroyed and stabilized with the help of anaerobic bacteria are called anaerobic biological units and may consist of anaerobic lagoons, Imhoff tanks, Septic Tanks etc.
- Out of these Anaerobic biological units, only anaerobic laggons use the primary settled sewage and therefore it can only be classified as secondary unit. Whereas imhoff tanks and septic tanks use the raw sewage and therefore, not classified as secondary units.
- The secondary systems generate excess biomass that is biodegradable through endogenous catabolism and by other micro-organisms process.
- Primary sludge and secondary sludge are generally combined for further treatment by anaerobic biological process. In this process, as an end products, methane, carbon di-oxide, liquids and inert solids are generated.
- The methane have significant heating value and can be used in treatment plants as in a form of power. The liquids contain excessive concentration of organic matters and are recycled through the treatment plant. The solid residue has a high mineral content and can be used as a soil conditioner and as a fertilizer on agricultural lands.
- Sometimes the Primary and secondary treatments can be accomplished together.
Also Read – Pre Treatment of Water
4. Final Treatment of Advanced Treatment
- This treatment is also known as tertiary treatment.
- In most of the cases of treatment of municipal wastewater, treatment till secondary stage is sufficient. But in some cases, additional treatment may be required to meet the standards. In tertiary treatment, removal of organic load which is left after the secondary treatment is done and also the pathogenic bacteria’s are need to be killed in this treatment.
- This treatment is normally carried out by chlorination.
- If we need to dispose treated sewage, then tertiary treatment may not be required. It is required if we want to reuse water for water supply or irrigations.
- Actually the distinction between primary, secondary and tertiary treatment is rather arbitrary, since there are many modern treatment methods incorporate physical, chemical and biological processes in the same operation. The choice of the treatment methods depends up on various factors, including the disposal facilities available.