The wast water treatment depends upon the impurities present. The impurities present are physical chemical and biological impurities and accordingly treatments given to the wastewater are physical, chemical and biological treatments.
Showing types of wast water treatment based on the impurities
In the above table the screen, clarifiers grit chamber are all physical treatment units because in all those units the removal of impurities is taking place due to physical processes like gravity settling, mechanical straining etc. Mechanical straining takes place in screens and gravity settling takes place in clarifiers. Clarifiers are also known as sedimentation basins or settling tanks.
Chemical treatment of wastewater is preferred when the wastewater discharge is very small like that produced by industries, which contains chemicals.
The chemical treatment takes place in units which are called reactors, Chemical treatments of the industrial wastewaters can be achieved from one or several of the following methods:
(i) Chemical coagulation
(ii) Chemical precipitation
(iii)Oxidation and reduction (Redox reactions)
(v) Ion exchange
The municipal wastewater discharge is very large because it is the wastewater of entire city and therefore chemical treatment of municipal wastewater is not considered.
The biological treatment fully utilizes the micro-organisms for treatment of the wastewater. Depending upon the type of microbes used, the biological treatment is classified as:
(1)Aerobic treatment using aerobic micro-organisms:
The example of the treatment units are Activated sludge plant, Trickling filters, Rotating biologicalcontactors, Oxidation ponds, etc.
Anaerobic Treatment uses the anaerobic microbes. The examples of anaerobic treatment units are sludge digesters etc.
UNIT OPERATION IN WASTEWATER TREATMENT
The wastewater through the outfall reaches the wastewater treatment plant.
There are various units in the wastewater treatment plant. The typical municipal wastewater treatment flow sheet is shown below.
All the units in the flow sheet are shown in same line for convenience but they may be arranged conveniently in the above stated order.
The processes taking place in each treatment unit like as S, GC, PST, TF, etc. are called as unit operations. So the unit operation in screen chamber is screening, in PST it is gravity settling, in TF it is aerobic biological growth of microbes.
In the above treatment plant, the treatment takes place in three stages:
(i) Preliminary treatment: screening and grit removal
(ii) Primary treatment: primary sedimentation tank
(iii)Secondary treatment: trickling filter and secondary sedimentation tank and disinfection unit.
When the treated wastewater is intended for recycling or reuse as domestic water supply or for swimming pool use, the tertiary treatment is given to the wastewater. The phosphorus and nitrogen content of wastewater are reduced to unobjectionable level in treatment is given after preliminary,primary and secondary treatment.
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wast water treatment