Pretreatment of waste water precedes regular treatment through filters when necessary. It may include one or more of the following processes:


a. Screening 
b. Plain sedimentation 
c. Aeration 
d. Chemical dosing and mixing 
e. Coagulation followed by flocculation 
f. Sedimentation 
g. Pre-chlorination at the appropriate stages.


Some physical and chemical factors and the presence of toxic substances may make pretreatment of water essential. Treatment of potable water depends on many needs such as:


a. Aesthetic reasons, such as 'the removal of taste, odor, color and turbidity'. 
b. Economic reasons like 'softening for minimizing laundry costs'. 
c. Industrial needs including 'the preparation of boiler feed water'. 
d. Recreational needs like filling a swimming pool. 
e. Occasionally for other purposes like, 'to reduce corrosiveness' or 'to combat fluorosis', etc.


The ultimate objective of pretreatment of water is to ensure potable water which is of safe sanitary quality. Stated differently, we want to supply water that is:


1. Free from significant concentrations of toxic substances. 
2. Of attractive nature, so far as tastes and odors are concerned. 
3. Reasonably soft and yet non-corrosive. 
4. With a very low content of iron and manganese, so that it is non-staining to laundry and plumbing fixtures.

Naturally available water cannot be readily used. It requires some treatment. Primarily, pathogenic organisms need to be removed for controlling water borne diseases and hence protect public health.

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