definition 112 causes, effects and control measures of

Control measures: 

One way of reducing the

pollution load on marine waters is through the

introduction of sewage treatment plants. This

will reduce the biological oxygen demand (BOD)

of the final product before it is discharged to

the receiving waters.

Various stages of treatment such as primary,

secondary or advanced can be used depending

on the quality of the effluent that is required to

be treated.

Primary treatment: 

These treatment plants use

physical processes such as screening and sedi-

mentation to remove pollutants that will settle,

float or, that are too large to pass through simple

screening devices. This includes, stones, sticks,

rags, and all such material that can clog pipes.

A 

screen consists of parallel bars spaced 2 to

7cms apart followed by a wire mesh with smaller

openings. One way of avoiding the problem of

disposal of materials collected on the screens is

to use a device called a comminuter which grinds

the coarse material into small pieces that can

then be left in the waste water. After screening

the wastewater passes into a 

grit chamber. The

detention time is chosen to be long enough to

allow lighter, organic material to settle. From

the grit chamber the sewage passes into a 

pri-

mary settling tank (also called as sedimentation

tank) where the flow speed is reduced suffi-

ciently to allow most of the suspended solids to

settle out by gravity. If the waste is to undergo

only primary treatment it is then chlorinated to

destroy bacteria and control odours after which

the effluent is released. Primary treatment nor-

mally removes about 35 percent of the BOD and

60 percent of the suspended solids.