80
Primary treatment :
Urban sewage contains more than 95% water and , in addition to
wastes of human origin. It contains domestic and industrial waste
water , solids , litter and run-off from the land . As preliminary
treatment , it is screened to remove large solids , comminuted or
macerated to produce a slurry , and grit is extracted .
For primary treatment the remainder is placed in settlement tanks .
The supernatant liquid , which still has a high BOD and has a certain
proportion of suspended solids , is discharged to the receiving waters .
The sludge thas has settled out is disposed of elsewhere.
Secondary treatment :
If a greater reduction of BOD is required , the supernatant liquid is
subjected to biological treatment .
Tertiary treatment :
If a high quality of effluent is required , it may be necessary to give
further treatment
The liquid derived from secondary treatment may be retained in
sedimentation
ponds or passed
through sand or earth filters to
remove suspended solids .
To reduce plant nutrients , nitrates may be removed by algal growth in
retaining ponds .
Desinfection treatment :
Some form of desinfection is increasingly used to remove pathogens ,
such as bacteria and viruses , if the human population is likely to be
exposed to the effluent that is discharged .
This can be achieved by the addition of strongly ractive chemicals sucha
s chlorine , ozone . another alternative is to subject the waste to
ultraviolet light or continuous microflitration treatment , both of which
remove viruses as well as bacteria.