119
cent suspended solids and 25 - 35 per cent BOD. Water is then chlorinated in
the next chamber to destroy bacteria.
Secondary (Biological) Treatment :
Secondary treatment is required for removal of BOD because primary
treatment can not remove BOD completely. Biological treatment takes
advantage of micro organism to convert wastes into stabilized, low energy
compound. Secondary treatment like trickling, filter and activated sludge
process are used after primary treatment of water. Oxidation ponds method
can be used directly to the raw sewage water.
Trickling Filters & Rotating Biological Contractor :
In trickle filter system, a rotating arm sprinkles waste water (after primary
treatment) over a rock or gravel bed, the pore spaces which are filled with
biological slime containing bacteria or algaes fungi, worms, snails, insects which
helps in removal of BOD. In case of rotating disc which consists of about 3 - 5m
diameter plastic discs which rotates around a horizontal axis. The biomers films
that grows on the surface of the disc which moves out of water and air as the
disc rotates. While submerged in water the organism is adsorbed and while out
absorbs oxygen.
Fig. 7.2 : Sketch of the cross section of a trickling filter
(After Gilbert, 1955)
