Sewage Treatment - 1

Grit chambers or grit channels or grit basins are used to remove inorganic particles having specific gravity of about 2.65. The detention time is based on the depth of water and settling velocity of inorganic particles. A detention time of about 40 to 60 sec is generally sufficient for a water depth of 1 to 1.8 m.

Coarse screens: Spacing is 50 mm or more Medium screens: Spacing is 6 to 40 mm.
Fine screens: Spacing is 1.5 to 3 mm

Detritus tanks are nothing but grit chambers designed to flow with a smaller flow velocity (of about 0.09 m/sec) and longer detention periods (about 3 to 4 minutes) so as to separate out not only the larger grit, etc., but also to separate out the very fine sand particles.

The upper chamber is called the sedimentation chamber or flowing through chamber, through which the sewage flows at a very low velocity and the lower chamber is the digestion chamber, in which the sludge gets digested due to anaerobic decomposition.

Bulking of activated sludge is due to the presence of filament micro-organism, e.g. fungi . Chlorination is some times used to destroy such organisms to avoid bulking of activated sludge.

Coagulation- It can assemble mainly inorganic particle to form large flocs.
Hypochlorination- It is used to kill mainly pathogens,
Reverse Osmosis- A process by which a solvent passes through a porous membrane in the direction opposite to that for natural osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure. With this process maximum impurities are removed including dissolved organic wastes.