RESULTS AND DISCUSSIONS
Sedimentation is accomplished either in horizontal-flow or vertical-flow tanks. The former are usually rectangular and the latter circular.
In a rectangular tank, sewage enters continuously at one end and passes at the other end, generally over a weir. Sludge is removed manually into sludge-digestion tanks. The scum formed at the surface is removed by the mechanical scraper with the aid of a second blade called skimmer, through a scum trough.
In the case of a circular or upward-flow tank, sewage enters at the center, rises vertically to be drawn off by flowing over a peripheral weir arranged at the surface. Such tanks are particularly designed to make use of the principle of flocculation whereby, small colloidal particles are agglomerated into bulky wooly masses, which are more easily settled as sludge on the bottom of the tank.
Mechanical scrapers collect the sludge, concentrating it towards the center, from where it is removed for further treatment. The effluent flowing over the outlet weir is collected in an outlet pipe for further treatment.
When only raw sewage is to be treated in these tanks, they may be generally termed as primary settling tanks or primary clarifiers.
While when a sewage that has received secondary treatment, as in trickling filters or aeration tanks, is to be treated in them, then they may be called as secondary settling tanks or secondary clarifiers.
For the sedimentation tanks, the capacity in water supply is determined by the volume of sewage-flow and the required detention period:
1. detention period: 1 to 3 hours. Longer periods result in higher efficiency than shorter periods but too long a period induces septic conditions and should be avoided.
2. velocity of flow: about 30 l/sec.
3. surface loading: it may be noted that the overall range of surface loading between 30,000 to 50,000 l/m/day is in conformity with that used in case of horizontal flow and vertical flow sedimentation tanks.
4. liquid depth of mechanically cleaned settling tanks should not be less that 2.1 m, and for the final clarifier for activated sludge, not less than 2.4 m.
The second unit is describing a septic (Imhoff) tanks. Designed by Karl Imhoff of Germany, an Imhoff tank is an improved septic tank in which the incoming sewage or influent is not allowed to get mixed up with the sludge produced. And, the outgoing sewage or effluent is not allowed to carry with it large amount of the suspended matter as in the case of a normal septic tank.