How To Remove Solids From Wastewater

Wastewater treatment has assumed a differentrectangular and the latter circular.
dimension today against the backdrop of the dangerIn a rectangular tank, sewage enters continuously at
of running out of fresh water. Wastewater isone end and passes at the other end, generally over
sewage, storm-water and water that have beena weir. Sludge is removed manually into
used for various purposes around the community.sludge-digestion tanks. The scumformed at the
Most communities generate wastewater from bothsurface is removed by the mechanical scraper with
residential and nonresidential sources.the aid of a second blade called skimmer, through a
Unless properly treated, wastewater can harm publicscum trough.
health and the environment.In the case of a circular or upward-flow tank,
Here I have discussed about removing the solidssewage enters at the center, rises vertically to be
from wastewater. How can we remove the settlingdrawn off by flowing over a peripheral weir arranged
solids from the wastewater?at the surface. Such tanksare particularly designed to
Simple. Thru a settling tank. It comprises of themake use of the principle of flocculation whereby,
following units:small colloidal particles are agglomerated into bulky
(a) Sedimentation tanks: either plain or chemicalwooly masses, which are more easilysettled as
precipitationsludge on the bottom of the tank.
(b) Septic (Imhoff) tanksMechanical scrapers collect the sludge, concentrating
(c) Sludge digestion tanksit towards the center, from where it is removed for
**Sedimentation tanks**further treatment. The effluent flowing over the
This is carried out with the objective to removeoutlet weir is collected in an outlet pipe for further
suspended mineral and organic matter from sewagetreatment.
after the wastewater has been subjected to passWhen only raw sewage is to be treated in these
through screens and grit chamber. These are thetanks, they may be generally termed as primary
units in which sedimentation is brought about. Thesettling tanks or primary clarifiers.
lighter organic sewage solids, which settle in theWhile when a sewage that has received secondary
sedimentation tanks, are termed as sludge, while thetreatment, as in trickling filters or aeration tanks, is to
sewage that has been partially clarified by the settlingbe treated in them, then they may be called as
out of the solids is known as the effluent. Bothsecondary settling tanks or secondary clarifiers.
sludge and effluent should be further treated in order**Design criteria for primary sedimentation tank**
to make them stable and unobjectionable.As with the sedimentation tanks in water supply, the
The settlement of the solids may either be causedcapacity is determined by the volume of
by gravity or by aggregation or flocculation ofsewage-flow and the required detention period.
sewage-particles. If the coagulating chemicals are not(i) detention period: 1 to 3 hours. Longer periods
added in the sewage, the tanks are referred as plainresult in higher efficiency than shorter periods but too
sedimentation tanks. whereas, if chemicals are usedlong a period induces septic conditions and should be
for the purpose of bringing the finer suspended andavoided.
colloidal solids into masses of large bulk, thus(ii) velocity of flow: about 30 cm square/min.
hastening the settlement process, these are then(iii) surface loading: it may be noted that the overall
known as chemical precipitation tanks. The chemicalsrange of surface loading between 30,000 to 50,000 l
used are alum, lime, ferric chloride, ferric sulfate,m / day is in conformity with that used in case of
chlorinated copper etc.horizontal flow and vertical flow sedimentation tanks.
**Types of sedimentation tanks**(iv) liquid depth of mechanically cleaned settling tanks
Sedimentation is accomplished either in horizontal-flowshould not be less that 2.1 m. And for the final clarifier
or vertical-flow tanks. The former are usuallyfor activated sludge, not less than 2.4 m.