Wastewater treatment has assumed a different dimension today against the backdrop of the danger of running out of fresh water. Wastewater is sewage, storm-water and water that have been used for various purposes around the community.

Most communities generate wastewater from both residential and nonresidential sources.
Unless properly treated, wastewater can harm public health and the environment.

Here I have discussed about removing the solids from wastewater. How can we remove the settling solids from the wastewater?

Simple. Thru a settling tank. It comprises of the following units:

(a) Sedimentation tanks: either plain or chemical precipitation
(b) Septic (Imhoff) tanks
(c) Sludge digestion tanks

**Sedimentation tanks**

This is carried out with the objective to remove suspended mineral and organic matter from sewage after the wastewater has been subjected to pass through screens and grit chamber. These are the units in which sedimentation is brought about. The lighter organic sewage solids, which settle in the sedimentation tanks, are termed as sludge, while the sewage that has been partially clarified by the settling out of the solids is known as the effluent. Both sludge and effluent should be further treated in order to make them stable and unobjectionable.

The settlement of the solids may either be caused by gravity or by aggregation or flocculation of sewage-particles. If the coagulating chemicals are not added in the sewage, the tanks are referred as plain sedimentation tanks. whereas, if chemicals are used for the purpose of bringing the finer suspended and colloidal solids into masses of large bulk, thus hastening the settlement process, these are then known as chemical precipitation tanks. The chemicals used are alum, lime, ferric chloride, ferric sulfate, chlorinated copper etc.

**Types of sedimentation tanks**

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.

**Design criteria for primary sedimentation tank**

As with the sedimentation tanks in water supply, the capacity is determined by the volume of sewage-flow and the required detention period.

(i) 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. (ii)
velocity of flow: about 30 cm square/min. (iii) 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. (iv) 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.


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