Sewage treatment has assumed a new magnitude itself now, against the background of the risk of fresh water becoming depleted. Wastewater comprises storm-water, water used for varied purposes, and sewage, enveloping the community.

Most urban social groups produce sewage from household as well as nonhousehold sources. Unless duly processed, sewage water can cause illness or disease to the public and pollute the environment.

Here I have expounded on removing solids from wastewater. How may we withdraw solids sinking to the bottom from the sewage?

Easy. By means of a settling tank. It is constituted of the parts given below:

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

**Sedimentation tanks**

This process is implemented with the goal of removing suspended organic and mineral matter from sewage. Subsequent to its being made to go past screens and granules chamber. These are the modules in which sedimentation is effected. The lighter organic wastewater solids, which settle in the sedimentation tanks, are labelled as sludge. Meanwhile the sewage water that has been partially clarified by the settling down 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 settling in of the solids may be attributed to flocculation, gravity, or aggregation of wastewater-granules. If coagulating chemicals are not added to the sewage, the tanks are referred to as plain sedimentation tanks. Whereas, if chemicals are used for the purpose of bringing the finer suspended and colloidal solids into solid masses of large volume, these are then known as chemical precipitation tanks. Chemicals are added to quicken the settlement process. The chemicals used are ferric chloride, ferric sulphate, chlorinated copper, alum, lime etc.

**Types of sedimentation tanks**

Sedimentation is effected either in horizontal-flow or vertical-flow tanks. The horizontal-flow tanks usually have 4 walls while the others are normally circular.

In a rectangular tank, sewage flows in steadily at one particular end and passes out at the other side, generally over a weir. Sludge is taken out manually and deposited in sludge-digestion tanks. Scum formed above the mass is removed by the mechanical scraper, with the aid of a second sharp edge termed skimmer, via a scum receptacle.

In a circular or upward-flow tank, sewage comes in at the middle, rises upwardly to be pulled out by steadily flowing over a weir on the boundary. Which is mounted on the surface. Similar tanks are specially made to make use of the procedure of flocculation. By Which, fine congealed granules are agglomerated into voluminous wooly masses, which in turn are settled without trouble as slush on the bottom of the tank.

Mechanical blades gather the slush, accumulating it towards the middle, from where it is taken out for further treatment. The sediment removed effluent flowing as a stream above the weir in the outlet is collected in a pipe in the outlet for further processing.

When only raw sewage is to be processed in the tanks being considered, They might well be ordinarily labelled as primary settling tanks or primary clarifiers.

Meanwhile when sewage receives secondary treatment, as in trickling filters or aeration tanks, similar tanks then 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 volume treatable is decided by the quantity of sewage flowing in and the required treatment period. The criteria are:

(i) time of detention: 1 to 3 hours. Longer periods cause greater efficiency than shorter periods, but extra long hours of treatment may cause septicemia and should not be permitted.
(ii) swiftness of flow: about 30 cm square/min.
(iii) surface loading: you can observe that the overall range of surface loading between 30,000 to 50,000 l / m / day matches that used in regard to horizontal flow and vertical flow sedimentation tanks.
(iv) depth of liquid of physically cleaned settling tanks should be ensured to be above 2.1 m. And for the final settling tank for treated sludge, above 2.4 m.

Author Bio:

Richard J. Runion is the President of Geostar Publishing & Services LLC. Rich loves net research & blogging. His new blog on "Wastewater Treatment" is fast becoming popular, as it is comprehensive and well-researched.

To learn all about diseases caused by wastewater, click: http://www.all-about-wastewater-treatment.com .

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