Secondary sedimentation tanks have the following functions
After secondary or biological treatment, the bacteria and other materials settle in secondary sedimentation tanks, settling basins, or clarifiers. This sludge is poured back into the primary sedimentation tanks' intake end, where it settles with the raw sludge.
After the biological reactor, a secondary sedimentation facility is given to aid in the sedimentation of the cells created during biological oxidation of organic materials. Complete treatment will not be completed if these cells are not eliminated, since they comprise around 40 to 60 percent of the organic matter contained in untreated wastewater in aerobic treatment. Depending on the reactor type, a portion of the settling cells is returned to the reactor, while the remainder is discarded as surplus sludge for further treatment.
What is the required area of sedimentation tank?
There are four types of sedimentation processes, depending on particle size and density, as well as the physical characteristics of the solids:
Type 2 — Flocculent and dilute (particles can flocculate as they settle),
Concentrated suspensions, zone settling, and delayed settling are all examples of Type 3.
Area requirements for SSTs
Based on the results of a single batch test, an area need has been established.
The following factors influence the ultimate overflow rate chosen for the sedimentation tank design:
1: Clarification is required in this area.
2: Thickening area required,
3: The pace at which sludge is removed.
Because the free settling zone requires less space than the thickening region, the rate of free settling is rarely the deciding factor. It is possible that the free or flocculent settling velocity of these particles might govern the design in an activated sludge process when light, fluffy floc particles are present.
For a column of height = Ho, and uniform solid concentration = Co, the position of interface as the time elapses is given in Figure below-
The area required for thickening:
At height H2 (Figure), where concentration is C2, the critical concentration controlling the tank's sludge handling capability occurs (C2 is determined by extending tangent and bisecting angle of intersection).
The time ‘Tu’ is calculated as follows:
a. At the depth 'Hu,' draw a horizontal line that corresponds to the solids' planned underflow concentration, 'Cu.'
Hu's value may be calculated as follows:
b. At point C2, construct a tangent to a settling curve.
c. From the intersection of the tangent and the horizontal line from 'Hu,' construct a vertical line.
d. 'Tu' will be determined by this vertical line.
The area required for thickening is calculated using relation & using this value of 'Tu'.
The region that has to be clarified is then determined. The controlling value is the greater of the two regions. Although 'Cu' will occur for a longer period in a settling test, due to continual withdrawal from the bottom of the tank, this time may not reach the settling tank, hence 'Tu' is calculated from tangent.
What can Netsol Water provide?
If you need help designing an efficient sedimentation system, contact Netsol Water. We can provide you design calculations, budgetary expenses, preliminary layouts, and a lifetime cost analysis, and much more.