efficiency of settling tank

Question 1

To determine eficiency of settling tank we have 2 methods

Answer 1

Particle size distribution method: same density and different size
Settling column analysis method:different size and different density

Question 2

Efficiency of settling tank between

Answer 2

60 to 75 percent

Question 3

Continue on booklet

Answer 3

true

Question 4

Exam question:
How do u determine the efficiency of settling tank where discrete settling governs. Explain and support your answer with diagram

Answer 4

When discrete settling governs (same density and different size), we use particle size distribution method; we can also use settling column analysis method.
In particle size distribution method we take a sample of water then we dry it. After that we do a sieve analysis analysis to find percent of weight greater than a given size of particle.
Given the above data , we compute the settling velocity using the discrete settling formula (v=g/18u (Rawparticle-Raw)dsquare. We also calculate reynolds number to make sure we have discrete settling at laminar flow Re<1.
Then we draw a curve of the fraction of settling velocity less than the stated versus terminal settling velocity. (U must show him Xs and vs on t he graph.
In order to calculate efficiency F;
F is equal to:
F= (1-Xs) + Sum (vdelatx)/vs) where vs is the design terminal settling velocity.
1-Xs represents the percent of particles having settling velocity higher than the design settling velocity.

Question 5

Exam question
How do u determine the efficiency of a settling tank where flocculant settling is involoved. Explain and support your answers by diagrams.

Answer 5

When we have different size and different densities we use the settling column analysis method.
In settling column analysis method a column of 300 mm width and a depth equal to that of the tank about 3m is used.
The test is performed by withdrawing samples at regular time intervals from multiple outlets along the column. (U must draw the picture).
After that the percent removals are calculated by the formula ((Co-C1)/Co *100 ).
Percent removals are then plotted as numerical values on a depth versus time graph. ( U must draw the graph)
At a desired detention time the efficiency is calculated as
F= (E1+E2)/2 * deltah1/h+…..

Question 6

Exam question
How do u determine the area of settling tank where compaction is governing.
Explain and support your answer by diagrams.

Answer 6

The area required for thickening is determined by measuring the rate of subsidence of the interface on the settling column analysis, which will result in the following graph (and u must draw it and defiine the elements Co,C2,Cu)
The rate of the subside of the interface is equal to the slope of the curve at that time.
The area required for thickening is given by the following equation:
A=Qtu/Ho.
Also u must explain the tangent thing and how did u get Cu.