Lab 1 Settling

8/10/2019 Lab 1 Settling

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Lab 1 - Preliminary Design of a Settling

Tank

Prof. Hofmann

Background

A town wants to install a conventional horizontal flow sedimentation tank (i.e. a simple, rectangular

tank) to treat its water such that the effluent TSS is less than 50 mg/L. The depth of the tank will be 1.5

m (which is actually unrealistically shallow!), and the flowrate in the plant is 10 MLD.

Assume that the surface area for the sedimentation tank(s) will be 200 m2. Conduct a settling column

test on the water sample provided to estimate whether the TSS criterion will be met.

If the TSS criterion is not met, estimate the surface area required.

Experimental Methods

You will conduct two settling column tests in parallel (it is always better to duplicate experiments, and

pool the data). While the dimensions of the two parallel settling columns are slightly different, you can

still pool the data and plot it together. Its up to you to decide the best way to do this.

In general, you will fill the two settling columns with water sample, and then collect TSS samples at the

different sampling ports over 1 hour. Note that the bottom sampling port lies in the sludge zone so do

not take TSS samples from this port since the solids are no longer settling.

It is recommended that you collect TSS samples at the following time intervals (the lab supervisor may

adjust these times as necessary): 0, 3, 6, 12, 20, 35, 50 minutes. Measure the TSS of the water sample

before filling the two columns. This will be the concentration at time t = 0.

Be sure to purge the sampling port of old sample before collecting the new sample.

It is up to you to learn how to measure TSS before conducting the experiment.

Data AnalysisApply methods (do bothType I and Type II settling) that you learned in class to solve this problem. It is

expected that the results be shown in a table as well as in graphical format as per class notes for Type I

and Type II settling problems. Show the data, and answer the design problem. Briefly discuss any

sources of uncertainty that you have, and how they might affect your final design solution or

recommendation. Explain why you think that Type I or Type II is the better analysis in this case.


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APPENDIX Examples of Settling

Calculations

TYPE 1 Example:

Data for the settling analysis of a Type 1 water sample is shown in the table below:

Time (min.) 0 60 80 100 130 200 240 420

Concentration (mg L-1) 300 189 180 168 156 111 78 27

The data will be used to determine the removal efficiency in a settling basin with a loading rate of 2532

*********

1. Calculate the mass fraction remaining and the settling velocity using the following equations.

Mass fraction remaining =CC0 where C0is conc. at time = o; Ciis conc. at time = ti

Settling velocity, vt=Z0t where Z0is distance travelled by particles, t is time taken

Time (min.) 60 80 100 130 200 240 420Mass fraction remaining 0.63 0.6 0.56 0.52 0.37 0.26 0.09

vtx 10-2

(m/min) 3.3 2.5 2 1.55 1 0.83 0.48

2. Plot the mass fraction remaining versus settling velocity as shown in the Figure 1 below.


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Figure 1: Removal Fraction vs Settling Velocity

3. Determine the velocity (v0) which equals the surface loading rate of 2532(1.7 x 10

2m min

-1)

4. The Removal Efficiency is given by (R) = (1 r0) + [area above curve but below ro]/overflow rate

5. From the graph (Figure 1) above, r0= 54%

5. Numerically integrate the area above the curve in the graph, up to ro.

Removal = (1 0.54) + 0.47/1.7 = 0.46 + 0.27 = 0.73 or 73%

Element r vtx 10-2

r . vtx 10-2

1 0.04 1.6 0.06

2 0.15 1.25 0.19

3 0.15 0.91 0.13

4 0.11 0.6 0.07

5 0.09 0.24 0.02

TOTAL = 0.47


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TYPE II Example:

Data for the settling analysis of a Type II water sample is shown in the table below. The initial total

solids concentration is 250 mg/L. Determine the overall removal efficiency for a settling basin 3 m deep

and a detention time of 1h and 45 minutes.

Depth

(m)

Time of Sampling (min)

30 60 90 120 150 180

0.5 133 83 50 38 30 23

1.0 180 125 93 65 55 43

1.5 203 150 118 93 70 58

2.0 213 168 135 110 90 70

2.5 220 180 145 123 103 80

3.0 225 188 155 133 113 95

**********

1. Determine the removal rate at each depth and time using the equation:

xij = 1 CC0

Normalised Concentrations - Percent

Depth (m)Time of sampling (min)

30 60 90 120 150 180

0.5 47 67 80 85 88 91

1.0 28 50 63 74 78 83

1.5 19 40 53 63 72 77

2.0 15 33 46 56 64 72

2.5 12 28 42 51 59 68

3.0 10 25 38 47 55 62

2. Plot iso-concentration lines as shown in Figure 2.

3. Construct a vertical line at t0= 105 min

4. Calculate the removal efficiency (R) using the equation:

R = Rintercept +1

H(ZiR)

where from Figure 2, Rintercept = 43%


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(Note: if calculating overall removal efficiency for a tank that is 2 m deep, your Rinterceptwould be at a

depth of 2 m on the graph (about 51% removal) not 3 m. You can explore removal efficiencies for

hypothetical tanks of any depth up to 3 m using this chart.)

Figure 2: Type II Settling Analysis


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From Figure 2, integrating gives the following:

Removal Efficiency (R) = 0.43 + 1/3 (0.588) = 0.63 or 63%

Element R Zi R . Zi

r1 0.07 2.55 0.179

r2 0.1 1.73 0.173r3 0.1 1.13 0.113

r4 0.1 0.72 0.072

r5 0.1 0.39 0.039

r6 0.1 0.12 0.012

TOTAL 0.588

Lab 1 Settling

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