INTRODUCTION
The former ANC group experimented on different designs for a lime feeder, including a column model, a conical vessel, a funnel-column and an inverted traffic cone model. These results can be found on pages 10 through 17 of ANC control with [lime.docx|^ANC control with Lime-1.docx] and in Table 1 below. The descriptions and diagrams pertaining to the experimental set-ups are explained in pages 4 through 8 of the above report.
The column succeeded in keeping the lime suspended for a few hours but the water began to flow in a preferential path after the lime settled on the bottom. For the conical column, the mixing at the bottom of the vessel proved to be insufficient in keeping all the lime in a suspended state. On the other hand, the funnel-column apparatus worked well for 20 hours but only because it was unclogged periodically, which would not possible in a real-time set up.
As a result, the inverted cone model that supplied saturated limewater at a pH between 11 and 12 for about 18 hours without having to be unclogged and without the above difficulties was selected as the best alternative among them. However, the main problem with it is that inverted cones are extremely difficult and expensive to construct, install and maintain. So the task of the ANC team is to search for a simpler solution for the lime feeder design. Since the upflow velocities were also highly variable (as seen in Table 1), another goal is to find the optimal upflow velocity for the limefeeder.
Reactor |
M_init (g) |
Q (L/min) |
Res.Time |
T_obs (hr) |
T(hr) |
Vb |
Vbottom |
|---|---|---|---|---|---|---|---|
HalfCone |
400 |
0.6 |
12.48 |
9 |
12.56 |
|
84.9 |
Funnel |
200 |
0.1 |
46.10 |
20 |
37.67 |
0.71 |
12.4 |
Funnel |
200 |
0.1 |
46.10 |
25 |
37.67 |
0.71 |
12.4 |
Funnel |
100 |
0.1 |
46.10 |
7.75 |
18.84 |
|
12.4 |
Cone |
300 |
0.4 |
31.95 |
6 |
10.69 |
1.5 |
19.6 |
Cone |
300 |
1 |
12.78 |
1.25 |
4.28 |
|
49.0 |
Cone |
500 |
0.88 |
14.52 |
4.75 |
8.10 |
2.35 |
43.1 |
Cone |
1800 |
0.55 |
23.24 |
18 |
46.65 |
|
26.9 |
Cone |
1000 |
0.52/0.84 |
22.02 |
11 |
|
|
|
Cone |
1000 |
0.67 |
20.00 |
12 |
|
1.65 |
32.8 |
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*Table 1. Results of 2006 Experiments. M_init is the initial mass of lime; Q is the flow rate; T_obs is the time the effluent had a pH of 12; T is the theoretical duration; Vb is the upflow velocity; and Vbottom is the velocity at the bottom of the vessel.
ALKALINITY IN HONDURAN WATER
The table below shows the actual measurements of pH and alkalinity in AguaClara treatment plants in Honduras. To more accurately research ANC, the conditions of Honduran raw water will be simulated in the laboratory. The results in the table demonstrate a decrease of pH during the treatment process. It is strongly visible on Cuatro Comunidades and Tamara plants. Therefore, one goal is to increase the alkalinity of the water, creating a buffer system against acidity.
OJOJONA |
RAW WATER |
TREATED WATER |
|---|---|---|
pH (UN) |
6.51 - 6.8 |
6.26 - 6.56 |
Alkalinity (mg/L CaCO3) |
34.7 - 36.3 |
17.3 - 19.4 |
MARCALA |
RAW WATER |
TREATED WATER |
pH (UN) |
6.44 - 7.28 |
6.07 - 6.45 |
Alkalinity (mg/L CaCO3) |
15.3 |
Before chlorination 8.7 |
CUATRO COMUNIDADES |
RAW WATER |
TREATED WATER |
pH (UN) |
6.34 - 7.00 |
6.80 - 6.85 |
Alkalinity (mg/L CaCO3) |
7.65 |
Before chlorination 4.59 |
TAMARA |
RAW WATER |
TREATED WATER |
pH (UN) |
La Chorrera: 6.44 |
6.56 - 6.66 |
Alkalinity (mg/L CaCO3 |
La Chorrera: 7.14 |
4.08 |
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*Table 2: Water Quality in Honduras
Source: Honduras water reports, 2009
PROPERTIES OF LIME
The pH of calcium hydroxide(lime) solution decreases with an increase in temperature. (see Table 3 below) The temperature at the Honduran plants is generally in the range of 19-21 degrees C but during summers it can increase to 27 degrees C. This temperature change can change the pH of lime and thereby affect the working of the lime feeder and needs to be taken into consideration while designing the lime feeder. (Explain how this is expected behavior of solute in water with respect to temperature. Why is pH of the saturated solution decreasing with increasing temperature?)
Temperature ºC |
pH |
|---|---|
0 |
13.423 |
5 |
13.207 |
10 |
13.003 |
15 |
12.810 |
20 |
12.627 |
25 |
12.454 |
30 |
12.289 |
35 |
12.133 |
40 |
11.984 |
45 |
11.841 |
50 |
11.705 |
55 |
11.574 |
60 |
11.449 |
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*Table 3: Changes in pH of lime with respect to Temperature changes
PROCEDURES AND RESULTS
One of the first tasks was to measure the relationship between the changes in pH and ANC with changing flow rates in the lime feeder. The analysis of lime feeder flow requirements 
was made with the help of MathCAD software.
The next tasks were a series of experiments carried on the column-based model of a lime feeder.
Experiment 1: Feasibility of using a Glass column as a fluidized Lime feeder
Experiment 2: Testing the lime feeder performance using a pH probe
Experiment 3: Addition of sloping glass column above the lime feeder and Tube-length Calculations
FUTURE TASKS
- Run several trials with the new design to determine the amount of lime required to maintain a pH of 12 for 24 hours. In addition the performance of the new design will be evaluated and revised, if necessary.
- Simulate Honduran water conditions. Honduran water is low in alkalinity and can be simulated using distilled water instead of the alkaline Cornell tap water. This will give us a true idea of the effectivenes and run-time of the lime feeder.
- Recalculate design dimensions while taking into account the natural variation of flow rate at the inlet tank in the water treatment plant.
