ANC CONTROL

FALL 2009 Research

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 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 . (Put link here and describe where in the previous report you would find these recommendations)
The column model 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. (Can you show visuals of these set-ups or are they shown in the report? Can you give the specific page numbers?)

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. The most effective design was the inverted cone because it. However, the main problem with it is was that inverted cones are extremely difficult and expensive to construct, install and maintain. So the task of the ANC team is was to search for a simpler solution for the lime feeder design. (Can you summarize their results in a table? What upflow velocities were most successful? What was the velocity of the jet at the bottom of the column?)
Since the upflow velocities were also highly variable (as seen in Table 1), another goal was to find the optimal upflow velocity for the limefeeder.

{float:left}*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.{float}

ALKALINITY IN HONDURAN WATER

The table below shows the actual measures measurements of pH and alkalinity in AguaClara treatment plants in Honduras. To have high accuracy in the lime feeder designmore accurately research ANC, the conditions of Honduran raw water were simulated in the laboratory will be simulated. The results in the table demonstrate a decrease of pH during the treatment process which . It is strongly visible on Cuatro Comunidades and Tamara plants.
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Table 1: Water Quality in Honduras
Source: Honduras water reports, 2009(What conditions are you going to design the lime feeder for (i.e. worse conditions)?)
. Therefore, one goal was to increase the alkalinity of the water, creating a buffer system against acidity.

{float:left}*Table 2: Water Quality in Honduras
Source: Honduras water reports, 2009{float}

PROPERTIES OF LIME

The pH of a saturated calcium hydroxide solution (aka lime) solution decreases with an increase in pHtemperature. (see figure 1 Figure 3 below) The temperature at the Honduran plants is generally in the range of 19-21 degrees C but during summers it can go up to about increase to 27 degrees C. This temperature change differential can change the pH of lime limewater 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?)
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Figure 3.
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Table 2: Changes in pH of lime with respect to Temperature changes

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PROCEDURES AND RESULTS

One of the first tasks was to calculate the dosage of lime required by the lime feeder and the 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.

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Experiment 3: Addition of sloping glass column above the lime feeder and Tube-length Calculations

 

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• 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. Once the lime feeder design is working successfully with distilled water, a new experiment will be required to analyze the performance with a lower pH concentration.
• Recalculate design dimensions while taking into account the natural variation of flow rate at the inlet tank in the water treatment plant.