ANC Control Mid-Term Report

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. The descriptions and diagrams pertaining 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.

The most effective design was the inverted cone because it. 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.

(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?)

ALKALINITY IN HONDURAN WATER

The table below shows actual measures of pH and alkalinity in AguaClara treatment plants in Honduras. To have high accuracy in the lime feeder design, the conditions of raw water in the laboratory will be simulated. The results in the table demonstrate a decrease of pH during the treatment process which is strongly visible on Cuatro Comunidades and Tamara plants.

Table 1: Water Quality in Honduras
Source: Honduras water reports, 2009
(Use a wiki table, not a poor quality image. Include a link to the data source. These reports are all posted on the AguaClaraES wiki.)
(What conditions are you going to design the lime feeder for (i.e. worse conditions)?)

PROPERTIES OF LIME

The pH of calcium hydroxide(lime) solution decreases with an increase in temperature. (see figure 1 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?)

Table 2: Changes in pH of lime with respect to Temperature changes
(give descriptive names to the images files. Replaces this image with a graph.)

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.