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.
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. 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.
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.
Source: Honduras water reports, 2009
PROPERTIES OF LIME
The pH of calcium Hydroxide or lime decreases with an increase in pH. (see figure 1 below)
PROCEDURES AND RESULTS
One of the first tasks was to calculate the dosage of lime required by the lime feeder and the relationship between the changes in pH and ANC with changing flow rates in the lime feeder. The
analysis of lime feeder 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
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. 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.
!pH changes with Temperature - Lime.JPG!