ANC CONTROL

Trial 2. TAP WATER, INCREASED AMOUNT OF LIME

INTRODUCTION

Despite the fact that the performance of the apparatus was as expected, in the previous trial, Trial 1, the pH could not be maintained at 12 for more than 20 min. So for the second trial, it was decided that the amount of lime used should be increased to 100 grams. It was projected that due to this change, more particles would stay (in virtual storage) at the middle of the apparatus providing hydroxides to increase the pH of the water, in order to achieve a stable pH of 12 for a much longer time.

OBJECTIVES

• To improve the results from the previous trial i.e. increase the time for which the effluent pH remains at 12.
• To check if there is scope for more improvement before starting the experiments with distilled water.

PROCEDURE

The same apparatus was used for this experiment as with the previous trial. The procedure used was that 100g of lime was made into a slurry and poured into the lime feeder from the top of the column (in order to avoid clogging). The flow rate was regulated by the peristaltic pump while the pH was recorded using a pH probe. Tap water was used for this trial. A more detailed explanation of the procedure can be obtained in the Materials and Methods section.

RESULTS

The apparatus worked well with the established upflow velocity, 4.42mm/s. The observed particle movements were satisfactory (as with the previous trial) i.e. a fluidized bed up to the middle portion of the lime feeder and a relatively clear effluent in the tube settler (sloped) region. The pH remained at 12 for 4hrs 20min which was much longer than the previous trial (only 20min), but it was still low compared to the desired run of 12hrs. The primary reason for its low duration was the hypothesized coating of calcium hydroxide particles (lime) with calcium carbonate present in tap water, exchanging surface sites of lime with HCO3-, and bringing HCO3- into equilibrium with solution as opposed to OH-, lowering the saturation pH.

Figure: Graph of Effluent pH versus Time

CONCLUSIONS

It was concluded that the sloped column model was working well, in terms of hydraulic design. Since no other technical complications were faced during this trial it was decided that the apparatus was fit to take on the next set of experiments - using distilled water, so as to eliminate the problem caused by tap water containing calcium carbonate. It was believed that doing this will help the experiment run at a pH of 12 for a much longer time.