ANC CONTROL

Experiment 4: Overloading the reactors with lower flow rate

Overview

According to the results of the third experiment the team discussed that the best way to achieve the saturation pH value in a longer period of time was by reducing the flow rate. Flow rate reduction will imply a reduction in the up flow velocity and will allow a longer retention time in the reactor. To probe this hypothesis, the team continued with the idea of increasing the volume of the lime in the reactor in order to provide more solid lime surface area, repeating the conditions of the third experiment but reducing the amount of lime to 100g and changing the up-flow velocity to 0.9 mm/sec.

Procedure

100 grams of lime was mixed with tap water and stirred to form a slurry. This was fed through the vertical column. The up-flow velocity was 0.9 mm/sec in both apparatuses. All the other procedures were the same as described on materials and methods.

Results and Discussion

This was the first experiment in which pressure and temperature gauges were implemented. Those devices control the influent water, avoiding floods and providing a continuous source of tap water.

The experiment ran for almost 80 hours; results are divided in two plots to facilitate their reading. As can be seen in the first figure, the effluent pH remained above 12 units for a short period of time in comparison with the total time. The 2.5 cm tube settler lasted for 12.5 hours and the 5 cm tube settler for 18 hours. It is longer than the results observed in experiment 3. However, it is important to remark that readings above 12.6 were observed; that is not chemically possible due to the saturation pH of lime. Those readings were associated with problems in the pH probes that the team wanted to fix for future experiments. The great drop in pH is due to a trial to calibrate the pH probe; that reading corresponds to buffer solutions.

The second half of the experiment corresponds to a new addition of 100 grams of lime after 45 hours. Low readings are due to pH probe calibration. It can be observed that the increase in pH was dramatic and that both reactors remained above 12 for 19 hours (2.5cm tube settler) and 27.5 hours (5cm tube settler). Despite these promising results, the high fluctuation and the readings above 12.6 make them inaccurate and nullify the data. It is necessary to repeat the experiment when there is more certainty regarding the pH probe readings.
 
 
In order to ensure accurate pH readings, the pH probes were grounded to reduce the potential for high voltage fluctuations that damage the data acquisition system. In addition, when non experiments are running, pH probes are placed in a 4 pH solution to avoid lime clogging into the pH probe membranes.

                                                                                                         Figure 1A. Experiment 4 performance. First 40 hours. (pH vs time)
                  The purpose of this experiment was to decrease the upflow velocity in order to increase residence time.  The mass of lime added was also decreased to 100 grams.

                                                                                                            Figure 2A. Experiment 4 performance. Second 40 hours. (pH vs time)

                      The purpose of this experiment was to decrease the upflow velocity in order to increase residence time.  The mass of lime added was also decreased to 100 grams.