The lime feeder demand
The lime feeder water flow rate required for the experiment was calculated with Mathcad MathCad file. The analysis was made according with the dosage of lime, the operation time, water velocities, water pH and other variables, most of them were taken from the previous team report.
The team used a raw water with hardness of 0.02gm/L, and the initial pH is 6.5 based on the previous team spreadsheet report, the lime dosage we want is to increase the pH to 7, while neutralizing the alum added. We caculate the Alkalinity under the orginial pH and pH at 7, so we get the hydrogen demand to rasie to this ANC, the lime consumed to neutralize the alum is caculate with the relationship between the equivalent weight of Alum and Lime, the total lime required is the sum of these two parts, the equations are shown below.
The relationship between the ratio of flow rates and pH
But in actual condition, even if we made the flow rate in lime feeder keeps constant, the flow rate of the plant can still change in a considerable degree in operation. The team made new calculations to figure out what will this affact on pH and ANC MathCad file.
For the calculations it was assumed the effluent pH of limefeeder is constant at 12.4, and the flow rate between lime feeder and plant changes from 10^(-5) to 0.01, the raw water condition we still use the hardness provided by the previous team report that could make the assesment comparable to our caculation of lime feeder demand based on the same raw water hardness, which is 0.02gm/L.This hardness is a good buffer for the system, from the graph we can notice that with the flow rate changes, the change of pH is not so dramatically, the case can be much different if there is no hardness in the raw water, the slope of pH changing will be much more steeper;We can find the change of ANC and pH of raw water in the plant sensitivity to this ratio between flow rate(see figure 1.).
Right now we haven't taken into consideration the change of pH after addition of Alum. So the final ANC is affetced by the initial ANC caused by the hardness of raw water and the hydrogen provided by the limefeeder, and we assume the additional alum will consume certain part of the hydrogen that could contribute to the ANC decrease.
The eqation to caculate balance OH in plant and the final ANC are also shown below:
balance OH in plant
final ANC of plant
Figure 1: Graph - Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH
All calculations were made based on the data provided by the previous team's spreadsheet report.
On this stage, the team also continued to search for other information including the cost of distilled water, lime properties (solubility with temperature), and Honduras' water regulation.