*The Lime dosage required in the Lime-feeder *
For our experiment, the
The lime feeder demand
The lime feeder water flow rate required for the experiment was calculated with Mathcad in a MathCad file.
The analysis was made according with variables required in the analysis, such as the dosage of lime, the operation time, water velocities, water pH and other variables, most of them etc. were taken from the values in the previous team's report.
The team used a Hence the raw water with was assumed to have a hardness of 0.02gm/L , and the an initial pH is of 6.5 based on the previous team spreadsheet report, the .
The lime dosage we want is should be able to increase the pH of water to 7, while neutralizing the effect of the alum that will be shortly added to it. We caculate By calculating the Alkalinity alkalinity under the orginial pH and a pH at of 7, so we get can find the hydrogen demand to rasie raise to this ANC, the . The amount of lime consumed to neutralize the alum is caculate with can be calculated using the relationship between the equivalent weight of Alum and Lime.
The , the total lime required is the sum of these two parts, the equations are shown below.
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The relationship between the ratio of flow rates and pH
But in Under actual conditionplant conditions, even if we made cannot keep the flow rate of water in the 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.constant. The water flow rate entering the lime feeder will keep changing in proportion to the actual plant flow rate because the doage of lime required in the plant will change as per the amount of water entering the plant. So the calculations must be based on the variable 'ratio' i.e. the fraction of plant flow rate entering the lime feeder.
The effect of this ratio on the ANC and pH are illustrated in the MathCad file.
It was assumed that For the calculations it was assumed the effluent pH of limefeeder is constant at 12.4, and the ratio of 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 . Other values in the calculations were kept the same as in the previous calculation.
A hardness of 0.02g/L acts as a good buffer for the system (see figure 1), from . From figure 1 we can notice that with it is observed that when the flow rate changes, the change of pH is not so dramatically, dramatic but the case can be is much different if the hardness become smallerdecreases. For instance, for the raw water of CUATRO COMUNIDADES , for which the inital ANC is 7.65mg/L, we use using the same method to caculate the ANC and pH sensitivity, the slope becomes much more steeper gives a much steeper slope (see figure 2);
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NOTE: For the present calculations, it has been assumed that water will remian neutral even after the addition of alum so Right now we haven't taken into consideration the change of pH after addition of Alum. So the final ANC is affetced only by the initial ANC (caused by the hardness of raw water) and the hydrogen hydroxide 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 lime feeder. But in reality, the alum addition will contribute to a further decrease in the pH of water and so one of our future tasks is to re-calculate the effluent pH required to negate the effect of the alum dose.
The equation to calculate balance OH ions in plant and the final ANC are also shown below:
balance OH in plant
final ANC of plant
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Figure 2: Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH (Raw water in Cuatro Comunidades, Alkalinity is 7.65mg/L)
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.