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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 figure 1 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 steeperthe hardness become smaller, for the raw water of CUATRO COMUNIDADES, the inital ANC is 7.65mg/L, we use the same method to caculate the ANC and pH sensitivity, the slope becomes much more steeper (see fugure 2);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 (Alkalinity is 20mg/L)
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.
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