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The lime dosage we want should be able to increase the pH of water to the degree we want, while neutralizing the effect of the alum that will be shortly added to it. By calculating the alkalinity under the orginial pH we can find the total carbonate e.g.CT in the system, and then we can find the relationship between the target ANC and the desired pH we want(see figure 1). The total lime dose required in tank is the sum of the amount using to neutralize the alum and to raise the raw water to the desired pH(see equation 2).
After we get the required lime dose demand in raw water, we want to find the lime feeder flow rate to satisfy this dosage, under the assumption that the effluent pH of lime feeder keeps at 12.4, and the plant flow rate keeps at 50 L/min, we get the required flow in limefeeder. Based on equation 3, we can go further to get the ratio between flow rate in lime feeder and the plant with the pH requirement in raw water, this relationship shows in figure 2.
(How did you obtain your initial estimate for initial alkalinity? Is this different for different plants?)
<equation 1>
<equation 2>
Figure 1: The target ANC with the pH change.
Figure 2: The ratio of required lime flow rate and plant flow rate with the required pH in raw water.
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A hardness of 0.02g/L acts as a good buffer for the system (see figure 3). (Explain more how hardness is related to alkalinity.) From figure 1 it is observed that when the flow rate changes, the change of pH is not so dramatic but the case is much different if the hardness decreases. For instance, for the raw water of CUATRO COMUNIDADES for which the inital ANC is 7.65mg/L, ,and we assume the same initial pH(6.5), using the same method gives a much steeper slope (see figure 4). (Can you show a comparison plot of the two?)
NOTE: For the present calculations, it has been assumed that water will remian remain neutral even after the addition of alum so the final ANC is affetced only by the initial ANC (caused by the hardness of raw water) and the hydroxide provided by the 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.
(In the future add in the effect that the alum dose will have)
The equation to calculate balance OH ions in plant and the final ANC are also shown below:
balance OH in plant <equation 3>
final ANC of plant <equation 4>
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