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To get the lime dosage we want it is necessary to know the initial total carbonate in the system. e.g CT, from the initial Alkalinity and pH of Marcala and Cuatro Comunidades we can find the CT based on equation 1 1:
And because we assume this is a closed system so the CT will not change during the process, so the relationship between Alkalinity and pH can be simply measured using equation 2:
In the next step, considering the flow rate of the plant will also change, the team would like to use the ratio between the lime feeder flow rate and the plant flow rate to get a more practical function, and from mass balance equation 3, we can acquire the relationship between this flow ratio and the concentration of hydroxide ion in raw water. The relationship between OH- concentration and alkalinity can be caculated with equation 4, the premise is that we have to know the equivalent of proton created by adding aluminum sulfate, which can be measured from the difference of initial alkalinity and final alkalinity from our data.
Based on all these datas and equations now we can make the model showing the change of pH and alkalinity as the funcion of flow ratio between lime feeder and plant. The model which predict these relationships of plants in Marcala and Cuatro Comunidades are shown below:
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).
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