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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:
Both of the two plants have almost the same increasing pattern of pH and alkalinity after we increase the flow rate ratio, and we can also notice the zero point of both plants represent when there is no lime feeded in water, and the pH and alkalinity are only affected by aluminum sulfate. What the model can show us is that the change of pH is affacted by the buffer intensity of bicarbonate, since the buffer intensity is simply the differential of alkalinity over pH, we can generate the buffer graph to show this relationship more clearly, the buffer graph are shown below:
The different buffer intensity in Marcala and Cuatro Comunidades is simply because they have different initial alkalinity. As the initial pH starts from 6, it just locate in the zone which the buffer intensity is greatest(the highest point is when pH equals to pK1 of carbonate system), as the pH continously increase the buffer intensity becomes weak and finally it can not control the pH increasement anymore, the pH starts increasing exponentially which would easily cause the coagulation mechanism faliure and undesirable effluent water quality, just as what the two models predict.
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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