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The lime dosage we want should be able to increase the pH of water to 7the 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 and a pH of 7, we can find the hydrogen demand to raise to this ANC. The amount of lime consumed to neutralize the alum can be calculated using total carbonate eg.CT in the system, and then we can find the relationship between the equivalent weight of Alum and Lime.target ANC and the desired pH we want(see figure 1). The total lime dose required in tank is the sum of these two parts, the equations are shown belowof the amount using to neutralize the alum and to raise the raw water to the desired pH(see equation below).
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, we know the effluent OH supply, and we suppose the plant flow rate is 50 L/min, we get the required flow rate in lime feeder with the pH requirement in raw water(see figure 2).
Relationship between the ratio of flow rates and pH
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A hardness of 0.02g/L acts as a good buffer for the system (see figure 13). 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 24).
NOTE: For the present calculations, it has been assumed that water will remian 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.
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
Figure 1: The target ANC with the pH change.
Figure 2: The required lime flow rate with the required pH in raw water.
Figure 13: Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH (Alkalinity is 20mg/L, good buffer.)
Figure 24: 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)
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