...
The lime feeder water flow rate required for the experiment was calculated with Mathcad. The analysis was made according with the dosage of lime, the operation time, water velocities, water pH and other variables, most of them were taken from the previous team report.
But in actual condition, even if we made the flow rate in lime feeder keeps constant, but the flow rate of plant can still change in a considerable degree in operation, so we have to figure out what will this affact on pH and ANC. We assume 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, we want to find the change of ANC and pH of raw water in the plant sensitivity to this ratio between flow rate(see figure 1.).
Rright 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
...