Co2

CaCO3

H2O i

Ca(HCO3)2

Carbon dioxide Calcium carbonate Water

Calcium bicarbonate

In contrast with the softening reaction, this process is suited to automatic pH control. Figure 7.42.11 shows this control system. The carbon dioxide feeder has two operators: one controlled by feedforward on the influent flow and the other by feedback on the effluent pH. The setpoint is about 9.5 pH.

Electrode fouling occurs as a result of the precipitation of crystallized calcium carbonate. Daily maintenance is required unless automated cleaners are used. The farther downstream (from the point of carbon dioxide application) the electrodes are placed, consistent with acceptable loop time delays, the less maintenance required.

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