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At the Four Communities plant, the chemical dose controller (CDC) adds alum to raw water in the entrance tank. The appropriate alum dose varies based on two factors, the incoming turbidity and the plant flow rate. The CDC is designed to maintain a constant dose regardless of changes in the plant flow rate.
The original CDC used in the Four Communities plant was the first generation of the technology used by AguaClara, (Figure ###). The CDC is made up of three components— a flow control module (FCM), lever arm and a sutro weir (FIGURE ###). The alum flow rate is set by the difference in height between the FCM inlet valve and outlet hose. This flow rate is constant regardless of the available head in the alum stock tank. The sutro weir is used to create a linear relationship between plant flow rate and the height of water in entrance tank. The lever arm unites the first two components of the CDC. The outlet hose of the FCM is connected to one end of the lever arm while a float balances the opposite end of the fulcrum. The float height varies with the water level in the entrance tank. When the float rises, the opposite end of the lever arm with the FCM outlet hose falls increasing available head in the FCM and therefore the alum dose. The coordination of plant flow rate and alum dose created by the CDC decreases the number of variables the operator must deal with to choose an appropriate alum dose.
Over the course of the summer, the alum flow rate was measured and the CDC system was monitored to detect any failure modes. The alum flow rate was every time the operator changed the dose by filling a graduated cylinder for thirty seconds. The results are shown in figure ###.
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