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Alum is dosed using a flow control module (FCM) developed by AguaClara. The FCM is a bottle fitted with a plastic float valve on the inlet, and a set length of tubing on the outlet. The float valve ensures a nearly constant head loss through the module, while the exit tubing provides a calculable head loss from friction. Plant operators adjust the alum dosing necessary for plant operation by adjusting the height at which the exit tubing connects with the raw water stream. The operators have a chart that relates influent turbidity to required alum dose, and required alum dose to the required tubing height change. The dose delivered by the FCM is therefore a function of the (constant) head in the bottle, the (constant) length and material of the outflow tubing, and the change in elevation across the tubing length, which is what the operators manipulate to adjust dosing.

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