OK. I can't give you every last detail because they don't provide all the details. But, it goes something like this. Each parcel of fluid passing through the reactor has to receive a certain minimum solar UV radiation dosage. To do this, they vary the water flow rate so that, as the solar flux decreases, the flow rate also decreases in proportion. There is going to be a certain minimum value of the total pipe volume and surface area necessary to achieve the desired cumulative radiation dosage over all radiation conditions and associated flow rates. This determines the design of the reactor.Yes, also given the inactivation constant, the pipe diameter and the initial water quality.