How a about providing the article, or at least a link to it, please.
Was the article that you read providing the rule of thumb 8 gph/watt written for an AquaUltraviolet UV system or was it generic as to make and model?
Generally speaking, slower water flow through the UV chamber does, indeed, provide more UV exposure to a particle passing through it. You can see that clearly in the table that @twillard inserted in post #8 that shows the design operating points for flow rates versus exposure for various wattages of AquaUltraviolet UV models. For example, for a 57 watt unit, a high flow rate of 3200 gph through the chamber provides 30,000 microwatts per square centimeter but at 1/3 of that flow rate triples the UV exposure. The article appears to discuss levels of sterilization which I presume equates to what the exposure level is for a particle passing through the device. I expect that the specific design of the UV unit will change that specific rule of thumb somewhat.
Also, the number of passes required for a level of cumulative exposure has to be taken into account. AquaUltraviolet bases its claims on 3 passes to get a certain high percentage of particles to have been irradiated sufficiently for sterilization.I do not recall the actual percentage number, itself. It could be that the article did not specify the same basis of 3 passes. In fact, as I recall, the table in post #8 assumes 3 passes to achieve the exposure rates shown at the heads of the columns of flow rates. You will notice the "EOL" in the column headers under exposure levels. That is telling you that the data provides the exposure levels when the bulb has reached its End Of Life.
My personal experience has been that most aquarists I have communicated with on the use of UV have not sized their flow rates properly to tank size and wattage. As a result most I have known are woefully low in their flow rates. If they are using the UV with reef systems they are overexposing with it and are killing off beneficial organisms. My understanding is that UV sterilization is only effective on single-cell organisms. Even those with hard cell walls may not be adequateley susceptible to UV. The UV does not directly kill the organisms; rather it tinkers with the DNA of a vulnerable organism, thereby preventing it from reproducing successfully. One other note is that exposing water to UV does no harm to it. There is no residual effect on the water passed through the exposure chamber beyond killing off vulnerable organisms.
