First, let me say that I am not a microbiologist so my knowledge in this area is extremely limited. My original interest in this LE was fluorescing salt deposits in crude oil heaters but I have interests in many areas of science.
I roughly measured the output of the UV LE in a Sundrop XR-U head to be 16mW/cm2 (using a Laserbee I). When you factor in that power was measured on an area of about 7cm2, irradiating to a target of 27J/cm2 becomes impractical in my mind.
Still, I figure there's a possibility for this to work. I took samples of run-off water from near my house in Pro-Clean bottles. I transfered 2 liters through a 5 micron filter to a translucent water bladder and suspended the UV Sundrop on high inside. I finished walking my dogs and transfered the water in the bladder into a new Pro-Clean bottle with a total irradiation time of 160 minutes. 5 day Biological Oxygen Demand was reduced enough for me to be comfortable drinking it (I'm not posting the actual numbers as I feel this one time test doesn't necessarily prove anything). As far as potable water goes, I would say you are better off using a SteriPEN. Dry surface irradiation is possible (SteriPEN does not function outside of water), but I feel the amount of time required to hit the target energy make it more of a novelty than a practical application.