UV light at the wavelength generated by the lamp in a Davey Steriflo® unit (254nm) is lethal to most micro-organisms as it damages their ability to reproduce. The organism is then no longer viable as it cannot grow to cause infection or be counted on a culture plate.
It has been shown that cysts such as Giardia and Cryptosporidium are inactivated by UV exposure. Water has to be very clean and free of particulate matter achievable through two stage filtration. For cyst control Davey recommend 1 micron filtration prior to supply of all drinking water.