SODIS doesn't sterilize water, just reduce the load of
living pathogens. Higher latitudes reduce the effectiveness, but when combined with other water purification methods might be useful in many situations.

"Since the early days of SODIS it has been widely accepted that SODIS only works in the “35 degree window,” that is, the region between 35° North latitude and 35° South latitude. There is plenty of evidence that the “middle” of our planet gets the most direct sunlight (which is, of course, intuitively obvious), but there has been very little investigation into the relative effectiveness of SODIS at different latitudes. It is known that latitude is not the only factor effecting solar intensity (geographic and climactic factors, as well as altitude are also important), and that solar intensity is not the only factor effecting SODIS (temperature can be an important parameter as well), so the 35° window is probably a simplistic approach. In fact, there is evidence that even at latitudes north of 40°, SODIS can be effective, at least in the summer months. Sunlight intensity in Toronto on June 21 is higher than at the equator."

http://www.waterschool.com/faqs/