IONIZING RADIATION (the real one) has been most tested in food processing applications, with some deep controversies arising (perceived or real). This backdrop has less relevance to water purification, but exists as a strongly emotive coloring on the subject. And the one with a ton of actual use in the market.
http://www.webpages.uidaho.edu/foodtox/lectures/lecture16/L16-Food%2520Irradiation_outline.pdf