Royal Doulton ceramaic water filters are as good as you can buy, with many advantages over seemingly cheaper filters. The Royal Doulton offers :absolute” rating, which means that it filters accurately as specified for the entire life of the filter. The filter can be used over and over again until it is waorn away! The Steracyl filter filters to 0.9 micron. This is effective in filtering out cysts such as Girdia, chryptosporidium and other microscopic bacteria.