These substances should be added after the necessary contact time. Sodium thiosulfate reduces iodine or chlorine to iodide or chloride which have no taste or flavor. Sodium thiosulfate is non-toxic and available in chemical supply stores. Filtration imparts no taste and requires no contact time. Filters also remove sand, clay, and other matter, improving water taste and appearance.
Filters effectively remove bacteria and parasites, including cryptosporidium and giardia.
Filters also remove most viruses, even though viruses are far smaller than the openings in the pores. The filters causes the viruses to clump together and adhere to other particles, forming masses to large to enter the pores. But some viruses may pass through. As few as ten viruses may cause illness. GI disease-producing viruses are frequently present in water in developing countries but less often in the U.S. wilderness.