Abstract: A system for purifying bodies of water, such as swimming pools, includes a prefilter upstream of a catalytic filter, and a pump for periodically circulating water to be treated through the prefilter and then through the catalytic filter. The catalytic filter includes a series of horizontally disposed filter elements each having a stationary layer of a permeable water-purifying biocatalyst composition. The preferred biocatalyst is a microbicidal surface compound formed by wet processing finely divided silver oxide, zinc oxide and lampblack. Water circulated through the prefilter is filtered to remove suspended solids or algae which would clog or otherwise disturb the biocatalyst filter cake. Water circulated through the catalytic filter is purified by surface contact with the biocatalyst. Repeated pool water contact with the biocatalyst generates a residual microbicidal condition in the pool water.