a package plant for treating raw water to yield potable water . the package plant includes an inlet for raw water , followed by an ozonator for disinfecting the water . next is provided an up flow roughing filter including a chemically active layer , on top . a slow sand filter follows , which is sized and shaped to promote growth of a biomass for water purification . the up flow roughing filter removes particulates , contributes to the removal of dissolved organics , and protects the slow sand biomass by removing disinfection residuals . in another aspect a method of maintaining the plant is provided , including washing the up flow roughing filter by draining water down through it , providing a vigorous up flow to agitate the top layer , and then washing the slow sand filter .