Designand construction specifications of a 3 mgd (12, 187 m3/day) demonstration plant to treat water by ozonation followed by diatomaceous earth (DE) filtration at New York City's Jerome Park Reservoir are described. The primary treatment objectives are to maintain a treated water quality of turbidity below 0.2 NTU, color below 3 color units, and iron and manganese below 0.3 mg/L. Three process trains will be employed, each operating at a maximum of 1 mgd (3,786 m3/day), to test variousmodificationsof theozone/DE process. The demonstration plant is scheduled to operate approximately one year, beginning in April 1986, to provide data for a full-scale plant to be constructed later in the decade.