Performance and process mechanisms of a high-rate direct filtration plant targeting 0.1 ntu (PDF)

The central filtration plant (CFP) of Israel's National Water Carrier treats 1.7 x 10(6) m(3)/d of surface water in direct filtration mode at a maximum filtration rate of 20 m/h through anthracite media with a depth of 2 m. The CFP produces filtered water of < 0.2 ntu, thereby receiving accreditation for 3-log removal credit for Cryptosporidium oocysts. Precise dosing, of chemicals in the pretreatment stage-particularly preoxidation with chlorine dioxide (ClO2)-was found to be a critical factor in achieving low filtered water turbidities and particle counts as well as effective removal of algae and parasitic cysts and oocysts. Although these findings were for the conditions at this specific site, the effect of preoxidation with ClO2 is considered applicable to other water types. This article describes the results of pilot-plant studies that were conducted to develop design criteria and performance of the full-scale CFP. The authors explain and discuss the processes and mechanisms involved in achieving low filtered water turbidities and high algae and parasite removals.