PREOZONATION TO IMPROVE AND OPTIMIZE DIATOMACEOUS-EARTH FILTRATION

The benefits of preozonation before diatomaceous earth filtration have been demonstrated in a City of New York research program to develop design criteria for treatment of its 300 mgd Croton water supply. In the treatment of a supply where particulate matter is predominantly of organic origin, preozonation provides two outstanding advantages that are not possible in diatomaceous earth filtration alone. 1. Removal of colloidal color to a true color of practically zero in the product water. 2. Significant extension of the length of filter runs between cleaning cycles. Color removal is achieved in the reaction of ozone with the suspended particulates in the ozone contact step in a manner similar to the microflocculation provided in the more familiar granular filtration. Extension of filter runs is accomplished through the action on the separated organic solids by residual ozone, carried over from the contact step, passing through the accumulated diatomaceous earth filter cake. This latter action results in continuous improvement of cake porosity to reduce the rate of head loss build-up.