High‐Rate Direct Filtration

The direct-filtration process differs from conventional sedimentation-filtration sys tems in that the total solids, both natural and added, must be stored in the filter. The early direct-filtration plants in Ontario were designed to filter at the maximum rate of 144 m/day (2.4 gpm/sq ft).* Some of the plants experienced oper ating problems at certain times of the year, and it became apparent that in-plant changes would be necessary to meet the rising demands of the municipalities. In 1967, the water-treatment plant-design filtration rates in Ontario were allowed to be increased to 288 m/day (4.8 gpm/sq ft). At the time of this change, it was believed that these rates would be achieved only with the proper pretreatment and sedimentation facilities. However, in the following years with the increased use of polymers and the wide use of multilayer filter media, directfiltration proposals utilizing the 288 m/day (4.8 gpm/sq ft) filtration rate were re ceived. In 1971 the Research Branch of the Ministry of the Environment (then called the Ontario Water Resources Com.) under took a program to study the operational variables of direct filtration. The primary purposes of this study were to rectify the