Crushed basaltic river rock, calcite limestone, and calcite-amended basalt were three types of media tested in pilot-scale horizontal roughing filters pretreating slow sand filters fed from the Santiam River over 60-day periods. Montmorillonite and kaolinite clay challenge tests were conducted on ripened and unripened roughing filters to test performance at turbidities greater than 150 NTU, and clay removal trends were qualitatively examined using clay peak area ratios derived from X-ray diffraction. Enhanced montmorillonite removal was evident in the calcite roughing filter, but schmutzdecke growth was inhibited in the calcite-fed slow sand filter. The basalt, calcite and calcite-amended roughing filters in combination with slow sand filtration achieved 88.0, 95.3 and 99.6% clay turbidity removal, respectively, during the challenge tests. The slow sand filter pretreated with a calcite-amended roughing filter was the only configuration to produce effluent that complied with the 1 NTU regulatory requirement using influent of <1 to 150 NTU.

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