Tests were conducted at the Contra Costa Water District's Bollman Water Treatment Plant to evaluate four alternative types of vertical shaft flocculation impellers: rake or gate-type impellers; the Rushton turbine; the 45 pitched blade turbine; and the foil-type impeller (pitched blade turbine with camber). The objectives of the study were to characterize both the hydraulic and flocculation performance of the impellers at each of three scales.