How Oversized Mixers Became an Industry Standard

Traditional design criteria for keeping mixed liquor in suspension suggest power level requirements in the range of 6 – 8 W/m3 (30 - 40 hp/mil gal). However, published criteria from controlled lab experiments for flocculation of mixed liquor translate to power levels two orders of magnitude lower. This suggests that lower power levels may be feasible for mixing mixed liquor than currently commonly applied. The chemical engineering industry's approach for keeping solids in suspension investigates each impeller individually. This stands in contrast to the wastewater industry, where commonly a minimum power level regardless of mixer type is defined, be it top mounted, submersible, pumped-mix, or other. Limited published data suggests that power levels much below the traditional design criteria (as low as 0.1 - 0.2 W/m3 or 0.5 - 1.0 hp/mil gal) are able to maintain adequate mixing, as indicated by the variation of mixed liquor concentration vertically in an activated sludge tank.Several studies indicate that power levels can be further reduced when different process feed streams are combined before being fed into the mixed zone. Projects that the authors have been involved in confirm that these low power levels are feasible, but not in all applications. This paper discusses six case studies from different full scale facilities to summarize experiences gained with lowering mixing energy to keep suspended solids in suspension or - vice versa - predict under which conditions mixed liquor will settle in anoxic or anaerobic zones, which may be desired to stimulate nutrient removal through mixed liquor in-line fermentation. Future research directed at determining the impact of different MLSS CoV values on biological reaction rates, so that minimum mixing objectives can be defined in relation to the targeted process performance.