Colloid Removal in Fluidized‐Bed Biofilm Reactor

A methanogenic fluidized‐bed biofilm reactor was successfully operated for the removal of 1‐μm organic colloids. The removal efficiency of total suspended solids was 72–76%, and the total chemical oxygen demand removal was 91–93%. Since some of the effluent suspended solids were biomass, the actual removal efficiency of the original organic particles was approximately 90%. Mechanistic filtration theory was modified to include bed fluidization, biofilm attachment to the collector surface, and effluent recycle. Independent filtration experiments showed that biofilm accumulation increased the cohesion coefficient from zero to 0.04. The predictions using a measured cohesion efficiency of 0.04 agreed with the findings from the methanogenic system: Removal of input suspended solids was 90% at 31% bed expansion; the recycle of effluent was an important determinant of the ability of the fluidized‐bed system effectively to filter the particle material; and, for the organic loading conditions of this study, the removal of organic particles did not vary with changes in the expansion of the bed. However, the modeling also predicted that increases in influent particle concentration should allow better overall particle mass removals for the same mass loading.