A number of recent studies have described new rapid biochemical oxygen demand (BOD) methods. However, most have not maintained the features that make the 5-day standard BOD assay particularly relevant to wastewater management – a high level of substrate bio-oxidation and use of wastewater treatment plant (WWTP) sludge as the biocatalyst. In a critical breakthrough, return activated sludge (RAS) from Coombabah WWTP, southeast Queensland, was successfully incorporated as the biocatalyst in a ferricyanide mediated-BOD (FM-BOD) bioassay. The bioassay was initially optimized for the measurement of highly variable and complex wastewaters, particularly trade wastes, by maximizing the analytical working range (10 – 170 mg BOD5 L-1) and extent of substrate degradation (96 ± 23% of measured BOD5 oxidation). A highly significant correlation (n = 35; slope = 1.07; R = 0.95; incubation time = 6 h) was found between this RAS FM-BOD and standard BOD5 assays using a range of real trade waste samples. The activated sludge FM-BOD bioassay was re-examined with the goal of measuring low–mid range wastewaters (i.e. treated effluents and WWTP influents) that comprise the bulk of all BOD samples analyzed worldwide. All experimental parameters were re-optimized, primarily to improve the detection limit of the FM-BOD assay to approximate that of the standard BOD5 assay (i.e. ≈2 mg BOD5 L-1). Primary influent sludge (PIS) from Coombabah WWTP was the most favorable sludge trialed, with the new bioassay having an analytical range of 2 – 40 mg BOD5 L-1. A highly significant correlation (n = 33; slope = 0.94; R = 0.96; incubation time = 3 – 4 h) was observed between the PIS FM-BOD and standard BOD5 assays using a range of treated effluent, influent and grey water samples.