A sensitive ferricyanide-mediated biochemical oxygen demand assay for analysis of wastewater treatment plant influents and treated effluents

Representative and fast monitoring of wastewater influent and effluent biochemical oxygen demand (BOD) is an elusive goal for the wastewater industry and regulatory bodies alike. The present study describes a suitable assay, which incorporates activated sludge as the biocatalyst and ferricyanide as the terminal electron acceptor for respiration. A number of different sludges and sludge treatments were investigated, primarily to improve the sensitivity of the assay. A limit of detection (LOD) (2.1 mg BOD5 L−1) very similar to that of the standard 5-day BOD5 method was achieved in 4 h using raw influent sludge that had been cultured overnight as the biocatalyst. Reducing the microbial concentration was the most effective means to improve sensitivity and reduce the contribution of the sludge's endogenous respiration to total ferricyanide-mediated (FM) respiration. A strong and highly significant relationship was found (n = 33; R = 0.96; p < 0.001; slope = 0.94) between BOD5 and FM-BOD equivalent values for a diverse range of samples including wastewater treatment plant (WWTP) influent and treated effluent, as well as several grey water samples. The activated sludge FM-BOD assay presented here is an exceptional surrogate method to the standard BOD5 assay, providing representative, same-day BOD analysis of WWTP samples with a comparable detection limit, a 4-fold greater analytical range and much faster analysis time. The industry appeal of such an assay is tremendous given that ∼90% of all BOD5 analysis is dedicated to measurement of WWTP samples, for which this assay is specifically designed.

► A new variation of the ferricyanide-mediated bioassay for biochemical oxygen demand (FM-BOD) is described. ► The new assay requires 4 h and has a comparable LOD (2.1 mg BOD L−1) to the standard BOD5 assay. ► Similar results (R = 0.96, slope = 0.94) were obtained with the FM-BOD and BOD5 assays for 33 samples. ► This new FM-BOD assay is suitable for wastewater, grey water and treated effluent samples.