COD fractionation and biological treatability of mixed industrial wastewaters

This study was conducted at a centralized wastewater treatment plant that receives discharges from nearly 160 industries. The chemical oxygen demand (COD) was fractionated for two objectives: delineation of the limits of the activated sludge process being used at the plant, and evaluation of the potential environmental impact of the treated effluent. Physico-chemical analyses, respirometric and biodegradation tests, as well as COD fractionation were carried out. Molasses-wastewaters were determined to be the major contribution to the plant. The influent was dark brown in color, with a relatively high content of both organics (2503 mg/L COD) and salts (5459 μS/cm conductivity), but a low biochemical oxygen demand (568 mg/L BOD5) and BOD5/COD ratio (0.24). The degradability of the organics was limited by the high content of inert soluble COD (SI). The COD fractionation pattern was 40–20–40% for SI, XI (inerts) and SH (soluble hydrolyzable), respectively. More than 90% BOD5 removal was obtained, which was sufficient for the plant to meet the national Standards. However, the effluent discharged into the river was intensely colored and polluted (>1000 mg/L COD, >5000 μS/cm), emphasizing the need for legislation regulating COD, color and salinity, and for upgraded treatment methods worldwide for molasses wastewaters.

► Dominant characteristics of the wastewater (WW) derive from the presence of molasses.
► COD fractionation of the WW showed 3 components (40–40–20% for SH, SI, and XI).
► The slowly biodegradable fraction was in soluble form (SH) and not particulate form.
► The effluent is legally discharged to a River (BOD5 met) but remains highly polluted.
► It is urgent to regulate COD and upgrade treatment methods for molasses WW worldwide.