Long-term, high-rate anaerobic biological treatment of whey wastewaters at psychrophilic temperatures

Two laboratory-scale anaerobic hybrid reactors, R1 and R2, treated low- (1 kg COD m−3) and high-strength (10 kg COD m−3) whey-based wastewaters, respectively, in a 500-day trial. The chemical oxygen demand (COD) removal efficiencies of R1 averaged 70–80%, at organic loading rates of 0.5–1.3 kg COD m−3 day−1, between 20 and 12 °C. The COD removal efficiencies of R2 exceeded 90%, at organic loading rates up to 13.3 kg COD m−3 day−1, between 20 and 14 °C. Lowering the operating temperature of R2 to 12 °C resulted in a decrease in COD removal efficiency, to between 50% and 60%, and a disintegration of granular sludge. The decline in performance, and granule disintegration, was reversed by decreasing the organic loading rate of R2 to 6.6 kg m−3 day−1. Specific methanogenic activity profiles revealed mesophilic (37 °C) temperature optima for biomass in both reactors, even after 500 days of psychrophilic operation, although the development of psychrotolerance in the biomass was noted.