UASB process consistently removed 84% of COD in wastewater with hydrolyzed proteins for loading rates up to 32 g-COD/L/day, corresponding to a food-to-microorganisms ratio of 0.81 g-COD/g-VSS/day, at 37°C and a hydraulic retention time of 9 hours. Of all the COD in the wastewater, about 74% was converted to methane, 16% was unhydrolyzed proteins which remained refractory to degradation, and 10% converted to biomass. The average sludge yield was 0.079 g-VSS/g-COD. There was no noticeable foaming and sludge flotation. The maximum specific methane production rate in the reactor was 0.60 g-methane-COD/g-VSS/day, which was comparable to the specific methanogenic activity (SMA) of 0.59 g-methane-COD/g-VSS/day observed by the serum vial test using hydrolyzed proteins as substrate. The SMA using acetate as substrate was 0.89 g-methane-COD/g-VSS/day, higher than those (0.39–0.59 g-methane-COD/g-VSS/day) using formate, propionate and butyrate, individually, as substrate. The granules did not have a layered structure nor a predominant type of bacteria. Instead, it had a densely packed structure with interwined bacteria of diverse morphologies with scattered microcolonies of Methanothrix, Methanosarcina, and justapositioned syntrophic associations.

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