A comparison of media types in acetate fed expanded-bed anaerobic reactors

A synthetic feed, containing acetate as the only carbon source, was used to start-up four different anaerobic expanded-bed reactors containing three different types of microbial attachment media. The media types used were low-density anthracite, granular activated carbon (GAC) and two sizes of sand. All media types were of the same average diameter, 0.7 mm, except for a smaller sand, 0.35 mm. These media types were chosen to compare surface roughness, macroscopic shear stresses due to upflow velocity and sphericity. The 0.7 mm sand required the greatest upflow velocity, 16 cm/s, while the other reactors had upflow velocities of 5.5-6.0 cm/s. Sand had the least surface roughness and GAC had the roughest surface, while anthracite had the most angular shape. At steady-state, the GAC reactor retained 3.75-10 times the attached biomass retained on the other media tested and the GAC reactor accumulated biomass at a faster rate during start-up. Shear losses reflected the biomass accumulation with the two sand and anthracite media having shear loss coefficients 6-20 times greater than that of the GAC medium. Sand induced the formation of sludge granules in both sand reactors with two species of methanogens and stability of the sludge blankets was critical to reactor performance. Scanning electron microscopy demonstrated that attached growth developed in crevices where biomass was protected from shear forces. Attached growth on the sand and anthracite media was located only in crevices, while the GAC medium is completely covered with crevices and biofilm developed on the entire GAC particle. Surface roughness was critical to biofilm development with the rougher surface providing the better attachment medium.