The HUASB reactor had highest organics accumulations because
it had the lowest up-flow velocity (i.e., lowest shear forces), which
allowed long-chain organic compounds to more readily accumulate
on the granule surfaces. To test this speculation, SEM was performed
on ‘‘seed’’ granules for the HUASB and AHR reactors, and
contrasted with granules collected after operations were completed.
Discrete microorganisms were apparent on the seed granules
(see Supplementary Data Fig. S1A); however, the granules
after treatment operations were significantly coated with amorphous
materials and individual organisms were not evident
(Figs. S1B and S1C). Although these observations are qualitative,
they are consistent with observed long-chain organics levels and
potentially explain poor methane production rates seen in the
granule-based reactors.