This paper discusses aspects of the formation, growth and detachment of sulphate-reducing aggregates, growing on H2/CO2 and H2/CO gas mixtures in a 4.5 L lab-scale gas-lift reactor. The biomass in the aggregates consisted predominantly of Desulfovibrio sp. and Acetobacterium sp.

Our experiments with H2/CO2 showed that aggregates with pumice particles, i.e. biofilms, as well as carrier-free aggregates were formed. At pH 7.0, all the aggregates had a very hairy structure. This structure, however, was pH dependent. At lower pH-levels the aggregates became smooth. At pH-values higher than 7.0 the hairy structure became even more irregular. The predominant microorganisms were rather heterogeneously distributed throughout the aggregates at all pH-values.

Growth on gas-mixtures of H2/CO led to the formation of smooth aggregates. Presence of CO also led to formation of a layered biomass structure in which the Acetobacterium sp. were more at the outside of the aggregates and the Desulfovibrio sp. were located at the inner part of the aggregates.

The observed changes in the surface structure of the sulphate-reducing aggregates are related to the growth rate of the microorganisms present. A high growth rate yields a rough and hairy aggregate surface. A low growth rate yields a smooth aggregate surface, since the low growth rate of the microorganisms allows detachment of the large protrusions from the aggregate surface.

The surface structure is not dependent on the type of bacteria present at the surface, i.e. sulphate-reducing bacteria or homo-acetogenic bacteria.

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