Comparison of rod- versus filament-type methanogenic granules: microbial population and reactor performance

Abstract

Two types of methanogenic granules capable of high chemical oxygen demand removal rates were developed in laboratory-scale upflow reactors at 35° C. One granule type (R-granules) had a rod-type Methanothrix-like species as the predominant species whereas the other (F-granules) had a filament-type M. soehngenii-like acetate-utilizer as the predominant species. These two types of granules were compared in terms of operational performance, physical-chemical characteristics and microbial population. The R-granules had a higher density [65–70 vs 39–43 g suspended solids (SS)/l], specific gravity (1.03 vs 1.01) and specific volumetric methane production rate (180 vs 120 l CH4/l granules per day) than the F-granules. Acetate, propionate and butyrate degraders in both types of granules had similar specific growth rates. The most probable number enumeration indicated that both types of granule had the same population levels (cells/g SS) in terms of methanogens (H2-CO2-, formate- and acetate-utilizing) and syntrophic acetogens. Hydrolytic-fermentative bacteria were present in greater number in the F-granules than in the R-granules. The R-granules had a higher cell density than the F-granules. The differences in operational performance were due mainly to their different microbial composition, especially the predominant acetate-utilizing methanogens in the granules. The long-filamentous M. soehngenii-like rods in the F-granules appeared to be responsible for their lower density and large-sized granules.