The bacterial population of a high-rate, anaerobic, fixed-bed loop reactor treating sulfite evaporator condensate from the pulp industry was studied over a 14-month period. This period was divided into seven cycles that included a startup at the beginning of each cycle. Some 82% of the total biomass was immobilized on and between the porous glass rings filling the reactor. The range of the total number of microorganisms in these bioftlms was 2 x 109 to 7 x 109 cells per ml. Enumeration and

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... cterization by microbiological methods and by phase-contrast, epifluorescence, and electron microscopy showed that the samples consisted mainly of the following methanogens: a Methanobacterium sp., a Methanosarcina sp., a Methanobrevibacter sp., and a Methanothrir sp., as well as furfural-degrading sulfate-reducing bacteria resembling Desulfovibrio furfuralis. Viable counts of hydrogenotrophic methanogens were relatively stable (mostly within the range of 3.2 x 108 to 7.5 x 108 cells per ml), but Methanobrevibacter cells increased from <5 to 30% of the total hydrogenotrophic count after transfer of the fixed bed into a second reactor vessel. Acetotrophic methanogens reached their highest numbers of 1.3 x 108 to 2.6 x 108 cells per ml in the last fermentation cycles. They showed a morphological shift from sarcinalike packets in early samples to single coccoid forms in later phases of the fermentation. Furfural-degrading sulfate reducers reached counts of 1 x 107 to 5.8 x 107 cells per ml. The distribution of the chief metabolic groups between free fluid and biofilms was analyzed in the fifth fermentation cycle: 4.5 times more furfural degraders were found in the free fluid than in the bioffilms. In contrast, 5.8 times more acetotrophic and 16.6 times more hydrogenotrophic methanogens were found in the biofilms than in the free liquid. The data concerning time shifts of morphotypes among the trophic groups of methanogens corroborated the trends observed by using immunological assays on the same samples. Recently, an increasing interest in the anaerobic treatment of industrial wastewater (26, 35, 48) has greatly stimulated research on advanced reactor designs such as the upflow anaerobic sludge blanket process, anaerobic filter, fluidized bed, and fixed-bed loop (FBL) (9, 12, 14, 26, 31, 38) . With sulfite evaporator condensate (SEC) from pulp industries, the high-rate FBL reactor allows very high loading rates of up to 100 kg of chemical oxygen demand (COD)/m3 per day at short hydraulic residence times of about 11 h and a COD removal of over 80% (1, 2). This type of reactor exploits the natural ability of microorganisms to attach to solid surfaces (36). The information on organisms encountered in high-rate anaerobic reactors is scarce (24, 29). A Methanobacterium sp., a Methanosarcina sp., and a Methanothrix sp. were identified in biofilms and sludges of fluidized-bed and fixedbed reactors by microscopic (8, 34) and microbiological methods (6, 7, 12) . The characterization and quantification of microbes in conventional anaerobic digestors has a longer tradition (16, 23). For example, numbers of 109 to 1010 cells of hydrolytic bacteria per ml have been found in anaerobic sludge (17, 23, 30, 42, 43, 51) , often accompanied by sulfatereducing bacteria at numbers of 106 to 107/ml, of which members of the genus Desulfovibrio have frequently been among the dominant species (8, 40, 42) . In reactor samples, about 108 hydrogenotrophic methanogens per ml have been * Corresponding author. found, cells of Methanobacterium spp. and Methanospirillum spp. being the most abundant (50, 51). Acetate-utilizing methanogens in sludge reached numbers of 106 cells per ml, with Methanosarcina and Methanothrix species as the predominant organisms (50, 52). Recently (29) , we reported on the methanogenic subpopulations measured with antibody probes in a high-rate FBL reactor treating SEC. SEC is an acid wastewater that contains acetic acid, methanol, furfural, sulfate, and sulfite. It arises in bulk quantities in paper mills operating the bisulfite process, and it can be very efficiently degraded to biogas (1, 2, 4, 5). Only two organisms would suffice to completely convert the organic constituents in SEC to methane and carbon dioxide (4, 5): a Methanosarcina sp. (equations 1, 2, and 4) and a strain of Desulfovibrio sp. (equations 3 and 5) that was recently classified as the new species Desulfovibrio furfuralis (10).