Influence of waste water composition on biofilm development in laboratory methanogenic fluidized bed reactors

Methane yield as a monitoring parameter for the start-up of anaerobic fixed film reactors

This paper describes the variation of the methane yield during the start-up period of an anaerobic fluidized bed reactor. After a lag phase, with acclimatized sludge, the methane yield increased with time during biofilm development up to the theoretical steady yield value, reported to be around 0.351 CH4/g CODdeg. The establishment of the biofilm required a high consumption of organic material through the microbial synthesis (anabolism), thereby reducing the proportion of substrate converted to methane. As a result, this yield could be an indirect metabolic parameter for evaluating a start-up operation. It could provide vital information about bacterial fixation processes and is easy to be applied to any biofilm reactor, such as anaerobic filters, where biomass sampling is impracticable. Monitoring this parameter could also give useful dynamic information about the different steps of colonization and biomass attachment, which could be used to improve the start-up performance.

Structural and functional dynamics of sulfate-reducing populations in bacterial biofilms

We describe the combined application of microsensors and molecular techniques to investigate the development of sulfate reduction and of sulfate-reducing bacterial populations in an aerobic bacterial biofilm. Microsensor measurements for oxygen showed that anaerobic zones developed in the biofilm within 1 week and that oxygen was depleted in the top 200 to 400 &mgr;m during all stages of biofilm development. Sulfate reduction was first detected after 6 weeks of growth, although favorable conditions for growth of sulfate-reducing bacteria (SRB) were present from the first week. In situ hybridization with a 16S rRNA probe for SRB revealed that sulfate reducers were present in high numbers (approximately 10(8) SRB/ml) in all stages of development, both in the oxic and anoxic zones of the biofilm. Denaturing gradient gel electrophoresis (DGGE) showed that the genetic diversity of the microbial community increased during the development of the biofilm. Hybridization analysis of the DGGE profiles with taxon-specific oligonucleotide probes showed that Desulfobulbus and Desulfovibrio were the main sulfate-reducing bacteria in all biofilm samples as well as in the bulk activated sludge. However, different Desulfobulbus and Desulfovibrio species were found in the 6th and 8th weeks of incubation, respectively, coinciding with the development of sulfate reduction. Our data indicate that not all SRB detected by molecular analysis were sulfidogenically active in the biofilm.

Microbial attachment and growth in fixed-film reactors: Process startup considerations

Optimal steady-state performance of any biofilm reactor requires a fully developed and mature biofilm. During fixed-film reactor startup phase, biofilm is in process of development and accordingly, process performance is difficult to quantify. Environmental, cellular and surface factors greatly influence the process of biofilm formation during reactor startup. Improved knowledge of nutritional, toxicological and environmental requirements of wastewater degrading microorganisms has helped define optimal microbial growth conditions. In case of anaerobic fixed film reactors the startup is hindered by low microbial growth rates, strict environmental requirements and limited ability of methanogens to adhere and form fixed biofilms. These obstacles could be overcome by proper support media selection and formulation of appropriate inoculation procedures and startup strategies.

Quantitative Microbiological Analysis of Bacterial Community Shifts in a High-Rate Anaerobic Bioreactor Treating Sulfite Evaporator Condensate

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 biofilms was 2 × 10 9 to 7 × 10 9 cells per ml. Enumeration and characterization 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 Methanothrix 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 × 10 8 to 7.5 × 10 8 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 × 10 8 to 2.6 × 10 8 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 × 10 7 to 5.8 × 10 7 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 biofilms. 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.

Early stages in biofilm development in methanogenic fluidized-bed reactors

Biofilm development in methanogenic fluidized-bed reactors with sand as the carrier was studied on a laboratory scale. The microorganisms present in consecutive layers of the biofilm of mature sludge granules were preliminarily characterized on the basis of their morphology, element composition and adhesion capacity and were compared to bacteria which take part in the initial colonization of sand. The early phase of biofilm development was monitored with reactors receiving waste-waters containing different mixtures of volatile fatty acids and inoculated with fluidized-bed reactor effluent for different lengths of time. The results obtained indicate that facultative anaerobic bacteria abundantly present in the outermost biofilm layers of mature sludge granules are probably the main primary colonizers of the sand. Methanothrix spp. or other methanogens were rarely observed among the primary colonizers. The course of biofilm formation was comparable under the various start-up conditions employed including variations in waste-water composition, inoculation and anaerobicity. However, omission of waste-water and thus of substrate resulted in rapid wash-out of the attached biomass.