Anaerobic granulation describes the self-immobilisation of
methanogenic consortia into dense,
particulate biofilms. This procedure underpins the operation of several categories of high-rate anaerobic wastewater treatment system. Full-scale anaerobic
granular sludge plants have been generally operated in the
mesophilic (20-45 degrees C) or
thermophilic (45-65 degrees C) temperature range. On the other
hand, recent studies highlighted the economic advantages of treating wastewaters at their discharge temperatures (mostly under 18 degrees C), removing a costly heating process and increasing net
biogas yield. However, as yet, relatively little information is available about the
microbial behaviour and interactions in anaerobic
granular sludge formed under
psychrophilic conditions. To this end, and in order to provide a
microbial insight into low-temperature anaerobic granulation, we monitored the changes in
methanogenic community structure, associated with the changes in process performance. Three, laboratory-scale, expanded
granular sludge
bed (EGSB)
bioreactors treating a synthetic
glucose wastewater were tested at two temperatures of 37+/-1 degrees C (R1) and 15+/-1 degrees C (R2 and 3). Quantitative
real-time PCR and specific
methanogenic activity
assays highlighted a community shift towards hydrogenotrophic
methanogens, particularly the order
Methanomicrobiales in the low-temperature
bioreactors. Corresponding to this, denaturing gradient
gel electrophoresis (
DGGE) analysis identified the emergence and maintenance of a Methanocorpusculum-like organism. Our results indicate that hydrogenotrophic
methanogens, particularly the Methanomicrobiales-related populations, are likely to play important roles in low-temperature anaerobic
granular sludge systems. This suggests that the process efficiency could be improved by facilitating the growth and retention of this group.
