Response of methanogen populations to organic load increase during anaerobic digestion of olive mill wastewater

Process performances of an upflow anaerobic filter treating olive mill wastewater and the response of methanogenic Archaea to increasing volumetric organic load (VOL) were studied. At a VOL of 15 g chemical oxygen demand (COD) L^-1 day^-1, 90% of the infiuent COD was removed. Following a VOL increase from 6 to 15 g COD L^-1 day^-1, the polymerase chain reaction (PCR) titre of hydrogenotrophic Methanobacterium, determined by magnetic capture of the target DNA and group-specific PCR based on the 16S rRNA gene, decreased from 10¹¹ to 10⁸ cells g^-1 sludge, while that of Methanomicrobiaceae and relatives increased from 10⁴ to 10⁶ cells g^-1 sludge. Methanosaeta-like acetoclastic methanogens were less affected by VOL variation and dominated at high VOL with a 16S rRNA gene PCR titre of 10⁹ cells g^-1 sludge. Single-strand conformation polymorphism analysis of the PCR-amplified archaeal 16S rRNA gene showed a stable band pattern, indicating that VOL variation affected the methanogen PCR titre but not the archaeal community structure.