Denitrification and methanogenesis in a co-immobilized mixed culture system

Abstract

A co-immobilized mixed culture system of denitrifying bacteria and methanogenic bacteria, obtained by entrapping two sorts of acclimated sludges in polyvinyl alcohol (PVA) gel beads, was developed. Denitrification and methanogenesis with methanol employed as a carbon substrate using the co-immobilized system were investigated under an anoxic condition. Methane formation could occur only after the completion of denitrification, which was associated with the removal of the surplus methanol. Enhancing the denitrification activity of the beads could accelerate the production rate of methane, while increasing the methanogenic activity did not affect denitrification. Substrate diffusion in the gel beads may play an important role in denitrification process. This was resulted from the effect of particle sizes and an observed dense growth of denitrifiers which primarily appeared on the surface of the beads. The simultaneous production of nitrogen and methane gases, in continuous culture, has elucidated that denitrification and methanogenesis could proceed with in one reactor. Microscopic observation revealed that a Hyphomicrobium-like bacterium (a denitrifier) grew mainly on the peripheral surface while a Methanosarcina-like bacterium (a methanogen) grew in the inner part of the gel beads. A metabolic process for synchronous denitrification and methane production of the co-immobilized mixed culture system in a PVA gel bead was proposed.