MICROBIAL ECOLOGY, BIOKINETICS AND THERMODYNAMICS OF METHYLOTROPHIC DENITRIFICATION

Methanol is a widely employed carbon and electron source for denitrification favored over alternate commercial carbon and electron sources such as ethanol or acetate since it is relatively cheaper. Although biokinetic parameters for methylotrophic denitrification in activated sludge have been previously reported (Purtschert and Gujer 1999), relatively little is known on the abundance and diversity of methylotrophic organisms in activated sludge. Current and previous methods for estimating methylotrophic denitrifying populations in activated sludge have relied chiefly upon modeling and mass balance approaches. In the absence of accurate knowledge on identity and abundance of methylotrophic bacteria in activated sludge, reactor design, monitoring and modeling efforts to optimize methylotrophic denitrification are severely handicapped.Iin this study, we applied a realtively novel technique, stable isotope probing of 13C labeled DNA to specifically identify the microorganisms that took up and metabolized methanol in an anoxic sequencing batch denitfiying reactor. Based on SIP, the major microorganisms were most closely related to Methyloversatilis spp. And Hyphomicrobium spp. Based on batch denitrification assays, methylotrophic nitrite reduction biokinetics were significantly lower than nitrate reduction biokinetics despite complete nitrate and nitrite removal in the parent SBR. However, little nitrite self-inhibition was observed in extant batch denitrification assays. Measured average yield coefficients for nitrite and nitrate reduction were statistically similar but were significantly lower than thermodynamics based estimates.Finally, as the terminal electron acceptor was changed from nitrate to nitrite (keeping the methanol dose the same), the specific denitrification rates on both nitrate and nitrite decreased significantly.