Denitrification with methanol: Fundamental study of the growth and denitrification capacity of Hyphomicrobium sp

Hyphomicrobrium spp was found as dominant organism in a two-sludge nitrifying-denitrifying wastewater treatment system with methanol as external carbon source. The optimal pH for growth was found to be 8.3 and the organism seemed to be rather temperature sensitive (Q₁₀ = 3.3). The denitrification rate was expressed as a function of pH and temperature since it was almost independent on the concentrations of methanol and nitrate-nitrogen. Identical growth rates are found when using either nitrate- or nitrite-nitrogen; the nitrite consumption rate, however, is twice the nitrate reduction rate. Nitrate to nitrite reduction is the rate limiting step in denitrification reaction and some inhibition by high concentrations of nitrite on the nitrate reduction is measured. The methanol/nitrate-N ratio is 2.55 and increasing to 3.5 at extreme pH values. The endogenous denitrification rate is only 10% of the normal denitrification rate measured.