The simultaneous removal of formaldehyde and ammonium in aerobic cultures and the inhibitory effect of formaldehyde on ammonium oxidation were investigated. The influence of a co-substrate, methanol, on formaldehyde biodegradation and on the nitrification process was also evaluated. Formaldehyde was completely removed at all concentrations tested (30-3890 mg dm(-3)) in assays with that compound as the single carbon source and in the presence of methanol as co-substrate. An initial formaldehyde biodegradation rate of 4.6 g CH2O g(-1) VSS d(-1) was obtained for 2000 mg CH2O dm(-3) as single carbon source compared with a rate of 7.3 g CH2O g(-1) VSS d(-1) when methanol was added. Formaldehyde was inhibitory to the nitrification process at initial concentrations higher than 350 mg dm(-3). Increasing the initial formaldehyde concentration or adding a co-substrate such as methanol resulted in a longer lag phase before ammonium oxidation and caused a decrease in the degree of nitrification. Nitrification was completely inhibited at initial formaldehyde concentrations higher than 1500 mg dm(-3). (C) 2004 Society of Chemical Industry.