The simultaneous removal of formaldehyde and ammonium in a lab-scale activated sludge unit was investigated. The unit was operated at a hydraulic retention time of 2.4 days with an ammonium concentration in the influent of 350 mg NH4+-N/L, maintaining ing the ammonium loading rate at 0.15 g NH4+-N/L d during the operation time. However, the applied organic loading rate was increased stepwise by increasing the formaldehyde concentration from 26 up to 3168 mg/L, corresponding to 0.01-1.40 g COD/ L d. High formaldehyde removal efficiencies, around 99.5% (+/- 0.38), were maintained at all the formaldehyde concentrations. Ammonium removal was also very high during the operation period, around 99.9% (0.01). The ammonium concentration in the effluent was lower than 0.1 mg NH4+-N/L at all applied organic loading rates, indicating that there was no inhibition of nitrification by formaldehyde. (c) 2005 Elsevier Ltd. All rights reserved.