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 〖NH〗_4^+-N/L, maintaining the ammonium loading rate at 0.15 g 〖NH〗_4^+-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 〖NH〗_4^+-N/L at all applied organic loading rates, indicating that there was no inhibition of nitrification by formaldehyde.
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