Simultaneous urea hydrolysis, formaldehyde removal and denitrification in a multifed upflow filter under anoxic and anaerobic conditions Original Research

A multifed upflow filter (MUF), working under anoxic or anaerobic conditions, coupled with an aerobic biofilm airlift suspension (BAS) reactor was operated in order to treat a wastewater with high formaldehyde (up to 1.5 g L−1) and urea (up to 0.46 g L−1) concentrations. In the MUF, formaldehyde removal, denitrification and urea hydrolysis took place simultaneously. The MUF was operated at 37°C, at a hydraulic retention time (HRT) ranging from 1 to 0.3 d. An organic loading rate (OLR) of 0.5 kg-formaldehyde m−3 d−1 was efficiently eliminated during anaerobic operation and transformed into methane, while a much higher OLR (up to 2 kg-formaldehyde m−3 d−1) was oxidised under anoxic conditions by the nitrite or nitrate from the nitrifying airlift. However, only 80% of urea was hydrolysed to ammonia in an anoxic environment while complete conversion occurred under anaerobic conditions. Moreover, formaldehyde concentrations higher than 50 mg L−1 provoked a loss of efficiency of urea hydrolysis, decreasing to 10% at formaldehyde concentrations above 300 mg L−1. Methane production rate during the anaerobic stage was adversely affected by accumulations of formaldehyde in the reactor causing lower formaldehyde removal efficiency. However, denitrification proceeded properly even at a formaldehyde concentration of 700 mg L−1 in the reactor, although nitrous oxide appears in the off-gas. The COD/N ratios required for complete nitrite and nitrate denitrification with formaldehyde were estimated at 2.1 and 3.5 kg-COD/kg-N, respectively.