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

A multifed upflow filter (MUF), working under anoxic or anaerobic condition s, coupled with an aerobic biofilm airlift suspension (BAS) reactor was ope rated 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 rem oval, denitrification and urea hydrolysis took place simultaneously. The MUF was operated at 37 degreesC, at a hydraulic retention time (HRT) ra nging 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 tran sformed 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 Fro m the nitrifying airlift. However, only 80% of urea was hydrolysed to ammon ia in an anoxic environment while complete conversion occurred under anaero bic conditions. Moreover, formaldehyde concentrations higher than 50 mg L-1 provoked a loss of efficiency of urea hydrolysis, decreasing to 10% at for maldehyde concentrations above 300 mg L-1 Methane production rate during th e anaerobic stage was adversely affected by accumulations of formaldehyde i n the reactor causing lower formaldehyde removal efficiency. However, denit rification proceeded properly even at a formaldehyde concentration of 700 m g L-1 in the reactor, although nitrous oxide appears in the off-gas. The CO D/N ratios required For complete nitrite and nitrate denitrification with f ormaldehyde were estimated at 2.1 and 3.5 kg-COD/kg-N, respectively. (C) 20 01 Elsevier Science Ltd. All rights reserved.