Formaldehyde and urea removal in a denitrifying granular sludge blanket reactor

Simultaneous formaldehyde biodegradation, urea hydrolysis and denitrification in anoxic batch assays and in a continuous laboratory anoxic reactor were investigated. In batch assays, the initial formaldehyde biodegradation rate was around 0.7 g CH2O g VSS−1 d−1 and independent of the urea concentration (90– 370 mg N–NH2CONH2 l−1). Urea was completely hydrolyzed to ammonium in the presence of 430 mg l−1 formaldehyde and complete denitrification took place in all cases (125 mg N–NO−3 l−1). Formaldehyde removal efficiencies above 99.5% were obtained in a lab-scale denitrifying upflow sludge blanket reactor at organic loading rates between 0.37 and 2.96 kg COD m−3 d−1 (625–5000 mg CH2O l−1). The urea loading rate was increased from 0.06 to 0.44 kg N m−3 d−1 (100–800 mg N–NH2CONH2 l−1) and hydrolysis to ammonium was around 77.5% at all loading rates. The denitrification process was always almost complete (100–800 mg N–NO3− l−1), due to the high COD/N ratio of 6.7 in the influent. A minimum value of 3.5 was found to be required for full denitrification. The composition of the biogas indicated that denitrification and methanogenesis occurred simultaneously in the same unit. A good granulation of the sludge was observed.