Denitrification under high nitrate concentration and alternating anoxic conditions

Denitrification under high nitrate concentration was studied in a fed-batch mode. Methanol, sodium acetate and effluent from anaerobic digesters (AD) were compared as the carbon sources. An alternating oxy–anoxy regime (10 min aeration over a period of 110 min) seems to promote the establishment of denitrifying bio-flocks of good settling characteristics. The bio-flocks produced in an anoxic reactor were of inferior settling characteristics. Bio-flocks at concentrations of 4 to 5 g VSS/l denitrify efficiently on all three carbon sources tested. A maximum nitrate removal rate was found to be 0.486 g NO3−-N/(g VSS day). Nitrite accumulated initially but its concentration decreased with time and eventually reached zero when acetate or AD effluent were used as the carbon source.