ACHIEVING SIMULTANEOUS NITRIFICATION AND DENITRIFICATION OF WASTEWATERS AT REDUCED COST

The Orbal process for the treatment of waste waters, claims simultaneo us nitrification and denitrification in the outer lane in the presence of a dissolved oxygen concentration of 1.5 mg/l. The potential of thi s process is to offer both a reduced cost and simplified operation for nitrogen removal in the Mediterranean environment. The objective of t his work was to investigate these claims and attempt to describe a mec hanism for the experimental verifications. Using mixed cultures grown in chemostats, simultaneous nitrification and denitrification was obse rved under oxic/anoxic cycling. A reduced pathway, or 'nitrite shunt' could not be demonstrated but it was possible to explain the observati ons based on the lag time of the denitrifiers under oxic conditions. D enitrification continued for some time before oxygen replaced nitrate as the terminal electron acceptor and the length of the lagged respons e was a function of the duration of the anoxic conditions.