Achieving simultaneous nitrification and denitrification of wastewaters at reduced cost

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