According to the electron acceptor, both oxygen and nitrate could be electron acceptor for the respiration for microorganisms in activated sludge. The oxygen is preferred over the nitrate because of its higher redox potential and more energy produced in respiration. If you remove the oxygen from the agar plate or cultivate in anaerobic chamber, most probably the nitrate reducer will survive and proliferate and denitrification will be observed. But you should well design the ratio of COD and nitrate concentration, over high COD in anaerobic condition would induce the anaerobic pathway as well.