What happens if all dissolved oxygen is exhausted during aerobic decomposition in sewage? Can aerobic & anaerobic decomposition occur together?

Anaerobic bacteria can survive as spores in an oxygenated environment. They can't do anything except wait for oxygen to disappear, then they come back to life and start metabolizing. The reverse is true as well, if oxygen suddenly becomes present.If sewage undergoing aerobic respiration becomes anaerobic, all the