Sewage treatment in chemistry!

So i have a test coming up and im so confused and i cant ask my teacher as it is the weekends. so in sewage treatment when the sludge is taken to the sludge tank does it use anaerobic respiration or aerobic respiration ? and in sterilisation where you add UV ozone and chlorine to kill microbes does aerobic respiration happens at that point? Also I wrote in my notes that aerobic respiration (not sure if its aerobic or anaerobic) is used to break down sludge but i didnt write when that happens, does it happen at sterilisation (bio treatment) or sludge tank? does this anaerobic or aerobic respiration happens at water treatment too?