Nitrification and Denitrification in Wastewater Activated Sludge

By Heather Jennings, PE

The great teeter–totter of wastewater is the nitrification and denitrification cycle in activated sludge wastewater systems. It takes both to convert ammonia to nitrogen gas! Both processes feed off of and support each other but, in some ways, they have competing needs.

Nitrification consists of ripping off the hydrogen in ammonia and adding oxygen to make nitrates and nitrites: this is accomplished by bacteria that we call nitrifiers. You might be already familiar with some of these nitrifiers, such as Nitrosomonas, Nitrobacter, etc. I, personally, call nitrifiers the “divas” of wastewater. They can only tolerate specific conditions to really thrive—such as a pH range of 6.5–8.0. They also require 7.1 lb of alkalinity for every 1 lb of ammonia that is oxidized, and they prefer temperatures at 77°F with DOs (dissolved oxygen levels) above 2mg/L. Retention times need to be longer than five hours, and the F:M (food to microorganism) ratios must ideally be less than 0.25. Basically, nitrifiers are screaming for their lattes to be a specific temperature and everything just right or they will refuse to work! Sadly, we can’t fire them. We have to learn how to meet their needs to get any nitrification done.  

Denitrifiers, on the other hand, consume the nitrates and nitrites created by nitrifiers. They basically rip the oxygen off the nitrates and nitrites created by nitrifiers and aren’t as picky about environmental conditions as the nitrifier “divas” are. They have their demands, though. When I described this process to my kids, my son looked at me and said, “So, denitrifiers eat nitrifier poop?” (It made for an interesting dinner conversation, to say in the least.)  

Denitrifiers prefer temperatures between 86°–95°F (30°–35°C) and DOs below 0.5 mg/L. Are you starting to see the conflicts yet? Denitrifiers are helpful, though; they not only remove the oxygen bound to the nitrogen in nitrates and nitrites (releasing the nitrogen as a gas), denitrifiers also recover half of the alkalinity used during nitrification! The competing needs of nitrifiers and denitrifiers are why many activated sludge systems have separate basins for nitrification and denitrification, or, at the very least, anoxic zones after aerated zones.  

Although it is possible to nitrify and denitrify in the same basin, you are going to have to find the compromise or balance point in your operating practices to accommodate both; hence, the teeter–totter comment at the start of this blog. But this balance is an essential part of the “art” of wastewater management, and it wouldn’t be fun without it!