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.   [Read more…]