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Nitrification and Denitrification in Wastewater Activated Sludge

By Heather Jennings, PE

The great teetertotter 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 nitritesthis 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 thrivesuch as a pH range of 6.58.0. 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…]

Nitrification 101

By Heather Jennings, PE

Today we are going to focus on nitrifiers, those wastewater treatment autotrophs that get energy from oxidizing ammonia. (Autotrophs are microorganisms that produce complex organic compounds using inorganic carbon from simple substances as a food source.) Oxidizing ammonia is a fancy way of saying ripping off hydrogens to stick oxygens onto nitrogen—the essence of nitrification.

In wastewater treatment we rely on bacteria to perform nitrification. While Nitrobacter and Nitrosomonas are the most commonly recognized, they are only part of a suite of autotrophic nitrifying bacteria that do the job. Mind you, there are also heterotrophic nitrifiers that are part of the floc-forming bacteria that help in the oxidation of ammonium, but we’re going to focus on the autotrophs this time around. [Read more…]

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