Nutrient Removal - Denitrification (nitrate/nitrite removal)

Often I have seen people confuse nitrification and denitrification during conversation.  In the last post, I covered nitrification which is the conversion of ammonium (ammonia nitrogen) into nitrate via a two-step biological pathway. The organisms in the nitrification process are most commonly Nitrosomonas sp. and Nitrobacter sp. (There are other microbes, but they occupy the same ecological niche and have similar growth characteristics). Key points are the nitrification microbes are:

1. Need for high levels of dissolved oxgyen
2. Consumption of alkalinity
3. Relatively slow growth
4. Susceptibility to numerous toxic chemicals

Once ammonia is converted to nitrate, it is good to remove the nitrate from the water. In receiving streams, nitrate can promote algae blooms. Nitrate is removed under anoxic conditions (dissolved oxygen needs to be at effectively 0). Many waste water bacteria when faced without free oxygen in the water can use nitrate/nitrite as an alternative electron acceptor. This just means that the bacteria use the oxygen from the nitrate/nitrite.

When it comes to nitrate/nitrite removal the following conditions are needed:

1. Effective 0 dissolved oxygen - anoxic conditions
2. A source of soluble "food" ~ often called BOD.  Note the solubility factor, as using nitrate as electron acceptor does not allow for rapid metabolism on complex or insoluble compounds such as fats, oils, or long chain hydrocarbons.

In wastewater treatment we usually find denitrification setup involving an initial anoxic step at the front of the system where return sludge rich in nitrate mixes with influent high BOD waste. The bacteria use the oxygen in the nitrate, producing nitrogen gas that escapes to the atmosphere.

Denitrification can also be a problem when there is excessive nitrate in the secondary clarifier. If the solids are held in the clarifier too long (usually >3 hours) - the bacteria can use the nitrate to degrade adsorbed organics. The sludge then floats to the top of the clarifier and can carry over the weir. In this case the most often used solution is to increase recycle rates which lowers residence time in the clarifier.