Denitrification

Recently we have a article about nitrate placed. Nitrate is the last step in the nitrification process , which is part of the (aerobic) The nitrogen cycle , the most important biological process in your pond. This article explains how nitrate is formed in the pond and what we need to do to keep the level below 50 milligrams per liter. Besides the three options mentioned in this article (water changes, plants, and nitrate resin), there's a fourth way to get rid of nitrate: denitrification.

What is it?

During denitrification, anaerobic bacteria convert nitrate into nitrogen gas. Anaerobic means that this process can only occur under oxygen-free conditions. The nitrogen gas thus formed can, as it were, evaporate (through a process called diffusion) from the pond. This is how you actually get rid of nitrate (via nitrogen gas).

4 NO3- + 5 C + 4 H+ <--> 2 N2 + 2 H2O + 5 CO2
Nitrate + Carbon + Hydrogen form Nitrogen gas + Water + Carbon dioxide

Oxygen-free?

Now you'll say that we don't want anoxic zones in the pond, and you're absolutely right. In these areas, rotting processes often occur, consuming all the oxygen in the environment and creating a breeding ground for harmful bacteria. So, we must avoid them. However, there's another place where anoxic zones develop, albeit at a microscopic level.

Biofilm

In the article on the nitrification process, we saw that our biological filters require carrier material to create the largest possible surface area for the growth of bacteria that convert ammonia via nitrite into nitrate. This colony of bacteria adheres to the carrier material and forms a biofilm. This is a greasy film, which you can also feel if you saw through the sink drain pipe and run your finger along the inside of the pipe.

Oxygen-free zone

Considering that nitrifying bacteria use a great deal of oxygen for this conversion, you can probably imagine that the deeper you penetrate the biofilm, the less oxygen there will be. With a sufficiently thick biofilm, an anoxic zone can develop around the carrier material, deep within the biofilm. In this zone, bacteria can develop that ensure denitrification.

Terms and Conditions

Denitrification will therefore only occur if the biofilm is sufficiently thick. It's often said that a biological filter can't be large enough in terms of attachment surface area. If you're aiming for denitrification, this isn't true. After all, if you have a large surface area, the biofilm will be thinner. The number of nitrifying bacteria won't increase due to a larger surface area, but will be more evenly distributed throughout the available space. The number of nitrifying bacteria depends on the food supply, not on the size of the filter.

Danger

Denitrification isn't something to strive for for the beginning hobbyist. You'll need to adjust the amount of filter material you use. There needs to be enough space for nitrifying bacteria to do their work (removing ammonia and nitrite), but at the same time, it needs to be just enough to form the thickest possible biofilm once denitrification is established. Too much carrier material will prevent denitrification; too little will risk an ammonia and/or nitrite spike.

Author: Joop van Tol (www.koitoday.nl)