So, you want to build a denitrifier (which is not too uncommon in the saltwater world). What I am about to post is simply a copy and paste operation from research I did some time ago, but it is a lot of reading. Also, you may want to find @Phil Edwards
, who has quite a bit of experience in this area. He has a post entitled something like “Ask me anything”, where you can posit some of your questions. I have not tried any of this, as I came to the conclusion that it was far too involved for a planted tank since nitrates can be controlled with many of the techniques we do normally in a planted tank, including letting the plants digest them. If you do try it, and it is a runaway success, please post your method and results.
First, some threads that may help and there are many others, as the topic has been explored many times:
Now for the regurgitation and I apologize to any whom I may have not credited on any of this information, but it is something I meshed together for my own use:
NO3 Reduction (denitrification)
Other than water changes or a very heavy and healthy plant mass, the only other way to reduce nitrates is with anaerobic bacteria. This requires very slow moving water, a carbon source and, obviously, an absence of oxygen.
Instead of water changes, there are two primary methods for denitrification, the carbon fed digester and the sulfur bed digester. Some strains of bacteria known as facultative anaerobes can use the oxygen in the nitrate (NO3) for energy, converting NO3 into atmospheric nitrogen (N2). The carbon digesters require the addition of an organic carbon source to feed the bacteria, typically a short chain alcohol (methanol or ethanol) or a simple sugar. This method has the drawback of having to measure the addition of the carbon compound. Over- or under-dosing can upset the balance of the process or, worse, carry unreacted alcohol back to the tank. The sulfur-based method relies on the activity of several bacteria strains that consume sulfur and nitrate without adding other chemicals. Thiobacillis denitrificans and other similar bacteria can use the oxygen in nitrate for energy. The trend in the industry is the sulfur bed digester because of its simplicity.
The NO3 to N2 reaction (called "reduction") consumes alkalinity, so the sulfur is typically mixed with crushed oyster shell or aragonite (a source of carbonate for buffering) to keep the pH stable. Monitoring D.O. is especially important since the environment inside the filter should be oxygen-poor, but not oxygen-free. If too much oxygen is present, the bacteria will respire aerobically (with oxygen) and no nitrate reduction will occur, but if too little oxygen is present, the filter can go anaerobic (without oxygen) and produce toxic hydrogen sulfide.
Red Sea NO3:PO4-X (NOPOX) is safe and effective means of adding dissolved organic carbon (DOC) to freshwater aquariums, although specifically designed for saltwater aquariums. It is more reliable than vinegar or vodka and safer than methanol (purity issues with all three of these additives). However, not sure if this is supposed to ‘activate’ anaerobic bacteria in our substrates or if some sort of denitrifies is expected to be established first.
Activated carbon (which can remove some dissolved oxygen) will not affect NOPOX. The NOPOX is only adversely affected by Phosphate removers since it is a phenol based supplement (as per Red Sea).
Media should be rinsed every few months to clear slime from the pores. Initially, soak the media and shake it to remove air bubbles (there should be no floating pieces).
Lava rocks greater than 1” can be used to harbor anaerobic bacteria, in the filter. Allow several months for development. Another possibility is to place several in a container, drilling a small (.125-.250”) hole in the bottom and a larger (.250-.500”) hole in the top to allow a small stream of water through. This can be placed in a canister filter.
Seachem claims that all of their pumice stone (DeNitrate, Matrix, and Pond Matrix) will culture both aerobic and anaerobic bacteria. The three products are all the same media, just different sizes. The different recommended flow rates relative to the sizes is simply that anaerobic bacteria can only live in an environment of little/no oxygen. This is relative to the depth of the pores in the stone and the flow of the water passing through and around it. So in theory, anaerobic bacteria could exist with Pond Matrix with a faster water flow, while there would need to be a very slow flow for DeNitrate.
Brightwell Aquatics Xport NO3 Cubes (supposedly limited to pH =/> 7.5) is a possibility for the denitrator.
Colonizing the media:
Add Stability or Microbacter7 and a carbon source (NoPoX): adding daily for about 7 days directly onto the media will place many colonies of denitrifying bacteria on the media. They will immediately begin colonizing the Matrix, therefore being able to consume the nitrates present. Place it in a separate container with very low flow if it can’t be reached in it’s final destination.
Bring nitrate levels down to 20 ppm via water changes. At that point de*nitrate will bring the nitrate levels down to 4 – 5 ppm after several days of use. Since de*nitrate™, Matrix™,and Pond Matrix™are all biological support media, they do not actually ever exhaust, but they can grow less efficient with use by pore clogging. Prefiltering the water before it passes through these products will extend its useful life.
Remove Existing Filters Designed To Facilitate The Nitrogen Cycle:
Such filters do a fine job of processing ammonia to nitrite to nitrate, but do nothing with the nitrate. It is not that any less nitrate is produced when such a filter is removed, it is a question of what happens to the nitrate after it is produced. When it is produced on the surface of media such as bioballs, it mixes into the entire water column, and then has to find its way, by diffusion, to the places where it may be reduced (inside of live rock and sand, for instance).
If nitrate is produced on the surface of live rock or sand, then the local concentration of nitrate is higher there initially, and is more likely to diffuse into the rock and sand where the anaerobic bacteria are in close proximity.