There are many ways that aquarists can maintain high quality seawater. Yes, we’re back to that again, but no apologies as it would be great if all aquariums were successful and high quality seawater is very important, in fact, the most important aspect for success.
A problem that regularly makes an appearance is nitrate. Nitrate is a product of the nitrogen cycle, where the bacteria convert the toxin ammonia to the toxin nitrite, which is then converted to the generally non-toxic nitrate. In the full cycle nitrate is converted to gas which can escape from the aquarium.
If live rock is in use then the full cycle should be present, provided there is sufficient live rock and it is of good enough quality. Live rock can be overloaded of course by overfeeding and overstocking, and also by having insufficient quality rock present in the first place. So balance is required – enough food for the livestock, not too heavy a bio-load (too much livestock), enough live rock (enabling there to be sufficient bacterial activity), and regular routine seawater changes of a sufficient amount. Each aquarium is different, and the aquarist should know his or her aquarium’s requirements. It is not unknown for nitrate to be a problem even with all the requirements as shown present, and the cause not discovered.
If bio-filtration is completed by a canister filter, or a fluidized sand filter, or other designs of a similar nature, then the nitrogen cycle only goes as far as the production of nitrate. It follows that as time passes there will be an increasing amount of nitrate present in the seawater. (Infrequently nitrate is not a problem with these filters – there are possible reasons for this that will not be gone into here.) The first action to control the nitrate level is regular routine partial seawater changes. This on its own is often sufficient. Feeding and livestock levels need to be correct as in the previous paragraph.
If nitrate is a persistent problem and the aquarist has failed in attempts to find the cause, and is satisfied that husbandry disciplines are fine, (feeding, the amount of livestock, and the amount of seawater changed at routine changes) then there is another way. It has to be emphasized that this is a way to be tried after all remedial efforts have failed; it is not a first resort. More advanced aquarists will usually wish to try caulerpa in the sump etc in an effort to ‘mop up’ nitrates.
The sulphur denitrator is, as the name implies, a filter that removes nitrate. It is normally in the form of an acrylic tube, 3″ or so in diameter, with a feed at the bottom and an outlet at the top. Seawater is pumped through the tube where the nitrate is attacked. Bacteria again are the workforce. The term used is ’autotrophic denitration’ as I understand it (ref: C. Soler. mars/reefkeepers.net).
The bacteria that convert ammonia and nitrite need oxygen and cannot operate without it. However, the bacteria that convert nitrate are kept in a very low to zero oxygen content environment. If oxygen were to be present the bacteria would use that and ignore the nitrate. However, if oxygen is insufficient the bacteria will extract it from the nitrate, thus breaking the nitrate down. Therefore in the denitrator oxygen is kept at an insufficient level forcing the bacteria to obtain their needs from the nitrate.
The seawater being pumped through the denitrator flows very slowly. In fact, the flow rate can be seen at the return end as a fast drip (when the denitrator is mature). Therefore the pump power need only be low. Often, aquarists divert some flow from the return of a canister filter or similar. If pumped direct from the aquarium or sump there needs to be a mechanical filter before the seawater enters the denitrator.
In the denitrator being discussed the media is sulphur. This is usually in the form of small beads, about ¼” or so. The bacteria live on this media and as they convert the nitrate they produce sulphate on a 1:1 ratio. The bacteria use the sulphur as an energy source rather than have a separate food supply as is required in other style denitrators (heterotrofic types) where additional feeding is required. Sulphate is not a problem to the seawater quality as sulphate is present naturally at a high level, about 2700 ppm (parts per million).
The seawater leaving the denitrator is acidified to a degree. Though it is unlikely to adversely affect properly buffered seawater, it is best to increase the pH. This can be done quite easily and at the same time obtain an advantage. The returning seawater, which is flowing very slowly as said, can be routed through a container with calcareous material in it. The acidified seawater will tend to dissolve this material increasing the pH, and at the same time calcium will go into the aquarium. The calcium flow cannot be relied upon, and tests if necessary still need to be made. It is very unlikely that the calcium needs of a hard coral (SPS) system would be met, for example.
Using a sulphur denitrator could and more than probably would be the answer to a difficult nitrate problem that the aquarist has not been able to resolve in other ways. It is easy to start up and quick to mature, and does not need any additional fiddly feeding as does the other type. It also tends to be more forgiving with flow rate than the other type, but within close boundaries. The sulphur also lasts a long time.