Considering the maintenance of high quality seawater there are certain items we do not want to see at all, or at least wish to minimise them.
One parameter that often rears its head is nitrate. In the fish only aquarium nitrate can be allowed to rise a little, though personally I believe efforts should be made to keep it at the same level as a reef aquarium (the fish live on the wild reefs after all). The reef aquarium should ideally have no measurable nitrate, and the maximum guideline limit is 10 parts per million (ppm). This is because the reef system contains livestock that are more susceptible to trouble with high nitrate levels. Nitrate is also a nutrient for nuisance algae.
So how does the aquarist deal with nitrate. Firstly, it must be said that live rock and a deep sand bed (DSB) are recommended as filtration. If there is sufficient quality rock, the DSB is properly constructed, and feeding and routine maintenance are correctly done then nitrate may well never be a problem.
What if all efforts (including a careful examination of practices) fail to solve a persistent nitrate problem? It is at this point that the aquarist can consider a denitrator. There are two main types, one requires feeding and is a little more fiddly, and the other is a sulphur type which does not require feeding and will be considered here. These denitration devices are available commercially or can be DIY.
Basically, a normally tubular acrylic column is used which is filled with sulphur beads. Seawater is pumped through the sulphur at a very slow rate – this is a very fast drip when the device is mature. The action, by bacteria, takes place in a very low oxygen environment. As the bacteria do not have a ready oxygen supply, they are forced to use the nitrate, from which oxygen is extracted, breaking down the nitrate. Generally one molecule of nitrate is replaced by one molecule of sulphur. The sulphur has no detrimental affect on the seawater as it is present in a high percentage anyway. The only result to watch is that the output from the sulphur can be acidic. This is easily remedied by running it through some calcareous material before it returns to the aquarium.
Sulphur denitrators are efficient and easy to run. Should there be further interest I have added two links, the first descriptive and the second a DIY forum thread with pictures (ignore the sign-in prompt, it will disappear in a few seconds).