Q. Do you know anything about nitrate reduction with the help of elemental sulfur? In Germany, there is a special aquarium filter promoted using small sulfur drops as a filter bed.
A. The device you mention is known as a sulfur reactor and uses a process called autotrophic sulfur denitrification. Sulfur-based denitrification in freshwater is based upon autotrophic denitrification by sulfur-oxidizing bacteria, such as Thiobacillus denitrificans and Thiomicrospira denitrificans. Under aerobic conditions, these bacteria use oxygen as an electron acceptor, but under anoxic conditions, they oxidize various forms of reduced sulfur to sulfate, while reducing nitrate to liberate the oxygen for their own use. In addition to nitrate, T. denitrificans can also use nitrite. Autotrophic denitrifiers utilize inorganic carbon compounds (e.g., carbon dioxide, bicarbonate) as their carbon source. The process in seawater is not fully understood yet, but it does seem to work with the same result: the total removal of nitrate.
Professor Guy Martin, a specialist in water treatment at the Engineer National School of Chemistry in Rennes, France, is credited with originating the idea of using elemental sulfur and autotrophic bacteria to eliminate nitrate, but he only applied it to fresh water treatment for drinking water. Beginning in 1991, Marc Langouet, a past student of Dr. Martin, tested the method with seawater on his home reef aquariums – a risk given that no one knew if it would have toxic effects. By the end of 1994, after three years of experiments without apparent toxicity in numerous aquariums, Langouet proposed this method to Michel Hignette, curator of the Musée des Arts Africains et Océaniens (MAAO) Aquarium in Paris. There, a pilot project was launched under his care. Since then, experiments have been done on a much bigger scale at the MAAO, as well as at the Grand Aquarium in Saint-Malo, where Langouet was technical and scientific director from June 1996 to December 1997.
The system uses small beads of elemental sulfur placed in a contact chamber. Aquarium water is introduced at the bottom of the chamber at a very slow rate and exits out the top. As the water moves slowly upward, oxygen is removed by bacteria, making the chamber more anaerobic the further up the water rises. At this point, the bacteria will use nitrate in the absence of oxygen so that by the time the water exits from the top of the chamber, most of the nitrate has been removed. The chemical reaction that takes place releases excess hydrogen ions, which makes the water acidic.
Nitrogen gas is also produced, and this leaves the water at the top of the chamber (one reason why a reverse-flow design is favored). It is believed that carbon dioxide is also produced in this process, further acidifying the water. To counter the acidity, the effluent from the reactor should pass through another column of crushed coral in which the acidic water will dissolve the coral gravel, raising the pH and generating calcium ions.
Another by-product of sulfur-based denitrification is sulfate. Passing the water through calcareous gravel precipitates this sulfate and prevents it from entering the aquarium. The sulfur beads can last a very long time, but the chamber of coral gravel will quickly turn to sediment and should be replenished on a regular basis. The use of a fluidized bed design for both the sulfur and the coral gravel reactors should help to reduce the build-up of sediments.
These reactors are very effective at removing nitrate. In a 16,000-gallon saltwater fish aquarium at the MAAO, aquarists found that a sulfur-based denitrification system was able to reduce the nitrate ion level from a high of 320 mg/L to less than 10 mg/L in a month, so these reactors are very effective. The question is, however, whether we really need them in a reef aquarium in which nitrate levels are generally quite low to begin with.
This depends on how low such filters can bring the nitrate. Most reef aquariums still have nitrate levels that are a magnitude greater than those found in nature. If these filters can bring down aquarium levels to natural reef levels, they may well be worth the effort to use. On fish-only systems, these filters can easily help bring down nitrates to much lower levels. In addition, with the increase in the numbers of fish and the amount of food being added to reef aquariums, a sulfur denitrator may be a necessary tool to remove excess nitrate. Finally, if aquarists want to keep many of the more delicate nonphotosynthetic corals, such as Dendronephthya, which require frequent feedings, these filters would again help keep nitrates under control.
Some of you may be wondering what all the fuss is about, since denitrification filters have been around for over 10 years. The difference is that those filters rely on heterotrophic bacteria to reduce nitrate to nitrogen gas. This means that you need to feed the bacteria a carbon source, usually methanol or ethanol, in a controlled manner, and adjust the amount in response to changes in nitrate levels. Too much alcohol, and you get hydrogen sulfide production. Too little, and you have nitrite and nitrate released back to the aquarium. These filters also produce a large amount of bacterial slime that needs to be removed on a regular basis.
The genius of the sulfur-based denitrators is that they do not need to be constantly adjusted, you don’t need to feed them (other than to add some new sulfur maybe once a year or less), and they can also be used in conjunction with a calcium reactor to add calcium and alkalinity to the aquarium. Put simply, they are much easier to use with much less risk than alcohol-based reactors.
It is also possible to use this method in conjunction with a Jaubert plenum system, and most likely, with a deep sand bed system. By adding a thin layer of sulfur beads just above the plenum, or bare bottom of the saltwater aquarium in a deep sand bed, you can use the anaerobic environment there to allow sulfur-based denitrification to occur. This might allow Jaubert or deep sand bed systems to carry a heavier bioload by increasing their denitrifying capacity. You could also set up a system in a refugium or adjacent sump to allow you to add such a system to an existing saltwater aquarium without having to tear it down first.
The advantage of this system is that you don’t need to add another piece of equipment, but I prefer the convenience of having a separate device I can remove without having to disrupt my saltwater aquarium. Several manufacturers are offering these systems in Europe, and some are now being imported to the United States (e.g., Knop, AquaMedic and Deltec). You can read more about sulfur denitrators in the Fall 2000 issue of SeaScope and on the Internet at mars.reefkeepers.net/ USHomePage/USArticles/SulphurDenitrator.htm. Julian Sprung and I will go into more detail, including how to design and properly operate a sulfur-based denitrator, in the upcoming third volume of The Reef Aquarium book series.