Thiel Book - Chapter 3

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THE MARINE FISH AND INVERT REEF AQUARIUM
Albert J. Thiel

3.5 DENITRIFYING FILTERS:

Although not the same type of filter as the biological filter we just discussed at length, another filter that operates on the premise that bacteria do all the work is used when filtering reef tanks: the denitrifying filter, also called denitrator (denitrification filter is yet another name, but assimilation filter would be the most correct one).

Introduced to the U.S. hobby by George Smit in 1986, as part of his Minireef System, this filter has gained quite a bit in popularity since, but is still not very widely used. This is so because it is a filter that is not easy to operate, and not easy to keep operating at an efficient level.

Denitrification is a process whereby NO3, nitrate, is built down, removed from the water that is, by means of bacteria that are facultatively aerobic and anaerobic (also called nitrate respirators); usually in the presence of a carbon source. Unfortunately, some manufacturers and stores still persist to sell such units without a nutrient supplement.

The principle is real simple: flow water slowly through a container, or compartment in which a medium on which these bacteria can grow is placed, and the latter will slowly, but surely break down the nitrate and remove it from the system. The problem lies in the "flow slowly". Usually this means that the water must be flowed drip wise through a container that houses a medium such as filter floss, or balls, or sponge. Because of impurities in the water itself, the drip rate has a tendency to vary greatly. It usually slows down and may stop altogether, before the hobbyist notices.

When no water flows through the medium for a while, the filter goes completely anaerobic, producing hydrogen sulfide, and possibly methane, as a by-product. The same can happen when the water flows too slowly. The filter can go anaerobic in a matter of hours and such is a very dangerous situation, insomuch as hydrogen sulfide, besides smelling like rotten eggs, is very toxic, even in low amounts, to both fish and invertebrates and can ruin a tank in a very short period of time.

Because nitrates are a cause of problems, especially so in reef aquariums, they should be kept low, which often means removing them in some fashion. Nitrates both affect the well being and appearance of corals and invertebrates, and give rise to the appearance of micro-algae. As a result hobbyists must do something about them.

My recommendation is that you keep nitrates below 1 ppm as measured by tests that read in N-NO3, or 5 ppm in tests that read NO3. At those levels problems are unlikely. Lower levels are even better. My own tank, at my house, has no measurable nitrate, even when using low range testing kits. The only way to determine the levels is to use photometry and tests that measure in the 1/100 of a part per million range.

If levels are to be kept that low, hobbyists have to resort to some method to do so. There are in essence only two ways that seem to give any acceptable result:

using a denitrator, to keep nitrates close to zero ppm
using a compound that absorbs or adsorbs nitrates

3.5.1 DENITRATORS:

A device that is run separately from other equipment attached to the tank, meant to biologically reduce the nitrates that are present and constantly generated in the aquarium, as a result of feeding, metabolism, life, and other process taking place in the aquarium.

Denitrators are, usually, boxes or devices with multiple compartments that are all interconnected, and through which water is made to flow very slowly. The compartments are filled with a medium that offers surface area for bacteria to grow on. Filter floss, plastic filter media, and some coarse sponges make excellent fillers for these chambers and for bacteria to populate.

Because the water flows through the denitrator very slowly, it is in contact for a long time with that bacterial bed, and is cleansed in the process of nitrates and some other compounds. To keep the bed active, and to make the chemical and biological processes that occur in the denitrator complete, a carbon source needs to be added to the filter as well. Lactose, mixes of lactose and other sugars, for example fructose, are used to achieve the desired result.

In certain filters alcohol is used instead of sugars, and is placed in bags with micron pores to make it migrate very slowly into the water. The need to add a high quality nutrient is not to be underestimated. Indeed, it will make the difference between a filter that is rather easy to operate and regulate, and one that is giving you constant problems.

Theoretically, if the denitrator is operated correctly the effluent should not have any testable nitrates whatsoever (using an aquarium test kit). Practically, however, most denitrators do not function at their optimal level, but at a rather high level of efficiency, yet not 100%. This is, in most cases, enough to keep nitrate levels very low, or low enough for them not to be a problem. Because different denitrators require different operational instructions, it is not possible to generalize here and give you one way that will cover all instances. Most do require about the same care though.

I have used units manufactured by Energy Savers Unlimited, Summit Aquatics, Marine Technical Concepts, Thiel*Aqua*Tech and Sera from Germany, and find that they all perform as expected from them, providing the hobbyist takes the time, and makes the effort to regulate the units as required. That in itself can be a rather time consuming task. Let's look at a typical unit, and what needs to be done to get it running properly.

Hook up the denitrator using the instructions you receive from the manufacturer.
Typically, this will involve running a water input line to one end of the unit, and attaching hose to the other end to return the water to the sump of the trickle filter, or to the tank.
Water will now flow into the denitrator at one end, go through all the compartments slowly, and exit at the other end, after having been in contact with the bacteria that have settled in the filter material.
Because the process is of biological nature, the filter needs to run in, very much like the biological filter that removes ammonia and nitrite from the water. The biochemical process that takes place is the reason that an additive, or nutrient, needs to be added to the filter.
Before starting up the unit, you should fill it with water and let it rest for about 24-36 hours. This will greatly reduce the amount of oxygen present in that water. This is a necessary step.
After such time, start up the drip system, based on the rate recommended by the manufacturer, but not faster.
Add nutrient as indicated in the instructions.
Be patient.
It may take as much as 4 to 5 weeks before the denitrator is actually denitrifying the water, as nitrate tests done on the effluent will show.

To determine exactly what is happening inside your filter, all you need to do is test the effluent for nitrate and nitrite, and smell the water to determine whether any hydrogen sulfide is present. If you want to be real accurate, you can buy a hydrogen sulfide test kit from La Motte Chemicals, or from some aquarium companies. The following scenarios can occur:

The effluent contains no nitrite. Nitrate levels are at about the same level as in the tank itself. The reason is that you are flowing the water through the filter too quickly, or your filter has not started functioning yet. You must slow the drip, or flow, rate down.
The effluent contains less nitrate than the tank's water, but nitrite is also present in levels greater than 0.1 ppm. Although you are on the right track, you are still flowing the water too rapidly through the filter. You may also need to add more nutrient. Slow the water flow down a smidgen and retest about 12 hours later. Slow the water down further if necessary.
The water does not show any nitrate content, no more than 0.1 ppm of nitrite, but it has a definite smell that is not normal. You must add more nutrient.
The water has lower nitrate levels than the tank's water, nitrite is very low, <0.1 ppm. Slow the filter down just a tad.
No nitrate is present, and nitrite levels are lower than 0.1 ppm: the filter is running close to high efficiency. Do not slow the flow down, but use a small amount of nutrient more than you were before.
No nitrate is found in the effluent. No nitrite is found either.

The filter is running as it should, and at 100 percent efficiency. This is the ideal situation, but the one that only occurs for shorter periods of time, reverting to the previous scenario where the filter is running at very high efficiency levels.

Why is the denitrating filter so often a problem ? There are a number of main reasons:

The hobbyist is flowing the water through the denitrator too rapidly and no biochemical reaction is taking place. The water is highly oxygenated. The denitrator has become a regular biological filter.
The hobbyist is flowing the water too slowly and the filter is operating partially anaerobically, with all the dangers that such presents.
The hobbyist is not adding any, or not enough, nutrients. The process takes place partially but not completely. The filter is not as efficient as it could be. Nitrites will always be present in the effluent.
The drip rate is hard to regulate. The water stops dripping on a regular basis. Because the water coming from the tank to the denitrating filter contains impurities, the restriction made to adjust the drip, gets blocked by small particulate matter. You must clean that piece of hose regularly or more frequently to prevent the drip rate from falling to zero and the filter going anaerobic.
The filter is too small; there are not enough compartments in the filter; you have bought the wrong type of filter and may have to look for a better one.
Or you may have to run two units in series. This will allow you to speed up the flow rate quite some, because the path the water now has to travel is now much longer.

With some care, and a little patience, you will be able to get your denitrator to do what it is supposed to: lower the nitrate levels in your reef. The key is not to get discouraged if at first you are not succeeding. Many hobbyists before you have had the same problems. Persevere and try again. Follow the instructions more accurately, or call someone in the hobby who can help you understand where you may have gone wrong. Denitrators can be very effective but you must take the time to regulate them properly.

3.5.2 COMPOUNDS TO LOWER NITRATES

Besides using denitrating filters to lower nitrates, hobbyists also resort to a method that is both simpler and more practical for most: a compound, resin, or special filter material is placed in line with the water filtration, or in a canister filter, or in an area with good water circulation. As water flows through the material, nitrates are removed slowly and reduced to within acceptable levels, usually in the 5 to 7 ppm of NO3, or in the 1 to 11/@ ppm of N-NO3. Note that I mentioned "through" and not over. It is important that a good throughflow is achieved. If water does not come in contact with the compound that is used, the latter will not perform as efficiently, and perhaps even not at all, depending on exactly how the flow occurs.

There are several such compounds on the market: Ex-Nitrate, HyperSorb, and Poly-Filters. Some work better than others, and some have a longer life than others, depending on tank conditions. You may want to experiment with a few until you find one that performs well for you. All are widely available. Some perform more than just the removal of nitrates, e.g. Poly-Filters from Poly Bio Marine Inc. remove a slew of other noxious compounds as well (see section on chemical filtration for more details on Poly Filters and molecular absorption discs).

I have a lot of experience with both Ex-nitrate and Poly Filters and can recommend both very highly. Ex-nitrate, a product sold by my own company, is highly selective and lowers nitrate levels. Poly Filters a polymer based sheet-like medium) on the other hand are much wider in their beneficial effect, and remove phosphates also, for example. Such is a boon, because phosphates are a problem compound as well, leading to the appearance of large amounts of undesirable micro-algae. As already indicated they remove a great deal more as we shall see in a later section.

Ex-nitrate is a granular compound that can easily be placed between filter floss in a canister filter. For example, Fluval, Marineland, Lifeguard and other canisters all can easily be filled with this material. Water is then flowed continuously through the canister, and as it comes in contact with the Ex-nitrate, nitrates are slowly removed. The hobbyist tests for nitrate levels from time to time, and when nitrate levels go up again, the compound is simply replaced. About 3.5 pounds of this material are recommended per 50 gallons of water. In an average tank this amount will last for about 2 to 3 months. More of the compound can be used if the hobbyist wants to lower the levels even further. We have never been able, however, to bring them down completely to zero with this method, but to levels around 1 ppm of N-NO3 with is totally acceptable even in a reef aquarium.

Thiel*Aqua*Tech, the company that markets the compound, recommends that you do not use Ex-nitrate if your nitrate levels are higher than 40 ppm of NO3. Not because Ex-nitrate does not work (it does), but because it will be less expensive for you to first bring NO3 levels down to about 10 to 15 ppm by performing water changes, and then using Ex-nitrate to lower the level somewhat more and maintain it there. Indeed Ex-nitrate is not an inexpensive product.

When sandwiching Ex-nitrate between filter floss in a canister, check the floss from time to time to know when it is necessary to clean it. Once a week is recommended. You may also rinse the Ex-nitrate and then place it back in the canister.

Special canisters for this compound can be made, and placed in-line with the main water pumps. Such canisters, usually fabricated out of acrylic tube, can be made to any size you require, either by yourself, or by other companies in the hobby, including my own. Standard canisters holding two and four 50 gallon treatments are available from Thiel-Aqua-Tech. Marine Technical Concepts is another company that constructs excellent equipment for this and other products.

Because the marine reef segment of the tropical fish hobby is in a time of rapid evolution right now, and because many new products are being introduced all the time, it behooves every hobbyist to keep on the lookout for progress made in this, and other areas. Read the hobby magazines, talk to other hobbyists if you can, call manufacturers, and ask what new products they have available. You would be surprised what you can sometimes find that way.

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