What is a nitrate filter
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In a nitrate filter, bacteria convert nitrate into nitrogen. This requires anaerobic, ie low-oxygen, zones in the filter. Because there is no oxygen available to the bacteria in these areas, they breathe in nitrate instead of oxygen. The flow rate of water in a nitrate filter must therefore be kept as constant and low as possible.
In addition to oxygen, the bacteria need carbon. Paraffin, alcohol, etc. are often used as a carbon source.
It makes no sense to use a nitrate filter on a plant tank with a nitrate content of less than 20 milligrams/litre. Unless the reducing effect is desired, ie the redissolution of trace elements, which is supposed to stimulate plant growth. A nitrate filter only makes sense if the nitrate levels are really high and cannot be eliminated in any other way, e.g. B. by changing the water.
Nitrate filters are not recommended for normal aquarists. A nitrate filter must also be checked and maintained. It’s easier to trim down and encourage fast-growing plants.
A nitrate filter can be useful for breeders who have to work with high stocking and intensive feeding during rearing. Changing the water does not replace a nitrate filter. In the best case, longer intervals between water changes or smaller amounts are possible.
Is a nitrate filter dangerous?
It is often written that nitrate filters are tricky or dangerous because they produce high levels of nitrite depending on the conditions:
- Operating nitrate filters is tricky because you have to constantly make sure that there is enough nitrate and oxygen available. Especially when there is no experience.
- A nitrate filter can easily become unbalanced and the fish can then die of nitrite poisoning relatively quickly.
There are three ways a nitrate filter can tip over:
- The flow is too high. The filter environment therefore becomes aerobic and nitrate reduction gradually ceases. However, the bypass does not produce any nitrite, but will break down the nitrite like a conventional filter. So nitrate is produced again. The bacteria in the main filter should be able to bridge the changeover time.
- The flow is too low. The environment becomes almost devoid of oxygen. As a result, the bacteria break down the remaining nitrogen compounds even more quickly and convert them into nitrogen. There is no nitrite as an intermediate product of denitrification. With an acute lack of oxygen, other compounds are reduced, e.g. B. sulfates to sulfides and possibly further to hydrogen sulfide.
- The flow is so high that part of the bacteria oxidizes and part reduces in another part of the filter. There can be a game of ping-pong between nitrite and nitrate and it’s a matter of luck which of the two comes out of the filter.
In all three cases, no dangerous amount of nitrite is produced. In the third case, however, hydrogen sulfide is produced, which smells strongly of rotten eggs. A nitrate filter or the flow rate should therefore be checked regularly.
However, practical experience does not confirm that nitrate filters are particularly dangerous:
- Experiments and experiences show that no dangerous amounts of nitrite are formed.
- The bacteria can presumably adapt quickly to aerobic and anaerobic conditions.
- Larger quantities of nitrite only occur when running in previously unvaccinated nitrate filters.
- Once a complete dentrification flora is present, at best some nitrite will come out if the filter is run too fast.
The operation of a nitrate filter
A nitrate filter is run in like a normal filter. After a few weeks, the water supply is reduced and the quick filter becomes a slow filter. The running-in period can be 6 weeks. If the nitrate filter is inoculated, the running-in period can be reduced to 3 weeks or even a few days. The filter sludge for inoculation is ideally from a run-in nitrate filter. Filter sludge from a regular filter works almost as well.
A slow filter solves z. B. trace elements back. Only when the bacteria are fed with carbon does the slow filter become a nitrate filter. In order for the bacteria to breathe nitrate, it must be ensured that there is practically no oxygen present.
Bacteria need oxygen to breathe. Either the oxygen is taken from the water. In the process, CO2 is emitted. Or the bacteria take the oxygen from oxygen-containing compounds such as nitrite and nitrate. Nitrogen is emitted as a non-toxic gas. When there is about 20 times as much nitrate as oxygen, the bacteria prefer nitrate to oxygen.
If the water is extremely polluted, the nitrate filter receives enough dirt and very little oxygen. However, aquarium water is seldom so badly contaminated. With clean, oxygenated water, the oxygen in the filter prevents dentrification.
Normally something has to be fed to the bacteria so that they first breathe the oxygen and then the nitrate.
In order for something to be able to be breathed at all, a breathable substance must be present. This substance is the carbon that z. B. is supplied with paraffin. The paraffin-eating bacteria consume all the oxygen in the upper zones of the filter. In doing so, they produce fragments of the paraffin chain. These fragments serve as food for the dentrifying bacteria in the following zone, the nitrate serves as an oxygen substitute.
When running in a nitrate filter, nitrite is produced first. A nitrate filter is therefore only put into operation when normal filtration is working properly. Nitrite can also be formed with sudden changes in pH. Experience shows that no nitrite is produced if the pH value drops slowly over a period of several weeks. Perhaps the microorganisms in the filter are sensitive to abrupt changes in the environment. Many die off and the remaining bacteria cannot cope with the resulting biomass quickly enough, so that nitrite is produced first.
Water must only flow slowly through a nitrate filter. The water should remain in the filter for at least 15 minutes. However, if too little water runs through the nitrate filter, hydrogen sulfide H2S can form. Hydrogen sulfide smells strongly of rotten eggs and is one of the most toxic gases. The water flow must therefore be constantly monitored.
If there is no more nitrate, the water flow remains the same and the bacteria continue to be fed, the bacteria breathe sulfate SO4, which becomes hydrogen sulfide.
Ion exchangers with nitrate resin are sometimes also referred to as nitrate filters. This can also include filter media such as Nitrex.
Does a nitrate filter make sense?
Whether a nitrate filter makes sense depends on the nitrate content of the aquarium water and the tap water.
In well-planted aquariums with adequate fish stock, there is usually no excess nitrate. If the nitrate level in the tap water is low but too high in the aquarium water, the nitrate level can be reduced by changing the water. The fish population and feeding must then be reduced. Improved plant growth consumes nitrate. An Efeutute can also be used for this purpose. These measures reduce the nitrate content permanently and more effectively than nitrate filters.
Biologically working nitrate filters require a certain minimum level of nitrate in the water in order for them to work. At least about 10 mg/l nitrate is necessary. This load can be higher than the desired nitrate level. Denitrifying filters are not easy to maintain, because under unsuitable conditions, e.g. B. can produce nitrite and hydrogen sulfide.
A nitrate filter can be useful if you keep fish or plants that are sensitive to nitrate and water changes, e.g. B. when rearing young fish that have to stand in the feed.
Nitrate filters are often used to keep fish stocked too high. A meaningful nitrate content is determined by the plants, which are more sensitive to nitrate than fish. Nitrate levels between 10 and 20 mg/l are suitable for most fish and plants. Many plants suffer from deficiency symptoms below 5 mg/l.
Can nitrate filters be regenerated?
Nitrate filters with nitrate resins, which work like ion exchangers, can be regenerated. To do this, a saline solution is slowly run through the nitrate filter over the course of an hour.
It is not a problem if the solution runs longer. If the solution runs faster, the capacity of the nitrate filter may be reduced.
This type of nitrate filter exchanges nitrate ions for chlorine ions. The ionic load of the water does not change. This means that the conductance also remains the same.
Nitrate is better removed from tap water with a full desalinator or an osmosis system.
Nitrate is best removed from the aquarium water by changing the water and dense plant growth.
Can a nitrate filter be controlled by the ORP voltage?
The redox voltage indicates the degree of oxidation of the water and allows conclusions to be drawn about the oxygen content in the water. The oxygen content in the water can be practically measured via the redox value. The flow rate in the nitrate filter can be adjusted accordingly. If there is too much oxygen in the water, this affects the redox value and the flow rate is reduced.
If the system is adjusted in such a way that oxygen-free water emerges at the water outlet of the nitrate filter, the bacteria in the nitrate filter are forced to breathe in the nitrate.
The flow does not have to be steplessly regulated for this. It is sufficient if the flow is switched on and off from -180 mV. This is even better than a constant flow. Complete emptying of the treated water would be ideal.
With redox values more positive than approx. -100 mV, the bacteria only process the nitrate down to the toxic nitrite. At more negative values than approx. -250 mV, the bacteria also breathe sulfates into hydrogen sulfide H2S, which smells like rotten eggs and is also poisonous.
When water containing nitrate gets into the filter, the redox voltage initially drops due to the bacteria. At around -180 mV all the oxygen from the nitrate has been breathed out and the water should then be discharged. The nitrogen escapes in gaseous form.
The values oscillate somewhat when the device is switched on and off. This comes close to emptying. If the redox electrode is cleaned and checked more often, such a nitrate filter works well.
An internal recirculation must ensure that the values in the filter are the same everywhere. Otherwise nitrite or hydrogen sulphide will be produced.
The discharged water must be well aerated and filtered. However, regulated operation of these filters is safer than unregulated operation.
The oxygen and nitrate content cannot be measured directly with the help of the redox voltage, because other factors also influence the redox voltage. This includes:
- phosphate
- Intermediates in the food chain as…