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Nitrogen - (Nitrogenium) - N
Ammonia
Nitrates
Filtration Media
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Nitrogen primary form on Earth is molecular nitrogen. With an average molar concentration of 2,25.10-5 mol/l is nitrogen the fifteenth most common element in sea water. Expressed in mass units, the nitrogen concentration ranges between 0 and 45.10-6 g/l (0 - 45 μg/l).



Version 2012-III

Nitrogen - (Nitrogenium) - N

N

Nitrogen primary form on Earth is molecular nitrogen. With an average molar concentration of 2,25.10-5 mol/l is nitrogen the fifteenth most common element in sea water. Expressed in mass units, the nitrogen concentration ranges between 0 and 45.10-6 g/l (0 - 45 µg/l).

Introduction

Nitrogen occurrence in seawater:

PON Particulate Organic Nitrogen (particles> 0,4 micrometers)
0,002%
DON Dissolved Organic Nitrogen (particles < 0,4 micrometers)
2,300%
DIN Dissolved Inorganic Nitrogen (97,698% in total):
- molecular (N2)
95,200%
- other (NO3-, NO2-, NH4+)
02,498%

Simplified Oceanic Cycle of Nitrogen

Nitrogen Cycle
1
PON Formation
Output
Live organisms obtain nitrogen either through the direct fixation of dissolved molecular N2 (only certain bacteria), through the assimilating reduction of DIN (NO3-, NO2-, NH3- - algae, phytoplankton), through the assimilation of DON (bacteria) and through the consumption of other organisms (fish, mammals, crustaceans, corals etc.). Excrements and carcasses of dead organisms release undissolved organic nitrogen-containing particles into seawater - PON (Particulate Organic Nitrogen).
PON
In reefkeepeing, major factor influencing this part of nitrogen cycle is indisputably feeding (supply of particulate organic matter - POM) and supply of dissolved nutrients in the form of dissolved organic matter (DOM). If the system is overfed (POM) or overdosed (DOM), non-utilized matter enters directly the cycle phases 2 (POM) and 3 (DOM).
2
DON Formation
Output
Also called Remineralisation. The carbon and nitrogen bonds in PON react readily, causing a fast PON degradation to DON - Dissilved Inorganic Nitrogen
DON
3
Ammonification
Output
DON is degraded to ammonia under not yet completely understood conditions with the aid of heterotrophic bacteria - an enzymatic reaction - leading finally through the reaction with H+ to the formation of ammonium.
NH3 / NH4+
4
Nitrification
Output
4a - Nitrosomonas bacteria oxidize ammonium to nitrite.
NO2-
4b - Nitrite is then further oxidized to nitrate by Nitrospira and Nitrobacter.
NO3-
Nitrification is a fast process and requires an oxidizing environment (oxic zones).
5
Denitrification
Output
Some heterotrophic bacteria (e.g. Paracoccus) assimilate nitrates. However a significant nitrate fraction is reduced to nitrite and consequently to molecular nitrogen, which is then released into the atmosphere and thus is not a part of newly developed biomass.
N2
Bacterial denitrification is slow process and requires reducing environment with either very limited or no presence of oxygen (anoxic or hypoxic zones), high energy supply in the form of quickly utilizable organic carbon and high supply of essential mineral nutrients for bacteria.

Together with molecular nitrogen, ammonia and nitric oxide also forms during bacterial denitrification. Molecular nitrogen and nitrous oxide are primarily released into the atmosphere, while a smaller fraction remains in water and together with ammonium enters at different phases of the previously described cycle.

 

Denitrification in Reef Tanks

Nitrogen Compounds in Reefkeeping

Some intermediates and by-products of nitrogen cycle might be of serious or fatal effects on aquatic animals:

PON

Precursor

Particulate Organic Nitrogen - not a problem in itself. However PON stays at the beggining of the cycle i.e. more PON removed on time - less problems in further phases of the cycle. This fact emphasize the importance of particle prefiltration.
DON

OK

Dissolved Organic Nitrogen - not a problem in itself, quickly enters the process of ammonification.
NH3
NH4+

Higly toxic

Ammonia and ammonium - highly toxic to fish and other animals even in trace concentration (NH3 is more toxic).
NO2-

Toxic

Nitrite - above certain small concentration toxic to fish and other animals
NO3-

Harmful

Nitrate - above certain concentration (animal-specific) may be harmuful. Furthemore the nitrates are assimilated my many bacteria, cyanophytes and algea, causing the formation of their colonies
N2

OK

Inert.

As results from the table above the smooth rundown of nitrogen cycle in reef tank is more than necessary. Further, it makes logically no sense to deal with particular parts of the cycle apart, but rather to take global care of entire nitrogen cycle. And nitrogen cycle in reef tanks indeed depends on human intervention as the differences between natural environment and reef tank prohibits the nitrogen cycle to run itself.

 

Methods of Denitrification

Biological - Natural Bacterial

Key Elements
Disadvantages
Bacteria
Bacteria
None

BIOLOGICAL - BACTERIA in a Medium

Key Elements
Requirements / Disadvantages
Bacteria
Bacteria
Medium
An extra tank or biological filter equipment is necessary. The key issue is the porosity and seawater resistance of used glass, ceramic or plastic medium.

BIOLOGICAL - Refugium

Key Elements
Requirements / Disadvantages
Bacteria
Algae
An extra tank is necessary.
Dead matter must be removed regularly.
The algae metabolism can significantly influence pH-level.

BIOLOGICAL - Plants

Key Elements
Requirements / Disadvantages
Bacteria
Mangrove
The plant is high-maintenance.
The required number of plants is hard to achieve.

Physico-chemical - Absorption

Key Elements
Requirements / Disadvantages
Bacteria
Sorbent
Non-linear time behavior of absorption.
Regular sorbent exchange.
Absoprtion capacity can never be utilized on 100%

Physico-chemical - Chemical Reaction

Key Elements
Requirements / Disadvantages
Bacteria
Reactor
Chemicals
Reactor needed.
Regular maintenance and chemicals exchange required.
Not animal-safe in case of malfunction.

Whatever method is used for denitrification, it’s necessary to check regularly the pH value and especially redox potential. Ideal redox potential value is 250 mV. For the values over 300 mV highly toxic hydrogen sulfide emerges as a by-product. To the other extreme at less than 50 mV dangerous NO2 is released, since the reduction process is not completed.

 

Optimal Solution - Natural Denitrification

It's clearly a great advantage for any reefkeeper to reach and maintain the natural bacterial denitrification process. In order to achieve this, following conditions must be met in the tank:

  • Presence of oxic, hypoxic and anoxic zones
  • Presence of nitrifying and denitrifying bacteria
  • Continuous supply of readily utilizable chemical energy for bacteria
  • Continuous supply of essential mineral nutrition for bacteria

Of course there is general requirement for proper composition and physical parameters of seawater. For the purpose of denitrification pH-value (8,1) and redox potential (250 mV) are most important.

 

Ad 1 - Oxic, Anoxic and Hypoxic Zones

Formation of all required zones can be achieved via:

  • Live rocks
  • Highest quality reef ceramics with required seawater resistance and porosity
  • Sand bed as long as it has a thickness of at least 6 cm and the right composition

Ad 2 - Nitrifying and Denitrifying Bacteria

A certain quantity of bacteria is basically present in every live rock – as long as it is a real natural live rock. The needed amount of bacteria is available either in imported live rocks, in imported live sands, in artificially cultured live rocks and sands or in commercially available biological catalysts containing a mixture of various artificially cultured marine bacteria.

Ad 3 – Water Flow

While nitrification is a rather fast process which demands a highly oxidizing environment and runs mostly at relatively high water flow rate through the filter, denitrification requires quite different conditions.

Since it is a slow process with a high energy demand, a fast water flow is undesirable. The optimal water flow in separate live-rock-tank is easy to manage, but the maintenance of required conditions in main tank is hard and may lead to certain complications. A massive water flow rate between main tank and the sump, which in today’s modern systems may be thousand litres of water per hour, is no longer a problem – the water flow rate is reduced thanks to friction at the live rock surface and at the tank ground and depends on the tank volume. Another problem that might occur is the formation of „dead zones”, where water flow is very slow. This may lead to the precipitation of organic sediments and consequently to very dangerous decay processes, which have essentially nothing to do with the denitrification.

Ad 4 – Chemical Energy and Essential Nutrition Supply

As already mentioned above, the denitrification process is characterized by a steady high demand on readily utilizable chemical energy in the form of organic carbon. Only carbohydrates, also known as saccharides, are essentially able to cover this demand. One could of course experiment with low-molecular “earth-born” carbohydrates and optionally alcohols, such methods are however not natural and therefore risky. Particularly in the case of alcohols there is an overdose-risk, while the application of certain “earth-born” saccharides is linked with serious side-effects. The only natural way to nourish the denitrifying bacteria is the use of a balanced mixture of those carbohydrates that naturally occur in seawater together with a mixture of biogenous trace elements..

Literature

An Introduction to Marine Biochemistry, Susan M. Libes, John Wiley & Sons, Inc.

CS Reefkeeping Concept

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