Nitrogen - (Nitrogenium) - N

Introduction

Nitrogen occurrence in seawater:

Simplified Oceanic Cycle of Nitrogen

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:

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

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 nitrification 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 riskfy. 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