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Aquarium.Net Natural Nitrate Reduction

Sam Gambles first article in a series on the hows and whys of sand bad filtrtion and NNR, Sept. 1996 Index for Aquarium Net, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist


By Sam Gamble

Getting nature to divulge secrets is not easy and we have been lax because of the success by more technical and unnatural methods. Thanks to several pioneers and new work that can be seen, there is a movement spreading. From better understanding and getting involved, we will hopefully in the future maintain improved aquariums. Just as importantly communication among us will help others.

Bob Goemans, author of Salt Corner in MFM has found success with implementing his own experiments, and published what he learned. With his conviction and counsel, I renovated a 90 gallon reef system. Methods for sampling were incorporated into the design. Thus it enabled us to learn more about important fundamental aspects currently assumed or unknown about the functions of the sand bed. The selection of methods and equipment was to hopefully provide relevant results to illustrate useful ideas.

The system was staged in its curing phase from empty to an inventoried bio-load. The reasoning was to get an understanding for the tank, the sampling methods and character of the sand filter. This calibration and precipitation of conventional wisdom would help direct technical methods. With this in mind I started the investigation by searching for relationships that are obvious and not so obviously at work.

Thanks to the success of others, we are fortunate to have seen some excellent aquariums filled with thriving corals and other invertebrates to illustrate the basic principles for a sand bed system. But, to be candid, at this point we don't have a surplus of information to consult. The results and observations were surprising in some cases and interesting in just about all instances.

To help understand the key features of natural nitrate reduction and the role of sand filters we must focus on the open systems in nature to compare our more finite marine application. Our goal is to maintain a nutrient poor environment as per the guide-line for coral reef eco-systems. In the case of our reef aquaria, we first build an environment trying to achieve eco-system's basic requirements. To do so successfully the crucial elements must be provided in the ratios necessary for balance .

The fundamental driving forces are chemical and radiant energy. These elements are interrelated complexly and mediated biotically and/or abiotically. With the topic of NNR and living sand filtration the process of denitrification is where we started. The process is mediated by bacteria (microbes). The deposition of organic material on the surface layer of marine sediments supports an elevated microbial metabolism and limits the penetration of O2 into substrata (Sand). It produces an ideal environment for microbial NO3- reduction. It is created in an environment where NO3 can be in ample supply to substitute for O2 in the process of organic matter degradation.

In sediments, this would involve only a very thin layer that interfaces with the bulk water. In systems using porous rocks for filtration i.e. Berlin Method, this would include the rock to water surfacial interface with slight modification. In natural marine sediments the oxic/anoxic (high oxygen/minimal oxygen) interface varies from a few millimeters to several centimeters. In other words less than half an inch. So then, what is a segregated sand bed, with a plenum space underneath going to do for the axiom of nitrate reduction (denitrification). Plenty ! !

The presence of the surfacial interface, the area at the sand's surface where oxic conditions are changed to anoxic conditions by microbial metabolism of organic matter, does not change much in the living sand filter. In fact it seems extended or enhanced. What goes on below is where the difference becomes apparent after measuring variables like nitrate, oxygen, pH, H2S, and alkalinity.

Normally in marine sediments, the area below the oxic/anoxic microzone is essentially anaerobic, with decreasing O2, pH, and redox. The diffusion of elements to and from the obligate anaerobes is slow and at a reduced capacity compared to the surface microzones. Any observed increased activity is most often the result of plant and animal infauna increasing the net volume of surfaces and availability of oxygen. In the aquarium's sand bed system the elements traverse horizontally and vertically throughout the sediment substrate and plenum. Oxygen can be measured at anoxic levels at varying times throughout the sand bed and plenum. Or oxygen can be present at one level even if the sand layer above is temporarily anaerobic, and partitioned from the aquarium water as a source of dissolved oxygen.

Nitrate concentrations have been seen to have definite movement toward the plenum and often accumulate there in higher concentration than the aquarium's water. Nitrate production from an organic load can be seen to diffuse gradually through the sand layers to the plenum. Associated with this transition is a gradual reduction of oxygen levels, and pH. Interestingly, total alkalinity is observed to increase slightly with proportional pH decrease.

To surmise the observations it would be possible to conclude one of the biggest contributions of this sand bed (NNR) system is to use facultative bacteria as opposed to obligate bacteria. Obligate anaerobes are found mostly in the natural marine sediment environments below the anoxic microzone. They only metabolize and grow in anaerobic conditions.

However, the facultative microbial populations can metabolize and grow with oxygen or reduced oxygen. This ability really enhances nitrate reduction capability. To implement a facultative design that is at least three inches deep by the length and width of the aquarium provides tremendous potential to filtration in this regard. It aids destructive denitrification and shunts assimilative denitrification. In the first case nitrate is broken down into its elements.

In the second case nitrate is rebuilt into ammonia for use by nitrogen fixers as fuel. We most often complain about the growth of nitrogen fixers like nuisance algae. They are very good at using nitrate that has been reassembled into ammonia. Unfortunately. The processes gets tremendously more complex as more nutrient pathways are investigated for inter-relationships. Perhaps this is a good place to stop and get into the design and building of a typical (generic) system next time.

Created by liquid
Last modified 2006-11-19 01:38