Biological Filtration


Denitrification occurs in an ordinary filter, when some parts become clogged with dense bacterial growth. Deeper parts of the filter are then starved of dissolved oxygen and the denitrification process starts as a result. This has often been mentioned in hobby magazines but an acurate picture of the process has never been provided. The resulting lower nitrate content was measured but the mechanics of the micro-environment were not explained.


Quite recently, a scientific paper from Denmark, published in Microbial Ecology, discussed the results of monitoring of the biofilm function on solid substrate, collected in two Danish streams. Both streams receive large amounts of effluent from a sewage treatment plant and run-off from agricultural areas, rich in nitrate and organic matter. These biofilm layers are similar to those in uncleaned aquariums, but they build up to 1 mm in thickness.


I found this information fascinating, for I have been trying to obtain data on the biomechanics of biofilms for years but even our sewage treatment laboratories have kept out of this field. Sewage aeration tanks, where digestion of organic matter occurs, depend on very high levels of solids present, where the bacteria can populate the surfaces of the whirling particles. If these particles are larger than the bacteria, then the process becomes more effective the more the dirt in the sewage (source: University Ph.D. thesis, U.S.A.). Aquarium water of any quality is crystal clear by comparison.


The Danish scientists actually measured the microprofiles of oxygen and nitrate by use of a microsensor. The object of the research was to measure the depthwise distribution of denitrification and oxygen respiration. Within the 1 mm of film thickness, penetration of oxygen and nitrification occurred to a depth of 0.33 mm; then followed a thin (0.03mm) layer of mixed nitrification-denitrification, and finally, the denitrification took over to a depth of 0.88 mm, where diffused nitrate was zero.


Denitrification always appeared when oxygen was depleted and the capacity of the bacterial community to perform rapid shifts from oxygen respiration to denitrification seemed to be the principal adaptation to fluctuations in oxic/anoxic conditions in the biofilm. It was also noted that the bacterium Thiosphaera pantotropha, abundant in some water purification systems, performs oxygen respiration and denitrification at oxygen concentrations near to air saturation levels.


The required supply of carbon was shown by these studies to be derived partly from algal cells and new research into biofilms of tricklefilters indicates that layers may switch from nitrification to denitrification, according to the light/dark regime.


Practical application of the denitrification process in the aquarium hobby faces the same problem as do the water treatment facilities. Actually, the similar levels of nitrate (100-200 ppm) are relatively low for a rapid treatment but the use of thick layers of bacteria is really a long-term process. French scientists have produced a simple battery of vertical pipes and they thereby achieved almost zero levels of nitrate. I am searching for more details, particularly the materials and grading used in their system, as a clue for similar filter construction.


At present, the solution lies in the installation of a separate tank (or chamber), which follows the trickle-filter oxidation (nitrification), since the denitrification stage must not receive free (dissolved) oxygen. In effect, oxygen must be consumed before or at least in the early part of the denitrification tank and this means an airtight lid and siphon inlet type of arrangement. The bacteria established in the denitrification stage also require a source of carbon, but methanol, with its combustible and explosive nature, is not the answer. Carbon dioxide is more convenient but unfortunately, its higher oxygen content reduces the efficacy of the process.


Water velocities through the denitrification stage should be slow and a bypass is recommended for regulation of the flow. The infill media should have even more area than in trickle-filters, because there is no airwater mixture to need larger passages. Properly graded sand, spent activated carbon, lava rock or expanded rock are suitable, as offering large surface areas for the growth of bacteria.


An example of an aerobic/anaerobic aquarium unit is shown in TFH Nov/88, together with a detailed description of its practical aspects. Certainly, the measured results are impressive.


The operation of a complex nitrification/denitrification filter is relatively reliable, except when power failures affect the circulation pump. A circulation stoppage of no more than 20 min is permissible, after which bacterial death will lead to an environmental disaster. A time-control switch, not allowing automatic re-start of the system, is probably the best solution, but unfortunately, cleaning and restart of a unit to re-establish new bacteria, is a slow process.


The design of the whole unit also requires provision for stepwise cleaning of the infill media in any part of the filter, to keep areas of established bacterial growth above the minimum working levels.


The main operational parts of a nitdfication/denitrification system are:-


Prefilter (strainer. foam or floss mat) to remove large particles. A combination of useful gadgets, such as surface skimmers, etc., certainly improves performance. The prefilter area should be cleaned frequently, the more often the better.


Nenitrification stage (trickle-filter, trickle trays, sprayed layers of foam or filter-wool). Cleaning is required to maintain an unobstructed flow. Excessive growth of bacterial film is culled to 1/3 of the maximal volume at any time.


Denitrification stage (anaerobic chambers). Cleaning is due whenever clogging obstructs flow. Temporary nitrate buildup after cleaning does not threaten life in the system. Bacterial colonisation of the cleaned elements is rapid, from the trickle-filters.


Settling well needed for settlement of shed layers of biofilm and mineralised matter derived from bacterial activity.


Recirculation pump to provide the needed flow through the system.


The above-described system provides the ultimate answer to closing the biological cycle. For many hobbyists it may possibly amount to an ‘overkill' but for serious ones and breeders, it is the only way to avoid massive wastage of water. In view of predicted future shortages of water resources, this is likely to become a very important consideration.

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