Bob replies...

(I decided to withhold this person's full name and company name)

Hi John,

Thanks for your email and I understand your thought process, however, by increasing the oxygen content in a major portion of the bed by forcing oxygen laden water upward through the bed, you're reducing the physical size of the plenum's sandbed anoxic zone. The only difference between your method and that of a reverse flow undergravel filter is the speed of the supplied water and the size of the sand grains.

As most hobbyists realize, the reverse flow UGF method sends aquarium water to the bottom of the bed, forcing it upward through the bed. This method somewhat increases the areas for nitrification, as oxygen laden water is now also available at the bottom grains in the bed. I say 'also' because its normally available at the bed/bulk water interface. In the standard UGF method, the water is withdrawn from the bottom of the bed, thereby drawing water downward through the bed. Therefore the nitrification process is generally more efficient in the upper reaches of the bed where oxygen-laden water is first pulled downward.

Before I go much further, the key to plenum efficiency is the somewhat small amount of oxygen that remains throughout much of its bed and 'the' actual amount of oxygen contained in the plenum space. After years of study, it became extremely evident that to interpret plenum mechanics it was first necessary to understand the different classes of bacteria existing in general sandbeds and that of plenums, and their environmental requirements/paths. And that's where the science of waste treatment plants and aquariums differ!

In waste treatment plants its important to reduce organic compounds (human waste) to inorganic compounds such as nitrite/nitrate. Therefore the stream of waste is stirred and aerobic heterotrophs reduce that matter to inorganic compounds. Not much different than what happens in aquariums with fish waste/detritus/dead animals. The resultant human waste stream is then further aerated in 'ponds' to reduce the ammonia laden products to nitrite and nitrate by encouraging growths of aerobic autotrophs. The result is sometimes used as a crop/golf course grass fertilizer. Of course, we aquarists want to minimize the "fertilizer" aspect of this process.

As you probably know, there are three classes of bacteria that should interest all hobbyists, i.e., aerobic heterotrophs that reduce organic matter to inorganic compounds; aerobic autotrophs that take that inorganic matter, such as ammonia and perform the nitrification process, and the anaerobic heterotrophs that perform denitrification. Unfortunately the anaerobic denitrification process has long been misunderstood and often misstated. In the course of our research, we (Sam Gamble and myself) found that the so-called anaerobic area/zone really consisted of 'two' different zones/paths each having they own class of bacteria and each accomplishing a different form of denitrification! We decided on using the word 'anoxic' to describe one area/zone and stay with the generally understood word 'anaerobic' to describe the other.

Research showed that dissimilatory denitrification occurred in zones having a small amount of oxygen, (0.5 to 2.0 ppm per Sam Gamble) which we called the 'anoxic' zone. It's accomplished by facultative anaerobic heterotrophs and they fully oxidize nitrate back to nitrogen gas.

In the other area/zone/path, assimilatory denitrification occurred in anaerobic areas (no oxygen/less than anoxic as we defined it), where nitrate is reduced ONLY to ammonium, no further and that's accomplished by obligate anaerobic heterotrophs.

Of course, in the past the 'anaerobic' area was said to be an area where little or no oxygen occurred. However, we found that definition did not suffice if aquarists were to better understand the natural processes occurring in their aquariums. And as for any reference to 'anaerobic' areas by any writer/author, for that to be of true value to the reader, it must define the two above described areas. Otherwise, the value of the term can be misleading.

With that now clear, the plenum (space) acts like a reservoir and temporarily holds nutrients that are not fully oxidized as they pass through the depths of the bed. And that's understandable as we hobbyists often have nutrient fluxes in our systems when maintenance lags or if the system is overfed. And since bacteria live in relation to the incoming food supply, they can't increase their numbers immediately to handle these peaks. This space also stores a small, variable amount of oxygen, usually about 0.8ppm. (Actual tests by Sam Gamble.) This small amount of oxygen tends to keep most of the sandbed above the plenum grid in an anoxic condition as we define it, thereby reducing nitrate to nitrogen gas.

If a highly oxygenated water source is introduced into the plenum, such as you describe, the bed above becomes a zone where aerobic autotrophs perform nitrification instead of the heterotrophs mentioned performing denitrification of one type or the other. Which is what happens mostly in UGF systems. Yes, depending upon the volume of flow there may be some so-called anaerobic areas remaining, however, I doubt very much the hoped for results will be anywhere near what you envision. One thing for sure, you have disrupted the natural downward dynamics of diffusion, and reduced the overall anoxic zones. Depending upon bioload, the nutrient level of the system will probably slowly increase. When or if that will reach a level that initiates unwanted algae growth or severe water quality problems, is impossible to quantify at this point in time.

You also say that water changes are accomplished by allowing the new water to enter under the plenum and slowly pass upward through the sandbed so as to keep the plenum (space) clean. It should be noted the plenum (space) doesn't normally get dirty or accumulate detritus or debris, unless it was incorrectly constructed. In fact, the plenum in all my past aquariums were spotless, even after five years of use. And as for supplying nutrients to the bacteria, a plenum system does that automatically without any outside help from the hobbyist. As noted above, the plenum contains a slight amount of oxygen. This oxygen appears to give the plenum a slightly lower negative electrical charge than the area above it, thereby sending its nutrients back to the sand above, which has a higher negative charge. Keep in mind that positive flows to negative, and nutrients are mostly positive charged, and that the deeper you go the greater its negative charges, except where plenums are involved! This automatic circulation of nutrients between plenum space and its sandbed is another reason why plenum beds far outperform deep beds directly on the aquarium bottom, as their greatest negative charge is on the bottom of their sandbed. And hopefully, after reading all of this its clear why a highly oxygenated water supply should not be introduced into the plenum area. It's simply counterproductive! But if experimentation is your goal, go for it and keep me posted.

Also, keep in mind that the majority of live rock internal areas, depending upon its porosity, is anaerobic (as I use the term), therefore, it can only reduce nitrate to ammonium, no further. And, since the surface of a sandbed is much like a filter pad, where it does it thing until water can no longer pass through it, why does anybody want to clog the sandbed surface with excessive amounts of rock! And for hobbyists that want a lot of rock in their aquariums, could be nitrate levels may be somewhat lower, but the nitrogen factor, i.e., ammonium, is still a viable factor to be concerned about. Keep in mind it can defuse upward in the bed, as can nitrate and be accessible to algae. I often get mail saying the hobbyist has little nitrate, but lots of unwanted algae! And with ammonium a better algae nutrient than nitrate, one has to question the value of so-called anaerobic areas that some in the hobby keep quoting as the way to reduce nitrate levels so as to reduce the threat of unwanted algae growths!

And you're right on about small grain sand/mud! Yes there's more area for bacteria to colonize, however, they are the wrong type bacteria! However, as to being fed or not, they do exist in numbers/live in relation to the available food source/supply. It's just that the result of their existence, such as possibly hydrogen sulfide and/or methane, are not generally thought of as a possible result associated with these very fine substrates.

Finally, I've personally dealt with many of the largest waste management firms when I was trying to find one that could handle hundreds of millions of tons of hazardous waste. Some were antiquated, and/or had the most ridiculous contact terms I had ever seen and/or their management was questionable in 'many' important categories, as was their capital holdings. Of course, this is not a personal view of your company since I'm not familiar with it. But one thing for sure, you're in what I consider a "thankless" business because people in general take what you accomplish for granted, and one that in some areas needs a major overhaul as our population continues to grow. And if I can help in any way, don't hesitate to contact me.

Hope this helps,

Bob