Bob replies...

Hi Gerald,

Thanks for an interesting letter and asking questions that I have not seen asked for quite awhile. You are quite right about differing opinions, and those vary from totally incorrect to extremely leading edge. However, sometimes those that appear to be fairly simplistic and quite logical are those that are incorrect. Also, sometimes certain positions are carefully worded to help promote certain ideas/products. It's really a buyer beware market.

Because it's not always possible to find someone quite knowledgeable and with nothing to gain from selling products, one has to weigh the responses about various subjects and think about the source of that information. As you probably know I've been in the hobby for 57 years, and have never worked for an aquarium product company or allowed my name to be used to promote products. My extensive background in the corporate world with mechanical and environmental engineers, and my connections with many of the finest scientists in the world, has helped provide me a wide array of knowledge that has helped answer many questions. And no, I don't have all the right answers, but I do go the extra mile to help my readers and viewers of my website.

I don't agree 100% with your position that sandbed processes can not be described with equations, and/or are not predictable or repeatable. After spending six years researching sandbeds of all types in both closed systems and in the wild, the scientists I had the pleasure of working with were in total agreement that the microbial and chemical processes where fairly straight forward. Where and why they occurred took some time to decipher. Once that was accomplished, we then had a very good handle on what would occur and where. However, that did not guarantee the result would be constantly repeatable.

All of this research is laid out in an easy to read format in the CD book titled "The New Wave" which leaders in the field of marine aquarium husbandry have called a 'leading edge' book on filtration. If you want all the necessary details to piece together a more "academic" position to satisfy yourself, then this is the book to read! I'm sure you'll come away much better informed and then find it easy to decipher who is giving you honest and factual answers regarding live sandbeds.

As to your position there are too many variables, I do agree with that, yet, they can be minimized if one understands the 'how and why' certain processes occur! In fact, that was the biggest leap we made during our six years of research!

Your statement that "the only difference between 4" of sand on the bottom of the aquarium and 4" of sand on a Plenum is what occurs in the water below the plenum. Every other external variable that impacts the systems should be identical" is not correct. Yes you are right that the water in the plenum is a 'control" factor, however, you fail to realize the entire value of that factor.

In a sandbed "directly on the bottom of the aquarium," its first inch (utilizing 2.0 - 5.0 mm size grains) contains aerobic heterotrophs living on the sand particles in that oxygen rich area. Of course they also live on the surface areas of live rock, and in fact any surface in contact with the bulk water (even the sides of fishes). They accomplish the nitrification cycle which reduces toxic ammonia laden waste products to far less potent products such as nitrite and finally to nitrate. Keep in mind the smaller the sand grain size, the less penetration of oxygen.

Once below about an inch in this type sandbed, there is a small area of depth where facultative anaerobic heterotrophs exist. This area contains approximately 0.5 - 2.0 ppm oxygen (per Sam Gamble and which we defined as the anoxic area). They produce dissimilatory denitrification where nitrate is reduced to its basic elemental form - nitrogen gas. Otherwise, everything below this approximately one inch of depth contains obligate anaerobic heterotrophs living in an area of less oxygen content than what exists in the anoxic zone, and this area is called the anaerobic zone. They only produce the ammonification process, or what is technically called assimilatory denitrification. This results in a nitrogen product (ammonium) being returned to the surrounding area. In other words, nitrate is not reduced to nitrogen gas, it is only reduced to ammonium, no further!

Now take the same aquarium and put a plenum under that 4" bed and things change quite substantially throughout most of the entire depth of the bed! Because the water in the plenum (water space) tends to collect a small amount of dissolved oxygen, generally about 0.8 ppm, it keeps the majority of the above sandbed in an anoxic state. Keep in mind this has all been fully tested and is not guesswork! It should be quite clear that instead of the majority of the bed producing ammonium such as when the bed is directly on the aquarium bottom, the majority of a plenum bed is fully reducing the incoming nitrate below the first half-inch of depth to nitrogen gas!

As for the plenum storing unprocessed nutrients, sometimes in greater amounts than in the bulk water, it is in many ways a blessing that no other type system can offer. Unfortunately those that do not understand the mechanics/processes that are occurring in the plenum often voice statements that sound like "Chicken Little the sky is falling." These non-informed people also recommend occasionally draining the plenum to prevent toxic compounds from accumulating. Simply more irresponsible babble! And it's exactly that because the honest facts have been known for years!

And yes, as our marine aquariums go through various stages, including overfeeding, lack of water changes, overcrowding, etc., there are numerous ways water quality is affected. During these times there is additional pressure on the microbial inhabitants to keep pace with an over abundance of nutrients. But they can not respond immediately, as their numbers are relevant to the incoming food supply so to speak and they take a day or two to increase their numbers. What slips by unprocessed to the plenum can be thought of as the rain from a major storm filling dried out reservoirs. Not all of the rain has soaked into ground and been used by trees. These unprocessed nutrients in the plenum reservoir, such as nitrate, are being eventually attracted back up into the sandbed because the plenum has an inherit capability that no other system has, one which took many years of research to figure out.

As an engineer, you probably understand the small electrical charges that accompany matter in many circumstances. However, many aquarists including myself were not aware of the value/importance of the electrical charge, measured in millivolts (mV), that accompanies matter in the depth of ordinary sandbeds. Even though the mechanisms and pathways associated with sandbeds are quite involved, it could generally be said that sandbeds of any type are basically a chemical sink where the diffusion of nutrients through them are influenced by electrical charge. (Now keep in mind that positive charges are attracted to negative charges.)

With that now understood, it's a fact the water's surface and the air above it are a negative mV. In the bulk water of the aquarium there is many charged molecules. Much of it is a positive mV. So is most of the living biomass, e.g., corals and fishes. Substrate surfaces are largely a negative mV. And the sandbed itself becomes even more negative with depth. The deeper the sand, the more negative it becomes and the more positive charged nutrients are naturally attracted to lower depths. In the DSB, unless there is sufficient and effective bioturbation there could easily be accumulation of these nutrients as the most negative charge is the sand at the aquarium's bottom.

But not so in a Jaubert plenum aquarium! Not only does the plenum (water space) retain some oxygen that keeps the majority of the bed above in an anoxic condition, this water in the plenum has a less negative charge than the sand above! Therefore, yet to be oxidized compounds in the plenum space are attracted back up into the sandbed for oxidation. Those in the know considered it a natural supply and demand process that retrieves constituents from the plenum reservoir. How much accumulates there seems to depend upon system bioload. Yet one thing for sure, those levels are always fluctuating, which indicates that the natural supply and demand process taking place between the plenum and its above bed is functioning normally and does not need any interference (draining or venting) from the hobbyist!

So the differences between a DSB and that of a plenum system should be quite evident. Yet, there is infauna to consider, as they are a 'wild card' when it comes to the efficiency of the microbial processes. They need oxygen to live and must link with the substrate surface, whereas microbes do not. Their tunneling processes can bring in differing amounts of oxygen and/or bring orthophosphate to the bed surface. Of course these results, good or bad, depend upon the size, type and number of infauna. One thing for sure, bacteria are more predictable when it comes to their location and processes, therefore one can not depend upon infauna to maintain an anoxic state in the lower reaches of deep substrates.

I tend to believe that if a closed system, no matter what its physical size (home or public aquarium), contained more anoxic area as we define it, the overall system could contain far less nitrogen products (nitrite, nitrate, ammonium), and of course far less hydrogen sulfide and/or methane. And this would help promote a more naturally balanced condition, and very possibly, less unwanted algae. And even though there are no true guarantees in this world, fact is that plenum systems contain less hydrogen sulfide because their beds have a greater oxygen content. In fact, they actually run slightly cooler than DSB and far cooler than mud-type beds, which are more swamp-like and where methane could really be a hidden problem!

Hope this helps.

Cheers,

Bob