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Very good question!!! Same question that many reefers are asking … including myself. My info is purely from Julian Sprung’s latest volume 3 installment of his book.
Julian also referred to Rob Toonen's study 2 times, and agreed that all the different system achieved filtration in the water ABOVE the sandbed. However, Julian feels that things are different WITHIN the sandbed. Thus, he mentioned that the denitrification process in both are different.
From Julian’s observation, he also disagreed that the plenum has no function. 2 functions mentioned are:
1. reducing the chances of hydrogen sulphite development in the sand bed.
2. helping to breakdown detritus within the sand bed. On this point, he is not sure if the reduced detritus is due to the plenum, or the fact that the Jaubert System has less rock on the sandbed.
Please note that Julian did not say that the plenum helps with filtration. This view is consistent with Rob’s observation.
Additionally, Rob Toonen's findings are for 6 months, where Julian Sprung’s observation are in terms of years. I don’t think 6 months is long enough to observe the differenced in the accumulation of detritus.
Please note that Julian AGREES with Rob regarding the filtering of water ABOVE the sandbed. And Rob did not really study the situation BELOW the sandbed, where Julian made many of his observations & assumptions.
In all honesty, I do not think anyone has the answer. However, all we can say is that it works well as a filtration method.
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