What lead me to this reading was a desire to biologically control both NO3 and PO4 buildup in a cost-effective way, rather than do weekly waterchanges. Occasional waterchanges, of course, still being a necessity to replentish Ca, I, or other minerals in lieu of dosing. I did a search here at NR and the only thing I found that mentioned these methods in more than a passing comment was the NR link in this list.
I got oriented toward RDSBs and coils b/c I don't think that a conventional in-tank DSB is desirable in a nano tank, mostly because 1) the footprints of our tanks don't allow much surface area for material exchange, 2) we would have to give up valuable real estate to several inches of sand, and 3) many people don't keep their nanos running undisturbed long enough to really glean the benefit.
Initially, I liked the coil idea until I got into the nitty gritty of tuning them to make sure you weren't pumping NO2 or H2S back into your system. Add the fact that most of these systems are designed and built for much larger systems with extremely heavy bioloads (100-300ppm NO3 in many cases) and you get something that doesn't look like it is easily applicable to nanos.
On the converse, it seems that RDSBs might have an nano application. They are simple to construct, easy to maintain, and don't require tuning. Depending on the size, they also may not need much in-tank bioload to keep them running.
I have read about two different RDSB setups, both of which were employed in larger systems, usually with either a rubbermaid trashcan or a standard 5g bucket. Some people used silica play sand from HD/Lowes, some used sugar-sized aragonite, some used a mix, and all were successful to varying degrees.
The topics of discussion here are: 1) Is biological denitrification worth trying for in a nano (6-55g)system? 2) Do you think that coils or RDSBs would have any real value in a nano system, or are they just one more gadget that we would have to muck around with? 3) Would a nano provide enough organic carbon to keep heterotrophic denitrifiers going or would you have to periodically dose methanol, EtOH, or sugar?