A number of discussions have prompted this thought pattern. I credit the bulk of this current burst of cranial activity to a comment made by Terri Mikkola (Thanks Terri) regarding the population mix of bacteria responsible for nitrification and those responsible for the mineralisation of solids. In UVI and Friendly style systems the multi-stage filtration system ensures that there is a separation from the zones where masses of solids accumulate. In my current mixed system re-vamp, I have a number of media filled structures (Gravel and netting) that act as first or second stage filtration. In gravel only systems, the addition of worms to the beds can greatly reduce the volume of solids building up on the gravel surface and therefore also make for a relatively good combination of nitrification and mineralization, but here is my current thought: Do we provide enough?
We characterise ourselves variously as plant or fish orientated folk, and describe fish tank to grow bed volume, but should we not be filter orientated folk? While it is great to be able to circumvent complex filter design in more basic units there is perhaps a reason why multi-stage filtration is so effective. When AP matures to the point that we have years worth of data on the performance of different designs, we might get the answer I am looking for now.
Until we get that information, my current philosophy in mixed design is newly influenced by my thoughts on effective use of space. There is obvious power implications of lifting water over a slightly greater distance to create a raised bed with filtration below, but if space is a problem, then it may be worth considering. If space is no concern, what is the operational efficiencies of a unit with minimum (not too little, but just enough) vs one with maximum filtration? By the time I'm done, my filter structures may be almost double that of my fish tank (I consider media filled beds / towers as part of filtration). For me, this boils down to having enough space for sufficient nitrification / mineralization bacteria to find a niche and do their work. And then some as insurance as I do not want to be in there cleaning it all every week.
I attached great value to a few second or third stage tanks / barrels just filled with floating media / nets in my new mixed system, but lack data at present to verify the results. Time will tell, but I think the question of filtration is a very important and often sidelined aspect of aquaponics.