The ultimate goal in integrating a media bed gravel filter with a DWC system is to eliminate the need for solids removal, essentially eliminating the labor factor as well as benefiting from the additional metabolization of those solids by introducing red worms. Is it possible to remove all solids via the media bed or will some degree of solids removal, ie. settling tanks or swirl filters still be needed? JD Sawyer just posed this question to me a couple days ago and quoted this from James Ebeling's book Recirculating Aquaculture,
"Carbon eating heterotrophic bacteria grow significantly faster than the autotrophic nitrifiers do. Their mass can double in an hour, while it takes nitrifiers days to double. This high growth and the associated oxygen demand consequently suffocate the nitrifiers buried deeper in the biofilms, resulting in death and sloughing of the biofilm from the bioreactor surfaces... You must have effective solids removal BEFORE the high ammonia water is transferred to the biofilter. As mentioned above, the heterotrophic growth will compromise the nitrifiers ability to oxidize ammonia, mostly because the heterotrophs consume the oxygen prior to the oxygen being able to diffuse into the biofilm to where the nitrifiers are. This results in nitrifiers being starved for oxygen and then they die off resulting in complete sloughing of the biofilm and loss of nitrification capacity. A recipe for disaster in RAS is to have poor solids removal" (Ebeling, Biofilter design, Recirculating Aquaculture 2nd Ed).
My theory is that stocking densities and feed ratios will have a tremendous effect on this and if managed correctly, should minimize the need for solids removal.
This is a very interesting discussion. I'm not sure if this will be applicable in an AP system but there is something interesting to note about the heterotrophic bacteria. While they can easily outgrow and smother the nitrifying bacteria they do not attach to surfaces nearly as well as the nitrifiers. This is part of the reason aquaculture systems "move" their biofilters so heavily. One reason is to keep the bacteria well oxygenated but it is also to slough off the heterotrophic bacteria.
Again I'm not sure how to use this in an AP system but I am always interested reducing waste of any kind in my system.