If I have one skill in this hobby it is being able to overstock tanks to their max. capacity and keep the fish healthy.
The bacteria with which you are familiar are not the same ones in our tanks. There is little or no Nitrobacter, it is Nitrospira that handle nitrite and which, according to more recent studies, can also process ammonia straight through to nitrate. Additionally, ammonia oxidizing Archaea have been discovered and to be present in some tanks.
Waste treatment is not a good comparison for what occurs in tanks. It is not possible to stock a tank to where it can produce the concentrations of ammonia or nitrite with which you work. Moreover, tanks, once stocked to their maximum capacity (meaning overstocked to where it would become unhealthy to add any more) do not surge ammonia etc. And lets not even bother talking about the volumes of water with which you work and those with which almost all fish keepers work. You talk about GPM and our filters are rated in GPH.
You talk about max. Flow rates in your bio-reactors. It seems to me that there are bigger pumps available. You operation might have considered upgrading the pumps. or how about adding a pump to increase the flow through the bio-reactor? My guess is this might have created to much flow and risked losing bio-film due to the shear forces that come with flow?
I understand about being able to calculate surface area needed for any bio-remediation. However, a tank an enclosed system and yours is a constant flow propositions. Only the flow rate of new water changes but it pretty much doesn't stop. In a tank we get the bacteria wherever it can thrive, not just in a filters. If there is too little media provided in a filter or filters, the bacteria will multiply elsewhere in a tank, especially the top half inch or so of the substrate.
A lot of waste water treatment is done using trickle filters, unless I am wrong. The major change over the last 60 odd years is the media now used is plastic. A trickle filter is what we in the hobby would call a wet/dry. Here is and interesting piece of info from Science Direct re Trickling Filter:
Biological Filtration
Miklas Scholz, in
Wetlands for Water Pollution Control (Second Edition), 2016
13.3 Basic Ecology
Trickling filters are well known for their ecological diversity of life forms participating in the wastewater’s stabilization. These include prokaryotic and eukaryotic organisms as well as higher life forms such as rotifers, nematodes, annelid worms, snails, and many insect larvae.
The bacteria are active in the uptake and degradation of soluble organic matter.
Nitrifying bacteria convert ammonia to nitrate. In a low-rate trickling filter, there is a high nitrifier population and the effluent is well nitrified. In a high-rate filter, there is more “sloughing” of the biomass due to higher fluid shear, and so little or no nitrification takes place.
You can rad more here:
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/trickling-filter
As I said in my first post, there is a lower limit to effective flow rates in a filter. However, most of the time filters never approach that rate. but an interesting clue can be found in the design of AquaClear power filters. These come with an adjustable flow rate. Unless the instructions have changed since I accumulated my ACs, they state that the flow rate can be reduced to 1/3 of the filter's rated capacity.
I highly doubt that Hagen would make a filter where one can reduce the flow rate to the point where the filter could not still provide sufficient capacity to meet what it claims to be the max. tank size fot the filter. Of course they do not have to keep more than normal full stocking safe. that responsibility belongs to us.
What I do know is the most effective filters I have now have the largest volumes of media and surface area and the some of the lowest flow rates i have used. I am not suggesting anybody shop from this site, that decision is up to you. But the information there is superb. I now have Hamburg Mattenfilters on 3 tanks with more to get converted. The flow rates are on the slower side, but more than sufficient to do the job better than most other filters i have used.
Read more about Mattenfilter.
One last comment, if you do read on the Swiss Tropicals site, you will see some things re filtration and "ecological diversity of life forms participating" and "prokaryotic and eukaryotic organisms as well as higher life forms such as rotifers, nematodes, annelid worms, snails, and many insect larvae." If you read this section:
Aquarium Biofiltration
