Freshwater Sump / Refugium

[thiswasgone]

I'll admit... this is getting a bit over my head. But I'm doing my best to keep up.

After a few google searches, do you agree that in laymen's terms heterotrophic bacteria/archaea is the bacteria that feasts on solid waste breaking it down into smaller parts more readily available to be processed by NH3-to-NO2 bacterial process we all know and love.

If so, isn't this bacteria mostly grown on the waste itself? It seems to me this bacteria will thrive in the mechanical chamber and will be a bit out of place in a chamber in the sump that does not target the collection of physical waste. And we wouldn't need to provide high surface area media, as it grows in, on, around the waste itself.

I also read: " Heterotrophic bacteria can reproduce very quickly, sometimes doubling in population every 20 minutes."

This misunderstanding seems to come from aquarist believing in a hard separation of mechanical and biological filtration. Fundamentally all mechanical filter media can act as biological filter media as beneficial bacteria (both heterotrophic & autotrophic) simply require water flow + surface area to attach onto and grow. Additionally, bacteria typically do not grow into standalone colonies, most micro-lifeforms have various co-existance relationships and in this case both will live in the same colony. We do not use mechanical and biological filter media interchangably because mechanical media either has enough surface area but is not porous enough to allow water to flow through or the reverse -> bacteria colonies either further clog up the filter or not enough surface area to grow effective colonies -> tank won't cycle properly. Thus, mechanical media is usually reserved for "polishing" the water quality by removing visible solids but not necessarily biological filtration.

This is why the best biological filter set ups use sand, beads, or kaldness media; they maximize surface area while also allowing for water to flow through/around them for tumbling or even in static formations. Thus, most of the heterotrophic bacteria will still be on your biological media ergo why "overfiltration" is a little bit of a myth but there definitely needs to be some cost-benefit analysis done to check if it is worth it compared to just more water changes.

 

[triamond]

So the 4ppm (daily) turned into 3 ppm, not 5

Yes… I thought about net water (total ‘external’ volume minus glass/acrylic, sand, free space at the top, rocks, wood, background, etc.)… it could be 75%. Similar for the sump, minus space for overflow/media/baffles/pump/sand, etc. – 50%.
3,785 * (195 × 0.75 + 75 × 0.5) = 695 (l) and about 4.3 ppm of NO3 daily (5.4 ppm for the aquarium only).

I think 60-65 ppm of nitrates per week for a single Oscar being properly fed in a 75 gal with no plants is high.
Not ridiculously high. But high. I'd expect 30-40.

Sounds reasonable
Complicating things...
Some of the nitrates (quantitatively) will be used for building bacterial mass.
Some will be held in not-so-labile organic substances that are not prone to mineralization (refractory DOC).
Tests will detect only the mineral final product of the process.
Some will be consumed by plants, fish growth, and algae.
Some will be removed during cleaning of mechanical media if present (but not much, as 85% of N is removed through gills in soluble form (NH3)).
The system (heterotrophs) is carbon (energy) limited, as fish produce energy from the oxidation of proteins, fats, and some carbs. Undigested ingredients are a source of energy. Rotting wood could be a source of carbon, so biofloc building should remove more nitrates.

But... will dedicating 40ish gallons of my sump to a refugium offset the waste to a degree that makes it worth having?

For educational purposes only If aquaponics is planned… no! Algae scrubbers, no!
Plants can use 10 times more light than algae; there is 1000 times more CO2 in the air. Aerial plants can grow 4.5 times faster than water plants, and water plants can grow 2 times faster than algae. There is much more water in algae than in aerial plants…

I also read: " Heterotrophic bacteria can reproduce very quickly, sometimes doubling in population every 20 minutes."

In water, it's easy to cause a bloom. But it takes time to build a proper life in the filter to starve out free-floating bacteria or feed on them. Enough space, flow, and oxygen are required.

s the bacteria that feasts on solid waste breaking it down into smaller parts

This is just a start – to break it into soluble components… then soluble organics need to be oxidized to CO2.
...
You don't need sand… just put a Pothos stem into water anywhere, and water roots will appear.
Sand will reduce the durability of the pump and will soon clog… bad idea.
I agree with duanes and jjohnwm on baffles (in terms of space), pump compartment, and overflow/dry run risk.
Things are even more complicated with a constant water changing system, as water goes down the drain when the pump stops (if it's on the pump side) or when it starts (on the sump input; not tested).
On the other hand, baffles increase speed through the filtration medium, which can be beneficial if there is not enough flow to cause turbulent movement close to the medium surface… No problem in MBBR.