Aquariums: Update on fluidized sand reactor system.

Y’all asked for an update—so here it is:

4 g NH₄Cl × 0.26 ≈ 1.04 g NH₃-N per hour

(That’s the current ammonia processing load.)

I honestly can’t believe I was able to build this. The system is processing 4 grams of ammonium chloride per hour, which works out to roughly 1 gram per hour of pure ammonia (NH₃-N).

I’m still pushing it to find the upper limit.

Right now dosing is:

• 10% ammonium chloride @ 40 mL/hr

• 10% sodium bicarbonate @ 80 mL/hr (to maintain alkalinity for nitrification)

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Core System Design

The system is built around a 6” diameter, 68” tall fluidized sand reactor containing ~60 lbs of sand. It’s fluidized to about 2× its settled depth, maximizing usable surface area while keeping all media active for nitrifying bacteria.

Additional biofiltration includes:

• \~6 gallons of fluidized K1 media (expanding as budget allows), driven by a high-flow wavemaker

• Two Swiss Tropicals Poret foam towers (30 PPI)

• Each tower: 19” × 13” × 5”

• Three airlifts per tower for strong circulation and oxygen delivery

• Submerged media: \~2 bags of lava rock + additional bioballs for extra surface area

For oxygenation, the system uses a large trickle filter, providing high gas exchange to meet the significant oxygen demand required at this level of nitrification.

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Next Phase: Denitrification (Plug Flow Reactor)

The next step is adding a denitrification plug flow reactor (PFR) to handle nitrate.

The goal is to create a low-oxygen (anoxic) zone where bacteria convert nitrate (NO₃⁻) into nitrogen gas (N₂), which then leaves the system.

Unlike the high-flow, oxygen-rich nitrification zones, this reactor will run:

• Slow, controlled flow

• Minimal oxygen intrusion

• Increasing nitrate reduction along its length

This will be powered by carbon dosing (likely ethanol, acetate, or similar), which provides the energy source needed for denitrifying bacteria.

The plug flow design means:

• Water enters one end and moves in a single direction

• Oxygen is consumed near the inlet

• Denitrification increases progressively

• By the outlet, nitrate should be significantly reduced

Flow will be controlled with a peristaltic pump to ensure proper contact time and prevent oxygen from penetrating too far into the reactor.

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Big Picture

If everything is tuned correctly, this system should:

• Handle very high ammonia loads

• Maintain stable nitrification

• Actively remove nitrate

Basically completing the nitrogen cycle in a more closed-loop, high-efficiency system.