by Marco G Pistrin, Founder and Managing Director, AquaOrganica SRL, Italy
Walk onto most farm floors and ask how the biofilter was chosen and you will hear a number: so many cubic metres of media, sized to a feed load, ordered from a catalogue. The biofilter is treated as a component, a box you buy, plumb in and seed. That is the most expensive misconception in land-based aquaculture because the biofilter is not a component. It is an ecosystem and you do not get to choose it. Your water does.
Two variables write the specification before an engineer draws a single line: temperature and salinity. They decide how fast the nitrifiers work, how much surface area you must give them, which organisms even show up to do the job and what it will cost you when the chemistry turns against you. This first article takes temperature and the system architecture it lives inside. The second takes salinity and the microbiology that has quietly rewritten the textbook.
The canvas comes before the filter
The same temperature behaves differently depending on how much water you reuse, so start with the architecture. The industry has built two answers to the same problem. In a recirculating system reusing 90–99 percent of its water, the biofilter is load bearing. There is nowhere for ammonia to go but through the bacteria and when those bacteria stop, the system stops, a biofilter crash in a full RAS is not a setback, it is a fish-kill timer. Everything couples: nitrification consumes alkalinity, demands oxygen and produces carbon dioxide, so the filter you sized for ammonia quietly sets your dosing, your aeration and your degassing load too.
The hybrid flow-through system answers differently. Running at perhaps 60–65 percent reuse with a continuous influx of freshly treated water, dilution does part of the work the bacteria do in a RAS. The biological load per pass falls, the biofilter shrinks and the consequence of a wobble is smaller. Some modern smolt designs lean so hard on flush that they minimise biological dependency on purpose, trading water for the elimination of long start-ups, off-flavour risk and the crash exposure that keeps RAS managers awake at night. Neither answer is more correct. They are different bets on the same chemistry and the reuse rate you choose sets the areal ammonia load every square metre of media must carry. That number, not the catalogue, is where sizing actually begins.
The first master variable: Temperature
Nitrification is a biological rate and like every biological rate it climbs and falls with heat. The measured relationship is steep. Surface-specific ammonia removal in moving-bed media rises roughly six-fold between 6 and 30°C, from around 0.04 to 0.25 grams of ammonia-nitrogen per square metre per day, before falling away above the low thirties. Nitrifiers keep working down to near 1°C but below about 4°C the kinetics drop off a cliff. For the designer this is not trivia. It is the single number that decides how big the filter has to be.
Figure 1 traces the shape of that relationship: a viable but sluggish floor down near 1°C, a steep rise through the working range, a peak in the optimum band around 25–30°C and a sharp drop-off once the water pushes past the low thirties. Read it as a map of where each species sits, the cold-water grower operates on the slow left-hand limb and pays for it in filter volume; the warm-water grower works near the peak and pays for it in oxygen. The curve, not the catalogue, sets the rate every design assumption is built on.
Cold-water species make the point. Atlantic salmon, rainbow trout, Arctic charr, all grown in water where the bacteria are sluggish by definition. You cannot make cold nitrifiers faster, so you compensate the only way physics allows with surface area. This is why cold-water RAS leans on moving-bed reactors carrying high specific-surface-area media, commonly several hundred square metres per cubic metre, filled to half or two-thirds of the reactor volume. The biology is slow, so you give it more real estate. The filter gets bigger for the same job.
Warm-water systems invert the problem. Grow yellowtail, barramundi or shrimp and the bacteria are fast, metabolism is efficient, removal rates are high, the footprint shrinks. But warm water holds less dissolved oxygen and now that oxygen must feed two appetites at once: the fish and the biofilm’s own biological oxygen demand. The constraint moves. In the cold you are buying surface area; in the warm you are buying oxygen. Same process, opposite bottleneck and a designer who carries a cold-water instinct into a warm-water build will undersize the oxygen system every time.
The filter type follows from the load
Temperature and load decide not only the size of the filter but its type. The moving-bed bioreactor has become the default in serious RAS for a practical reason: its carriers are kept in constant motion by aeration and that motion continuously shears excess biofilm off the media. The filter cleans itself, never clogs and carries almost no head loss, so it tolerates the heavy, swinging loads a grow-out system throws at it. Fixed-bed and trickling filters trade that robustness for a different benefit, a trickling filter degasses carbon dioxide and re-aerates in the same pass that it nitrifies, which in a tall enough column is genuinely useful. But fixed media clogs and goes septic under load and a septic biofilter is a hydrogen-sulphide and oxygen-crash risk you never want inside a recirculating loop.
Whichever type you pick, the same hard truth governs commissioning: nitrifiers grow slowly. Building a mature biofilter is a matter of weeks, not days and through that window ammonia and nitrite swing while the community establishes itself. This is the long, delicate start-up that flow-through designs are content to avoid and it is why a RAS is brought up with fish loaded gradually or the filter seeded from a running system, rather than switched on like a pump.
The trap that closes slowly
Temperature also sets a trap that springs in slow motion. Every gram of ammonia the filter oxidises destroys roughly 7.05 to 7.14 grams of alkalinity, measured as calcium carbonate and throws off carbon dioxide and acidity as it goes. Alkalinity falls, pH drifts down and nitrification, which dislikes low pH, slows in response. A slowing filter lets ammonia rise, which is exactly the wrong direction.
Figure 2 follows that loop stage by stage, nitrification consuming alkalinity and releasing carbon dioxide, pH sliding, the removal rate dropping, ammonia climbing and the cycle tightening on itself and names the three moves that break it. This is the feedback loop behind most ‘mysterious’ biofilter declines and it bites hardest in soft, cold freshwater where there was little buffering capacity to begin with. Pilot work on brackish post-smolt systems has shown how tightly biofilter and degasser performance track alkalinity together.
And alkalinity is only half of the gas problem. The same reaction floods the water with carbon dioxide, which depresses pH further and stresses the fish directly. A moving-bed filter strips perhaps half the CO₂ per pass at high loading, useful but rarely enough on its own, so intensive systems run a dedicated degasser alongside the biofilter. Manage the filter and ignore the gas balance around it and the filter will still fail; it will simply take the blame for a problem that was never biological.
The defence is unglamorous and non-negotiable: watch alkalinity as a leading indicator, dose bicarbonate to hold it and strip CO₂ deliberately rather than hoping it leaves on its own. A biofilter rarely crashes because the bacteria died. It crashes because the chemistry around them was allowed to drift while everyone watched the ammonia number, the lagging one.
What temperature decided
Temperature does not merely influence the biofilter. It writes half the specification: how fast the bacteria work, whether your limiting resource is surface area or oxygen and how quickly the chemistry can turn on you. Get it right and the filter is a quiet engine. Get it wrong and no amount of media will save you.
But temperature is only the first variable. The second, salinity, does something stranger. It does not just slow the biofilter down. It changes who is living in it. That is where the textbook most operators learned from turns out to be wrong and where the next article begins.
Stay tuned for Part 2 — Salinity, the New Microbiology and the End of Nitrate.
