What Nobody Tells You About Sizing a Biofilter
The biofilter is probably the least glamorous part of a live seafood holding system. It's not the chiller, which has all the mechanical complexity, or the tank array, which is the most visible piece of infrastructure. The biofilter just sits there, quietly turning toxic ammonia into something harmless. When it works, nobody talks about it. When it doesn't, you lose product.
The most common way biofilters fail isn't equipment malfunction it's under-sizing. And the most common reason for under-sizing is that the design was done at half-capacity conditions and nobody ran the numbers at full load. This post covers the key variables that drive biofilter demand and a practical rule of thumb for checking your sizing.
Where the Ammonia Actually Comes From
Ammonia in a lobster holding system has two primary sources. The first is respiration lobsters excrete ammonia directly across their gills as a metabolic byproduct. A lobster at rest at 38–40°F will excrete roughly 0.3–0.5 mg of total ammonia nitrogen (TAN) per kilogram of body weight per hour. At warmer temperatures, that rate goes up. At 50°F, you're looking at closer to 0.8–1.0 mg/kg/hr. The second source is waste decomposition. Dead animals, uneaten food, and accumulated organic matter all break down into ammonia. Even in a well-managed facility where mortality is low, you're getting a continuous baseline ammonia input from organic waste. This is often underestimated because it's diffuse and hard to attribute to any single event.
For a facility holding 50,000 lbs (about 22,700 kg) at 40°F, a conservative TAN generation estimate is roughly 10–12 kg per day. That's the load your biofilter needs to handle. And that's before you account for any incidental feeding, seasonal temperature swings, or high-density conditions that stress the animals and increase metabolic rate.
Nitrification Basics — What Your Biofilter Is Actually Doing
Biological filtration relies on two groups of bacteria: Nitrosomonas, which converts ammonia (NH3/NH4+) to nitrite (NO2-), and Nitrospira (or Nitrobacter in some systems), which converts nitrite to nitrate (NO3-). Both live in biofilms on the media surface. The overall reaction is called nitrification.
The key design parameter is the surface area available for biofilm colonization, expressed in square meters. Nitrifying bacteria are not especially dense a well-established biofilm will process roughly 0.3–1.0 g of TAN per square meter of media surface per day, depending on temperature, dissolved oxygen levels, and pH. Cold water (below 42°F) pushes you toward the low end of that range. Warm water and good aeration push you higher.
For the 50,000 lb facility above with a 10–12 kg/day TAN load, and using a conservative 0.4 g TAN/m²/day nitrification rate, you'd need roughly 25,000–30,000 m² of biofilm surface area. That's not a typo. Biofilter sizing is always larger than operators expect.
Why "It Worked Fine at Half Capacity" Is a Trap
This is where most under-sizing problems originate. A facility commissions a system at 25,000 lbs and runs it for a season. Water quality is excellent. The operator is satisfied. Then they expand to 50,000 lbs and assume the biofilter will scale proportionally just run it harder.
It doesn't work that way. Nitrification is a biological process, not a mechanical one. The bacteria take time to grow and colonize media. If you double your ammonia load overnight, the biofilm doesn't double in 48 hours. You get a ammonia spike that can persist for days or weeks while the biology catches up if it catches up at all before you start losing product.
There's also the oxygen competition problem. Nitrifying bacteria require dissolved oxygen (DO) to function. In a heavily loaded system, heterotrophic bacteria (which break down organic matter) outcompete nitrifiers for available oxygen if DO drops. Below about 3 mg/L of DO, nitrification efficiency drops sharply. Below 2 mg/L, it can stall almost entirely.
This is why biofilter performance is not linear with loading it's highly sensitive to the conditions at the margins.
Moving Bed Bioreactors — A More Forgiving Design
A moving bed bioreactor (MBBR) addresses many of the limitations of static biofilter media. In an MBBR, plastic biofilm carrier media typically small cylinders or rings made of polyethylene are kept in suspension by aeration or mechanical mixing. Each carrier has a protected interior surface area where biofilm colonizes, and the constant movement keeps the surface active and prevents clogging.
The practical advantages are significant. MBBRs are self-regulating: excess biofilm sloughs off naturally, which prevents the media fouling that gradually degrades static systems. They handle fluctuating loads better because the mixing keeps the biofilm exposed to consistent oxygen and substrate concentrations. And they're easier to expand you simply add more media volume.
Typical MBBR carrier media provides 500–800 m² of protected surface area per cubic meter of media at a 50–60% tank fill fraction. That same 50,000 lb facility would need roughly 40–50 m³ of MBBR volume at typical fill fractions a manageable footprint.
A Rule-of-Thumb Sizing Check
The following table gives rough starting-point estimates for biofilter sizing based on facility capacity. These are sanity-check numbers, not substitutes for a full engineering calculation temperature, species, feeding regime, and water exchange rates all shift these figures. These assume a water temperature of 38–42°F and a conservative nitrification rate of 0.4 g TAN/m²/day. If your system runs warmer or your animals are actively fed, adjust upward. If you're in partial recirculation with high daily water exchange (>20%), you can adjust somewhat downward because you're diluting the ammonia load.
Don't Forget the Startup Period
A new or thoroughly cleaned biofilter is essentially sterile. Nitrifying bacteria need to colonize the media before the filter can do its job, and that colonization takes time typically 4 to 8 weeks to reach stable capacity under a continuous ammonia supply.
Loading a new system to full capacity on day one is one of the most reliable ways to kill product. The ammonia spike during startup can reach 5–10 mg/L or higher well above the safe limit of around 0.5–1.0 mg/L unionized NH3 before the biology is established.
This is covered in more detail in our preseason preparation guide, but the short version is this: if you're seeding a new biofilter, plan for 4–6 weeks of cycling before you put full load on the system.
The Bottom Line
Biofilter sizing is one of the most consequential design decisions in a live holding facility, and it's one of the most frequently gotten wrong. If your facility is running fine at partial load but showing ammonia stress signals during peak season, the biofilter is the first place to investigate.
At Aqua Production Systems, we design MBBR and other biological filtration systems as part of complete recirculating aquaculture systems. If you're not sure your current biofilter can handle your peak load or if you're planning an expansion and want to size it right from the start give us a call. We're at aquaproduction.ca.
