How to Prevent Biofilter Crash in Commercial RAS

Why Biofilter Crash Matters

The biofilter is the biological conversion engine of a RAS. When nitrification slows or collapses, ammonia and nitrite can rise quickly enough to damage gills, reduce feeding, and cause mortality. Many crashes are not sudden accidents; they are the final stage of solids overload, low alkalinity, oxygen limitation, chemical exposure, or poor startup discipline.

Common Technical Triggers

• Solids entering the biofilter because mechanical filtration is undersized or poorly maintained
• Low alkalinity causing pH to fall below the active range for nitrifying bacteria
• Chlorine or disinfectant exposure from source water or cleaning procedures
• Oxygen starvation inside packed or clogged media beds
• Overfeeding before the biofilter is mature
• Uneven flow that creates channeling and inactive media zones

Warning Signs Operators Should Track

• TAN or nitrite trending upward after feeding
• pH and alkalinity declining faster than expected
• Biofilter media becoming pale, slimy, or clogged
• DO differences between inlet and outlet increasing abnormally
• Fish reducing feed intake without obvious disease symptoms

Design Controls

Biofilter protection starts upstream. Solids removal must be sized for peak feeding rate, not average flow. The system should also include alkalinity management, stable aeration, bypass or isolation logic for cleaning, and water quality alarms that trigger before the system reaches a mortality event.

Recommended controls include:

• Drum filtration or equivalent fine solids capture before biofiltration
• Reliable aeration or oxygen supply for MBBR and packed media systems
• Alkalinity monitoring and sodium bicarbonate dosing procedures
• Commissioning protocols that ramp feed and biomass gradually
• Manual verification of automated ammonia or pH readings

Emergency Response

If a biofilter crash is suspected, the immediate goal is to reduce nitrogen loading and stabilize fish welfare.

1. Stop or sharply reduce feeding.
2. Verify ammonia, nitrite, pH, alkalinity, and DO manually.
3. Stabilize alkalinity and pH without aggressive swings.
4. Increase aeration or oxygen where appropriate.
5. Inspect mechanical filtration and biofilter flow distribution.
6. Add seed bacteria only as support, not as a substitute for load reduction.

Related Engineering Review

Biofilter sizing should be reviewed together with solids removal, feeding plan, peak biomass, temperature, alkalinity, and operator response procedures.