Once favorable water quality is established, the rate of nitrification in a biofilter is largely determined by the amount of nitrifying bacteria present in the biofilm. In bead filters, the biofilm is often dominated by heterotrophic bacteria, which grow quickly on organic material from fish waste. While heterotrophs are important for breaking down organic matter, they can overgrow and suppress nitrifying bacteria, which are slower-growing but essential for ammonia removal.
Controlling Heterotrophic Competition
- Reduce organic loading to limit heterotrophic growth
- Low organic load with high TAN → high nitrification rates (CA > 100 mg/ft²/day)
- High organic load → heterotrophs outcompete nitrifiers, reducing efficiency
The Backwashing Paradox
- Frequent backwashing removes excess heterotrophic bacteria but also washes out nitrifiers
- Infrequent backwashing allows heterotrophs to clog the filter
💡 Key point: Optimal nitrification requires balancing the need to remove heterotrophs with maintaining enough biofilm age for nitrifiers to reproduce.
Optimizing Backwash Intervals
- Low organic load (<1 lb feed/ft³ beads/day): wide range of optimal backwash intervals
- Moderate/high organic load (1.5–2 lb feed/ft³ beads/day): narrower window; typical warm water interval: 1–3 days
- Additional factors: mixing duration, water quality, feed rate variations affect optimal frequency
