Nitrification performance and robustness of fixed and moving bed biofilters having identical carrier elements

- Four equal biofilter comprising of replicated fixed bed and moving bed biofilters were used to compare nitrification performance.
- Equal filling ratio and specific surface area of identical carrier media was used.
- Application of 50 mg/l hydrogen peroxide to RAS happened without biofilter by-pass.
- TAN and nitrite removal was found to be superior in fixed bed biofilter.
- Apparent nitrite production from moving bed biofilters was documented.

This study compared moving bed (MB) and fixed bed (FB) biofilter performance. The experimental recirculating aquaculture system had four equal biofilters in parallel. Each of the two replicated FB biofilters (with heavy elements) and the two MB biofilters (with neutral elements) had 200 l carrier media with a surface specific area to volume ratio of 750 m2/m3. Total ammonia nitrogen (TAN) and apparent nitrite removal rates were measured during identical steady-state conditions and during a water treatment event where 50 mg/l hydrogen peroxide was applied. FB biofilters were found to perform equal to or better than MB biofilters during the experimental phases. The average (n = 2) surface specific TAN removal in the FB biofilters was significantly higher than the MB biofilters (0.20 vs. 0.14; g N/m2/d) at steady state. The FB biofilters had a positive apparent nitrite removal (0.02 g N/m2/d) at steady state in contrast to MB biofilters releasing nitrite (−0.02 g N/m2/d). Application of H2O2 caused a transient five-fold TAN increase up to 1.05 mg N/l in the system. Prolonged elevated nitrite levels up to 2.85 mg N/l (>15 fold increase) was observed for one week due to reduced nitrite oxidation particularly in the MB biofilters. The findings indicate FB biofilters to be more resistant against the sanitizer applied, due to increased organic matter in the biofilters compared to MB biofilters. Aspects of the experimental setup are discussed in relation to other studies and commercial biofilter operations.