Effective nitrification and removal of particulate organic matter in biofilters with compressible polyurethane foam

This study evaluated the treatment efficiency and backwashing capabilities of a novel fixed-bed biofilter utilizing compressible polyurethane foam media. The novelty of the biofilter system lies in its custom-designed backwashing mechanism, in which the foam media is mechanically compressed to remove retained solids. The system was tested at two hydraulic retention times (HRTs), 2.4 min (low HRT) and 16 min (high HRT), within a pilot-scale freshwater recirculating aquaculture system (RAS) stocked with rainbow trout over an 18-week period. Assessment included single-pass removal efficiencies and volume-specific removal rates based on inlet–outlet concentrations, as well as particulate matter retention rates derived from sludge collected during backwashing. Results demonstrated immediate treatment effectiveness, operational simplicity, and practical applicability. High HRT reactors consistently achieved higher single-pass removal efficiencies, with peak reductions of 90 % for total ammonia nitrogen (TAN), 66 % for nitrite-nitrogen (NO₂-N), 64 % (Bactiquant) and 48 % (H₂O₂ degradation assay) for bacterial activity, 44 % for turbidity, and a 10 percentage point improvement in ultraviolet transmittance (UVT) — approximately 1.5–3.5 times higher than values observed in low HRT reactors. Conversely, low HRT reactors outperformed in terms of volume-specific TAN and NO₂-N removal rates (67.9 ± 18.6 g TAN/m³/day; 22.5 ± 12.5 g NO₂-N/m³/day), due to increased substrate loading. Low HRT reactors also retained more particulate organic matter in sludge, with average retention rates of 38.2 ± 1.1 g/week for dry matter and 25.7 ± 1.2 g/week for total chemical oxygen demand (COD), compared to 24.8 ± 0.7 g/week and 7.7 ± 0.7 g/week under high HRT, respectively. Backwashing enhanced nitrification without compromising the system stability, however, a temporary reduction in NO₂-N removal was observed following a four-week operational pause.

Overall, this type of fixed-bed biofilter concept demonstrates strong potential for aquaculture applications, offering rapid effectiveness, operational resilience, and modular flexibility for continuous or intermittent treatment of nitrogenous compounds and particulate matter.