Hollow Convective Carrier Design Enhances Biofilm Retention and Mass Transfer for Improved Nitrification Performance

This study developed a novel carrier configuration (i.e., hollow convective carrier [HCC]) to enhance biofilm retention and mass transfer in biofilm-based wastewater treatment. The HCC was fabricated by stitching two layers of permeable nylon mesh to form internal hollow compartments, enabling biofilms to grow within protected spaces while maintaining diffusive and convective transport. Long-term reactor operation showed that HCC-based pure biofilm systems achieved faster start-up and better performance than single-layer convective carriers (CCs) and commercial polyethylene carriers (PEK5). The superior performance was mainly attributed to the improved biomass accumulation and mass transfer. HCC-based reactor exhibited higher biofilm densities (2.4-4.1 g/m²) than CC (1.6 g/m²) and PEK5 (0.6 g/m²) carriers. Biofilms on the surface and inside the hollow compartments of HCC carriers had high nitrification activity and also accumulated more slowly-growing bacteria of Nitrospira than CC and PEK5 carriers. Nitrate tracer experiments confirmed that substrates could penetrate the thick convective biofilms on HCC carriers, with a mass transfer flux of 61.3 mg-N/(m² h) and overall mass transfer coefficient of 1.58 × 10^-6 m/s. Both mass transfer flux and overall coefficient in convective biofilms increased linearly with mixing intensity, supporting the role of convective transport in alleviating diffusion limitations. Overall, the hollow convective design integrates enhanced biofilm retention with improved substrate accessibility, providing a promising carrier strategy for stable and efficient treatment in biofilm reactors.