Carrier-Induced Formation of Diverse Partial Denitrification-Anammox (PD/A) Biofilms: Characteristics, Assembly Mechanisms, and Synergistic Nitrogen Removal

This study clarified the assembly mechanisms and coupling performance of typical carriers─fiber ball (FB), cubic sponge (CS), and hollow cylinder (HC)─with PD-Anammox (PD/A). From the perspective of carrier structure, FB’s lower porosity created a dense network that stabilized the biofilm (biomass shedding rates: 33.48 ± 2.31% at 80 rpm and 64.30 ± 3.22% at 200 rpm) but limited biomass enrichment (biomass density: 8.83 ± 0.32 mg/cm3). In contrast, HC supported more biomass (biomass density: 18.21 ± 0.79 mg/cm3) but exhibited poor biofilm stability under similar conditions. Regarding carrier material characteristics, FB’s weaker hydrophobicity enhanced Lewis acid–base interactions, promoting extracellular polymeric substance (EPS) secretion and improving biofilm adsorption capacity, ultimately stabilizing the FB biofilm. Microbial analysis revealed that DB preferred to retain in the weakly hydrophobic carrier biofilm (FB), while anaerobic ammonia-oxidizing bacteria (AnAOB) were predominantly concentrated in suspended sludge, supporting selective microorganism enrichment. FB hosted the highest abundance of denitrifying bacteria (DB) among the three carriers, which was corresponds to relative high adsorption capacity with sludge. Furthermore, DB enrichment on weakly hydrophobic carriers reduced the risk of excessive dissimilatory nitrate reduction to ammonium (DNRA) in the suspended sludge by providing enough NO2–, thereby enhancing PD/A process stability.