Study on rapid start-up and stable nitrogen removal efficiency of carrier enhanced continuous flow PD/A granular sludge system

Anaerobic ammonia oxidation (anammox) is a vital process for nitrogen removal, but challenges like slow start-up and unstable operation persist. To tackle these issues and assess carrier efficiency in improving the partial denitrification/anammox (PD/A) process’s start-up and stability under low nitrogen loading rates (NLR) (0.11–0.19 kg/(m3·d)), two continuous flow PD/A systems were set up: one with self-immobilized granular sludge (R1) and the other with carrier-based granular sludge (R2). The results showed that R2 achieved over 70% total nitrogen (TN) removal within 45 days, 16 days shorter than R1. Scanning electron microscope (SEM) characterization revealed that the ecological maturity of biofilm granular sludge was approximately 14 days earlier in R2 than in R1. Comparative analysis of protein/polysaccharide (PN/PS) ratios within extracellular polymeric substances (EPS) and volatile suspended solids/suspended solids (VSS/SS) demonstrated that carrier-based granular sludge exhibited superior settling characteristics and structural stability compared to self-immobilized granular sludge. Moreover, nitrogen removal stability within the R2 reactor system was superior to R1 under diverse operating conditions, highlighting carrier reinforcement’s efficacy in enhancing the PD/A process’s operational stability. The increased abundance of anammox bacteria Candidatus_Kuenenia in R2 ensured complete nitrogen removal in the PD/A coupling process, elucidating the microscopic mechanism of carrier-enhanced nitrogen removal. The hydrophilic porous biological carrier not only expedited the PD/A system’s start-up in the upflow sludge bed (USB) reactor but also enhanced the enrichment and collaboration of effective functional microorganisms. This study provides technical reference and theoretical evidence for rapid start-up and long-term stable operation of PD/A process.