FORMATION OF HAP-PNA GRANULAR SLUDGE AND THE ENHANCEMENT OF NITROGEN AND PHOSPHORUS REMOVAL

Abstract

 Anammox is attracting attention as an energy-efficient and low-carbon nitrogen removal technology and it has been applied worldwide. However, due to the unstable operation and sludge washout, the application of anammox-based process is slow. Recently, the utilizing of the hydroxyapatite (HAP) granules in the anammox process comes with the advantages, including simultaneous removals of nitrogen and phosphorus and better settleability of the sludge. In this study, two continuous one-stage partial nitritation/anammox (PNA) reactors with HAP-enhanced granular sludge were used to treat the synthetic wastewater with varying influent nitrogen concentration and HRT, respectively, to explore the removal performance and operational strategies of nitrogen and phosphorus at 25 °C in a long-term operation. As a result, the formation of HAP granular sludge enabled the maintenance of granular sludge with excellent settling performance, achieving a high nitrogen removal rate (NRR) of 2.0 ± 0.1 kg N/m³/d when the influent ammonium nitrogen concentration was 1200 mg/L, and the highest NRR of 4.8 kg N/m³/d, to the best of our knowledge, under a low hydraulic retention time (HRT) of 2 h and an influent ammonium nitrogen concentration of 500 mg/L. When the calcium concentration was 150 mg/L, the phosphorus removal efficiency (PRE) of 90% could be achieved. To achieve an high nitrogen removal performance stably, sufficient formation of HAP granules was necessary (phosphorus removal rate (PRR)/NRR ratio above 0.02), and a low HRT allowed the discharge of poorly settling sludge for the desired sludge settleability and concentration. The spatial distribution of the functional bacteria in the granular sludge was analyzed with fluorescence in situ hybridization (FISH). Anammox bacteria (AnAOB) and the ammonium oxidizing bacteria (AOB) were the two most abundant bacteria. The study also provides the enhancement strategies for the stability and an excellent nitrogen removal performance in a one-stage PNA process.