Ultrafine partial nitritation-hydroxyapatite granules facilitate stable high-rate partial nitritation of anaerobic digestate of waste activated sludge

In two-stage partial nitritation-anammox processes for sludge digestate treatment, low oxygen and organic matter often limit partial nitritation (PN) rates. To address these limitations, we developed a novel PN-hydroxyapatite (PN-HAP) coupled granular sludge process for high-rate PN of sludge digestate. The process leverages the high alkalinity and elevated phosphorus concentration (15 mg/L) of the digestate to induce in situ HAP precipitation in the PN reactor, forming ultrafine PN-HAP granules (100 um), which enhanced sludge settleability and yielded high biomass concentrations (17.10) within the reactor. With an influent ammonia nitrogen concentration of 310.85 mg/L and hydraulic retention time (HRT) of 0.73 hours, the nitrogen loading rate reached 10.22 kg/m3/d. Scanning electron microscopy and energy-dispersive X-ray spectrometry have revealed an entirely new structure of granular sludge, characterized by a mineral-rich, porous surface structure. HAP is uniformly distributed both on the surface and within the interior, rather than being concentrated at the center. Microorganisms predominantly resided inside the granules. The HRT directly influenced the granule structure. Furthermore, the PN-HAP process exhibited complete inhibition of nitrite-oxidizing bacteria (NOB). In summary, this study presents a novel approach for high-rate PN operation in sludge digestate, offering substantial application potential.