Reactivation performance and sludge transformation after long-term storage of Partial denitrification/Anammox (PD/A) process

This study explores the startup of innovative Partial denitrification/Anammox (PD/A) process using long-term stored sludge (>2 years at 4 C). Results indicate a swift recovery performance, characterized by a progressive increase in the activity of functional microorganisms with improved nitrogen volumetric loading rate during operation. Stable nitrogen removal efficiency of 99.6 % was attained at 14.2 C under influent nitrate and ammonium of 120 and 100 mg/L, respectively. A distinctive transformation was observed as the initially black seeding sludge transitioned to brownish-red, accompanied by rapid sludge granulation with size notably increased from 263.1 mu m (day 4) to 1255.0 mu m (day 128), significantly contributing to the rapid PD/A performance recovery. Microbial community analysis revealed substantial increases in functional bacteria, Thauera (0.09 %-10.4 %) and Candidatus Brocadia (0.003 %-1.98 %), coinciding with enhanced nitrogen removal performance. Overall, this study underscores the viability of long-term stored PD/A sludge as a seed for rapid reactor startup, offering useful technical support to advance practical PD/A process implementation.