Impacts of packed bed configuration on nitrate removal efficiency and microbial spatial distribution in solid-phase denitrification of partial nitritation/anammox granular reactor effluent

Abstract

Nitrate residues pose challenges to the application of the partial nitritation/anammox (PN/A) process in treating ammonium-rich wastewater. This study aimed to develop multilayer packed beds comprising poly-3-hydroxybutyrate-cohyroxyvelate (PHBV) particles and volcanic rocks to facilitate efficient nitrate reduction from the effluent of a PN/A granular reactor via solid-phase denitrification (SPD). The results indicated that when the influent concentration of ammonium-nitrogen was 300 mg/L, the SPD packed beds with a 60 % PHBV packing ratio completely eliminated the nitrate generated in the PN/A reactor without excessive organic release. Consequently, the combined processes achieved a stable nitrogen removal efficiency of 97.2 ± 2.3 %. The SPD packed bed configuration significantly affected both the nitrate removal behavior and biomass spatial distribution. High-throughput sequencing and functional gene predictions related to carbon and nitrogen metabolism revealed that the PHBV biofilm effectively drove the hydrolysis of solid carbon sources for denitrification, with synergistic effects between hydrolytic bacteria (Clostridium_sensu_stricto_7, Sporomusaceae, and Sporomusaceae) and denitrifiers (Comamonadaceae, Rhodocyclaceae and Thauera). Moreover, the biofilm attached to the volcanic rocks was crucial in reducing nitrate and organic carbon within the SPD packed beds, which were enriched in denitrifying genes. These findings facilitate combining PN/A and SPD processes to treat ammonium-rich wastewater.