Low-concentration organics mitigate the inhibition of free nitrous acid on nitrification in biofilters for gaseous ammonia removal

The accumulation of free nitrous acid (FNA), a by-product of nitritation, restricts biofilter ammonia removal under medium-to-high inlet ammonia loading by inhibiting nitrifiers. Preliminarily testing under varying C/N ratios validated that the addition of low-concentration organics carbon (OC, C/N = 1.0-1.5) effectively alleviated FNA accumulation, thereby improving biofilter performance. Subsequently, three parallel ammonia removal biofilters with different inocula were set to investigate the underlying mechanisms. Upon elevating the inlet ammonia loading rate to 22.8 +/- 1.2 gN m-3h- 1, the biofilters removed only 60.3 +/- 10.0 % of gaseous ammonia under inorganic carbon (IC) supplementation. However, the introduction of low-concentration OC (C/N = 1.0) facilitated nitrogen loss, enriched denitrifying bacteria (DNB), and bolstered the denitrification functional abundance. The successful coupling of denitrification significantly reduced FNA concentrations by 2 to 3 orders of magnitude. Therefore, the revival of nitrifiers contributed to the enhanced ammonia removal efficiency (94.2 +/- 8.4 %). Nitrite-oxidizing bacteria (NOB) and DNB subcommunities were identified as the driving factors for the mitigation of FNA and improvement of biofilter performance by the Mantel test. Activated heterotrophs facilitated ammonia conversion as well when OC was supplied, supported by Pearson correlation analysis. This study elucidated the synergistic dynamics between nitrifiers and DNB, emphasizing the favorable cooperation arising from the low-concentration OC addition (C/N = 1.0-1.5). Overall, this study highlights the cooperative interaction between heterotrophic denitrifiers and autotrophic nitrifiers under low-concentration OC supply, which played a vital role in alleviating FNA inhibition and enhancing the biofilter ammonia removal efficiency.