Insights into the influence of organic and salinity on the two-stage partial nitritation/anammox process in treating food waste digestate

<b>ABSTRACT</b>Food waste digestate (FWD), which contains significant levels of ammonium, organic matter, and salinity, can interfere with treatment performance of the anammox process. In this study, a two-stage partial nitritation/anammox (PN/A) process was established to investigate nitrogen removal and microbial response in treating FWD at a nitrogen loading rate (NLR) of 0.27 ± 0.02 gN/L/d. High concentrations of free ammonia (58 mg/L) and free nitrous acid (0.3 mg/L) facilitated the initiation of the partial nitritation (PN) process, achieving an average NO₂^-/NH₄⁺ ratio of 1.28 ± 0.08. For the anammox process, a nitrogen removal rate (NRR) of 0.72 ± 0.13 gN/L/d was achieved. Free ammonia (NH₃) stripping, Anammox pathway, and denitrification pathway contributed 4.1 ± 0.3%, 5.1 ± 0.2%, and 84.0 ± 1.5% of the total nitrogen removal, respectively. <i>Nitrosomonas</i>, a salt-tolerant ammonia-oxidizing bacteria (AOB), was enriched to 1.0%, while <i>Nitrospira</i>, a nitrite-oxidizing bacteria (NOB), was effectively suppressed to 0.003%. The salt-tolerant anammox genera <i>unclassified_f__Brocadiaceae</i> (13.9%) and <i>Candidatus_Kuenenia</i> (4.8%) dominated the nitrogen removal pathway. The high enrichment of <i>unclassified_f__Brocadiaceae</i> ensured stable operation of the anammox process at 0.62 ± 0.11% salinity, even with a high initial FA inhibition concentration of 40 mg/L. Additionally, <i>norank_f_A4b</i> (1.34%) and <i>norank_f_norank_o_SBR1031</i> (52.1%) facilitated the hydrolysis of refractory organic matter. Denitrifying bacteria, including <i>Hyphomicrobium</i>, <i>Truepera</i>, and <i>unclassified_c__Alphaproteobacteria,</i> played significant roles in nitrate removal, with a COD_consumed/NO₃^-_removed ratio of 2.7 ± 0.2. This study highlights the application of a two-stage PN/A process for rapid startup and effective nitrogen removal from FWD.