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

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 NO/NH 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 (NH) 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. , a salt-tolerant ammonia-oxidizing bacteria (AOB), was enriched to 1.0%, while , a nitrite-oxidizing bacteria (NOB), was effectively suppressed to 0.003%. The salt-tolerant anammox genera (13.9%) and (4.8%) dominated the nitrogen removal pathway. The high enrichment of 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, (1.34%) and (52.1%) facilitated the hydrolysis of refractory organic matter. Denitrifying bacteria, including , , and played significant roles in nitrate removal, with a COD/NO 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.