One-year stable pilot-scale operation demonstrates high flexibility of mainstream anammox application

Abstract

Mainstream nitrogen removal via anammox is widely recognized as a promising wastewater treatment process. However, its application is challenging at large scale due to unstable suppression of nitrite-oxidizing bacteria (NOB). In this study, a pilot-scale mainstream anammox process was implemented in an Integrated Fixed-film Activated Sludge (IFAS) configuration. Stable operation with robust NOB suppression was maintained for over one year. This was achieved through integration of three key control strategies: i) low dissolved oxygen (DO = 0.4 ± 0.2 mg O₂/L), ii) regular free nitrous acid (FNA)-based sludge treatment, and iii) residual ammonium concentration control (NH₄ ⁺ with a setpoint of ∼8 mg N/L). Activity tests and FISH demonstrated that NOB barely survived in sludge flocs and were inhibited in biofilms. Despite receiving organic-deficient wastewater from a pilot-scale High-Rate Activated Sludge (HRAS) system as the feed, the system maintained a stable effluent total nitrogen concentration mostly below 10 mg N/L, which was attributed to the successful retention of anammox bacteria. This study successfully demonstrated large-scale long-term mainstream anammox application and generated new practical knowledge for NOB control and anammox retention.

Keywords: Autotrophic nitrogen removal from wastewater; Bioenergy recovery; Effluent quality; Mainstream anammox; NOB suppression.

Graphical abstract

Fig. 1

Profiles of influent and effluent TN concentrations, temperature (a) and ammonium, nitrite and nitrate nitrogen concentrations in the final effluent (b) throughout the one-year stable operation of the pilot A/O system. The dashed line represents a nitrogen concentration of 10 mg N/L.

Fig. 2

Maximum activities (a), abundance (b), and in-situ activities (c) of anammox bacteria in the A- and O-biofilms during the stable operation of the pilot system.

Fig. 3

Profiles of ammonium, nitrite and nitrate in anoxic batch tests using sludge flocs (a), A-biofilms (b) and combined biofilms and flocs (c). (d) The ratios of NO₃⁻ removed to NH₄⁺ removed calculated in the tests of (b) and (c).

Fig. 4

(a) Measured maximum AOB and NOB activities in sludge flocs using ex-situ batch tests under non-limited substrate conditions. (b) Representative FISH images showing significant dominance of AOB over NOB in sludge flocs. EUB mix counterstaining is in green, probes specific for Betaproteobacterial AOB (Nso1225) in red), Nitrobacter (Nit3) in blue) and Nitrospira (Ntspa662 and Ntspa712) in blue).

Fig. 5

Measured TN removal and nitrate production rates of the O-biofilms under five different residual ammonium concentrations using ex-situ batch tests. The DO concentration was fixed at 0.4 mg O₂/L during the tests.