Feasibility of packed-bed trickling filters for partial nitritation/anammox: Effects of carrier material, bottom ventilation openings, hydraulic loading rate and free ammonia

[Display omitted] •The feasibility of PN/A was tested in a passively aerated trickling filter.•Most of the total nitrogen removal took place in the top compartment.•The ventilation at the top combined with an optimum HLR is recommended.•The expanded clay carriers outcompeted the commercial plastic carriers.•The proposed process can be a suitable low-energy alternative for nitrogen removal. This study pioneers the feasibility of cost-effective partial nitritation/anammox (PN/A) in packed-bed trickling filters (TFs). Three parallel TFs tested different carrier materials, the presence or absence of bottom ventilation openings, hydraulic loading rates (HLR, 0.4–2.2 m3 m−2 h−1), and free ammonia (FA) levels on synthetic medium. The inexpensive Argex expanded clay was recommended due to the similar nitrogen removal rates as commercially used plastics. Top-only ventilation at an optimum HLR of 1.8 m3 m−2 h−1 could remove approximately 60% of the total nitrogen load (i.e., 300 mg N L-1 d−1, 30 °C) and achieve relatively low NO3–-N accumulation (13%). Likely FA levels of around 1.3–3.2 mg N L-1 suppressed nitratation. Most of the total nitrogen removal took place in the upper third of the reactor, where anammox activity was highest. Provided further optimizations, the results demonstrated TFs are suitable for low-energy shortcut nitrogen removal.