Highlights•FNA inactivates NOB to a much larger extent than it does on AOB.•Sludge
treatment using FNA is effective in establishing the nitrite pathway.•Sludge treatment using
FNA does not increase N 2 O emission or SVI.•FNA-based method could potentially improve
nitrogen removal and methane production simultaneously.

Nitrogen removal via nitrite (ie the nitrite pathway) is beneficial for carbon-limited biological
wastewater treatment plants. This study presents a novel strategy for achieving the nitrite
pathway, which involves recirculating a portion of the activated sludge through a side-stream
sludge treatment unit, where the sludge is subject to treatment with free nitrous acid (FNA ie
HNO2). The strategy is proposed based on a novel discovery reported in this work that in the
concentration range of 0.24-1.35 mg HNO2--N/L, FNA is substantially more biocidal to nitrite …