The　biocidal　effects　of　free　nitrous　acid　(FNA)　have　found　applications　in　multiple　units　in　an　urban　wastewater　system,　including　sewer　networks,　wastewater　treatment　processes,　and　sludge　treatment　processes.　However,　these　applications　are　associated　with　chemical　costs　as　both　nitrite　and　acid　are　needed　to　produce　FNA　at　the　required　levels.　The　recent　discovery　of　novel　acid-tolerant　ammonia　oxidizers　offers　the　possibility　to　produce　FNA　from　domestic　wastewater,　enabling　the　development　of　next-generation　FNA-based　technologies　capable　of　achieving　self-sustaining　FNA　production.　In　this　study,　we　focus　on　the　concept　of　in　situ　FNA　generation　facilitated　by　acid-tolerant　ammonia　oxidizers　and　highlight　the　multiple　benefits　it　creates,　after　a　brief　review　of　the　historical　development　of　FNA-based　technologies.　We　will　discuss　how　wastewater　systems　can　be　made　more　energy-efficient　and　sustainable　by　leveraging　the　potential　of　acid-tolerant　ammonia　oxidizers.[Figure　not　available:　see　fulltext.].　©　2024,　The　Author(s).