Use of free nitrous acid from partial nitrification reactor for the sanitization of digester effluents and Class A biosolids production

Mesophilic anaerobic digestion alone typically fails to meet the pathogen reduction criteria required for Class A biosolids (<1000 MPN g⁻¹ TS of fecal coliforms), limiting their agricultural reuse. Since many wastewater treatment plants (WWTPs) have implemented partial nitrification/anammox (PN/A) processes in their sludge lines, this study proposes using free nitrous acid (FNA), generated in situ within a partial nitrification (PN) reactor, as a cost-effective biocidal agent to achieve Class A biosolids. A PN reactor was directly fed with the effluent from an anaerobic digester treating thermally hydrolyzed sludge and operated at hydraulic retention times (HRTs) between 1.5 and 4 days. The reactor achieved ammonium oxidation efficiencies of 46–49 %, generating FNA concentrations of 0.95–2.40 mg HNO₂ L⁻¹ that simultaneously promoted pathogen inactivation within the reactor, resulting in total coliform removals of 77–89 % and meeting Class A standards. Batch kinetic assays simulating mild acidification (pH 5–6) showed that > 99 % coliform removal could be achieved within 2 h, confirming the feasibility of a compact post-treatment step if needed. Economic evaluation indicated that the higher aeration energy demand was compensated by lower sludge management costs and potential revenue from Class A biosolids. Overall, the proposed configuration integrates nitrogen removal and sludge sanitization, providing a sustainable, energy-efficient, and economically viable strategy for biosolids reuse in WWTPs.