A novel combined use of FeCl3 and NaNO2 markedly improves the efficiency of aerobic waste activated sludge treatment

• Ferric iron and nitrite pretreatment enhanced biodegradability of waste activated sludge • Proposed strategy significantly improved sludge reduction efficiency of aerobic digester • Aerobic digestion performance was markedly enhanced in terms of sludge stabilization • Released phosphate was captured by iron and subsequently recovered as vivianite The management of waste activated sludge (WAS) remains a major challenge for wastewater treatment plants (WWTPs) due to its large volume, high treatment costs, and limited biodegradability. Conventional aerobic digestion, although simple and widely applied in small-scale WWTPs, often provides insufficient sludge reduction and stabilization, limiting its effectiveness. In this study, we propose a novel multi-stage strategy that integrates ferric salt–based acidification with nitrite addition as a pretreatment step, followed by aerobic digestion and subsequent anoxic recovery of iron–phosphate compounds. Batch tests confirmed that ferric salt–nitrite pretreatment substantially enhanced sludge solubilization, while long-term reactor operation demonstrated significant improvements in digestion performance. Pretreatment with 1,200 mg Fe/L and 5 mg N/L increased total solids and volatile solids reduction by approximately 32.2% and 46.1%, respectively, and improved sludge quality by lowering specific oxygen uptake rate (SOUR) and pathogen content to meet Class A biosolids standards. The dosed iron also captured phosphate released during digestion, which was subsequently transformed into vivianite in an anoxic bioprocess and successfully separated from sludge through acidic leaching. Overall, the integrated process achieved simultaneous sludge minimization, quality enhancement, and resource recovery, highlighting the potential of ferric salt–assisted pretreatment as a sustainable WAS management strategy within a circular economy framework.