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

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 1200 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. © 2026 The Author(s). Published by Elsevier Ltd.