Energy　savings　via　achieving　the　reduction　of　aeration　and　excess　sludge　is　required　to　realize　energy　self-sufficiency　in　wastewater　treatment　plants.　A　novel　partial　nitritation　+　simultaneous　anammox　denitrification　and　sludge　fermentation　(PN　+　SADF)　process　was　operated　for　nearly　two　years,　during　which　simultaneous　energy-efficient　nitrogen　removal　and　waste　activated　sludge　(WAS)　reduction　was　achieved,　with　a　stable　nitrogen　removal　efficiency　of　80%　and　external　WAS　reduction　of　40%-50%.　In　the　PN　reactor,　presence　of　ammonia　oxidizing　bacteria　and　absence　of　nitrite　oxidizing　bacteria　ensured　the　stable　nitritation.　In　the　SADF　reactor,　nitrogen　was　removed　via　denitrification　and　anammox　by　using　nutrients　and　organics　released　from　WAS　solubilization.　Comparable　performance　of　the　SADF　reactor　at　ambient　temperature　(12-32　degrees　C)　to　that　at　30　degrees　C　indicated　a　practical　application　potential　for　the　PN　+　SADF　process.　An　initial　estimation　of　a　full-scale　PN　+　SADF　process　serving　a　population　of　100000　showed　that　it　could　save　economy　and　energy　in　comparison　with　conventional　nitrification-denitrification　process.　Despite　some　challenges　in　implementation,　this　paper　highlights　the　potential　implication　for　sustaining　mainstream　nitritation-anammox　towards　energy-efficient　operation　with　excess　sludge　reutilization.　(C)　2018　Elsevier　Ltd.　All　rights　reserved.