Side-stream　sludge　treatment　using　free　nitrous　acid　(FNA)　is　a　novel　strategy　to　achieve　nitritation　in　mainstream　of　wastewater　treatment　plants　(WWTPs).　To　optimize　nitritation,　the　effect　of　starvation　on　this　strategy　was　investigated　in　this　study.　The　results　showed　that　pre-starvation,　which　is　the　starvation　before　FNA　treatment,　enhanced　the　resistance　of　sludge　to　FNA.　This　led　to　a　decrease　in　the　nitrite　accumulation　rate　(NAR),　which　dropped　from　70%　to　27%　after　aerobic　pre-starvation.　This　was　further　confirmed　in　the　FNA　treatment　using　the　sludge　collected　from　the　secondary　settling　tank　(anoxic　pre-starvation)　and　the　aerobic　zone　(without　starvation)　of　an　Anaerobic-Anoxic-Oxic　system.　The　post-starvation,　which　was　the　starvation　after　FNA　treatment,　decreased　NAR　from　63%　to　14%.　To　obtain　a　higher　NAR,　the　sludge　used　for　FNA　treatment　should　be　collected　from　aerobic　zone,　and　be　returned　to　aerobic　zone　after　treatment　to　avoid　prestarvation　and　post-starvation.