This　study　investigated　the　effect　of　sludge　concentration　(expressed　by　mixed　liquor　volatile　suspended　solids,　MLVSS)　on　free　nitrous　acid　(FNA)　pretreatment　strategy　to　achieve　nitritation.　Results　showed　when　FNA　was　0.308　mgHNO(2)-N/L,　nitrite　oxidizing　bacteria　(NOB)　activity　increased　by　70.2%　as　MLVSS　increased　from　8.4　to　16.8　g/L.　Nitrite　accumulation　ceased　as　MLVSS　increased　to　12.6　g/L,　indicating　that　FNA　inhibition　of　NOB　gradually　weakened　with　increasing　MLVSS.　When　FNA　was　higher　than　0.770　mgHNO(2)　-N/L,　NOB　activity　was　completely　inhibited　and　the　effect　of　MLVSS　on　FNA　inhibition　was　negligible,　with　nitrite　accumulation　potential　(NAP)　varying　from　88.1%　to　90.0%.　Mechanism　investigation　demonstrated　flocs　sizes　distinctly　declined,　with　more　extracellular　polymeric　substances　(EPS)　released　to　resist　FNA　inactivation.　Linear　fitting　showed　NAP　increased　with　FNA/MLVSS　increment.　Therefore,　MLVSS　affected　FNA　pretreatment　performance,　with　FNA/MLVSS　proposed　as　a　more　valuable　criterion　in　FNA　pretreatment　strategy　development,　than　the　solely　FNA.