A　ratio　control　strategy　was　implemented　in　a　continuous　moving　bed　biofilm　reactor　(MBBR)　to　investigate　the　response　to　different　temperatures.　The　control　strategy　was　designed　to　maintain　a　constant　ratio　between　dissolved　oxygen　(DO)　and　total　ammonia　nitrogen　(TAN)　concentrations.　The　results　revealed　that　a　stable　nitritation　in　a　biofilm　reactor　could　be　achieved　via　ratio　control,　which　compensated　the　negative　influence　of　low　temperatures　by　stronger　oxygen-limiting　conditions.　Even　with　a　temperature　as　low　as　6　degrees　C,　stable　nitritation　could　be　achieved　when　the　controlling　ratio　did　not　exceed　0.17.　Oxygen-limiting　conditions　in　the　biofilm　reactor　were　determined　by　the　DO/TAN　concentrations　ratio,　instead　of　the　mere　DO　concentration.　This　ratio　control　strategy　allowed　the　achievement　of　stable　nitritation　without　complete　wash-out　of　NOB　from　the　reactor.　Through　the　ratio　control　strategy　full　nitritation　of　sidestream　wastewater　was　allowed;　however,　for　mainstream　wastewater,　only　partial　nitritation　was　recommended.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.