Dynamic　modelling　and　simulation　is　increasingly　being　employed　as　an　aid　in　the　design　and　operation　of　wastewater　treatment　plants　(WWTPs).　In　this　paper　a　simulation　model　is　used　to　investigate　the　control　of　an　activated　sludge　process.　Two　different　control　strategies　to　optimise　nitrogen　removal　in　a　predenitrification　process　are　presented,　which　are　based　on　the　control　of　the　sludge　blanket　height　(SBH)　or　sludge　age　(sludge　retention　time,　SRT)　and　mixed　liquor　suspended　solids　(MLSS)　in　the　reactor,　by　adjusting　the　sludge　wastage　and　sludge　recycle　flow　rates,　respectively.　It　is　shown　how　the　recycle　and　wastage　flow　rate　variables　can　be　employed　to　continually　maintain　plant　operation　in　the　presence　of　disturbances.　The　performance　of　the　control　strategies　has　been　assessed　by　the　COST　624　benchmark　plant　and　a　pilot　plant.　The　results　have　shown　the　robustness　of　the　implementation　and　the　efficiency　of　the　proposed　control　strategies,　which　have　been　operated　automatically　in　a　safe,　stable　and　optimum　operating　point,　improving　effluent　quality　and　reducing　energy　costs.　Consideration　is　also　given　as　to　how　plant　operation　could　be　optimised　in　the　long-term　in　order　to　meet　the　yearly　total　nitrogen　standard.　(c)　2005　Society　of　Chemical　Industry.