In　this　paper,　six　different　strategies　for　integrated　control　of　nitrate　recirculation　flow　rate　and　external　carbon　addition　in　a　predenitrifying　biological　wastewater　treatment　system　are　proposed　and　evaluated　using　the　COST　simulation　benchmark.　The　proposed　control　strategies　consist　of　two　or　three　feedback　control　loops,　which　manipulate　the　nitrate　recirculation　and　the　carbon　dosage　flow　rates　in　an　integrated　manner,　such　that　the　consumption　of　external　carbon　is　minimized　while　the　nitrate　discharge　limits　are　met.　The　control　systems　require　measurements　of　nitrate　concentrations　at　the　end　of　both　the　anoxic　and　the　aerobic　zones.　A　number　of　simulations　are　performed　and　the　results　show　that　the　control　strategies　can　maintain　the　designated　effluent　quality　in　the　presence　of　external　disturbances,　while　obtaining　excellent　results　in　terms　of　consumption　of　external　carbon,　as　well　as　meeting　the　general　plant　performance　and　controller　criteria　as　defined　by　the　benchmark　group.　(c)　2005　Elsevier　Ltd.　All　rights　reserved.