Fuzzy　logic　can　in　several　ways　be　applied　to　improve　the　control　of　the　activated　sludge　system.　In　this　paper,　fuzzy　logic　based　control　strategies　for　external　carbon　flow　in　the　anoxic　zone　and　dissolved　oxygen　(DO)　concentration　in　the　aerobic　zone　are　presented　and　evaluated　by　the　simulation　of　the　predenitrification　layout　of　the　COST　624　benchmark　implemented　by　Matlab/Simulink.　The　aeration　fuzzy　controller　was　designed　as　high-level　controller　that　determines　the　DO　set　points　in　the　aerated　basin;　conventional　PI　controller　was　used　to　maintain　the　DO　concentration　at　the　set　point　levels.　The　ammonia　concentration　in　the　influent　and　effluent　were　used　as　input　variables　for　the　fuzzy　controller.　Compared　to　the　operation　with　constant　DO　concentrations,　the　required　airflow　could　be　reduced　up　to　7%,　the　effluent　ammonia　concentration　could　be　reduced　up　to　18%　by　using　fuzzy　logic　based　control　strategy.　At　the　same　time,　ammonia　peaks　in　the　effluent　could　be　reduced　significantly.　The　external　carbon　flow　fuzzy　controller　which　includes　two　inputs　variable,　the　error　(E,　nitrate　in　the　second　basin　minus　the　preset-point)　and　change　in　error　(E-C),　and　one　output　variable,　which　adjusts　the　external　carbon　flow,　compared　to　the　normal　PI　operation,　the　external　carbon　flow　could　be　reduced　up　to　24.2%.　Experiments　show　that　the　fuzzy　controllers　can　be　easily　implemented　in　modern　control　and　supervision　systems,　to　improve　the　effluent　quality　and　reduce　operating　cost.　(C)　2005　Elsevier　Ltd.　All　rights　reserved.