Different　time　scales　are　inevitable　for　the　controlled　variables　of　wastewater　treatment　process　(WWTP),　which　may　degrade　the　operation　performance　or　even　destroy　the　stability　of　the　closed-loop　system.　In　this　paper,　a　hierarchical　nonlinear　model　predictive　control　(HNMPC)　strategy　is　developed　to　deal　with　the　different　time　scales　of　controlled　variables　to　improve　the　operation　performance　of　WWTP.　The　main　merits　of　HNMPC　are　three-fold.　First,　a　hierarchical　control　structure　is　developed　to　obtain　reasonable　control　laws.　Then,　the　controlled　variables　with　different　time　scales　can　be　tracked　by　different　level　controllers.　Second,　a　gradient　method　is　developed　to　solve　the　hierarchical　optimization　problem　of　HNMPC　to　reduce　the　computational　cost.　Then,　the　fast　response　of　HNMPC　can　be　achieved　to　improve　the　operation　performance.　Third,　the　stability　of　HNMPC　is　proved　in　theory.　Then,　the　corresponding　stability　conditions　are　given　to　guide　the　practical　application.　Finally,　the　testing　results　on　the　benchmark　simulation　model　verify　that　the　proposed　HNMPC　can　achieve　suitable　operation　performance　in　terms　of　control　accuracy.　(C)　2021　Elsevier　Ltd.　All　rights　reserved.