In　this　paper,　the　online　optimal　control　is　discussed　to　control　the　dissolved　oxygen　concentration　(DO)　in　wastewater　treatment　process　(WWTP).　The　echo　state　network　(ESN)　and　the　online　adaptive　dynamic　programming　(ADP)　(ESN-ADP　for　short)　are　combine,　which　is　adapted　to　the　nonlinear　and　dynamic　conditions　of　WWTP.　Firstly,　the　ESN　is　proposed　to　replace　the　conventional　BP　network,　which　is　not　proper　for　the　nonlinear,　dynamic　property　of　WWTP.　Then,　the　online　training　algorithm　is　designed　for　ESN　to　ensure　the　adaptive　adjustment　ability　of　ADP　controller.　The　simulation　results　illustrate　that　the　ESN-ADP　controller　has　obtained　better　control　performance　other　control　schemes.　©　2022,　The　Author(s),　under　exclusive　license　to　Springer　Nature　Singapore　Pte　Ltd.