Zinc　oxide　nanoparticles　(ZnO　NPs)　are　widespread　emerging　pollutants　raising　global　concerns　about　their　influences　on　biological　wastewater　treatment　processes.　However,　the　impacts　of　ZnO　NPs　on　aerobic　sludge　digestion　that　is　a　major　sludge　treatment　process　remain　unknown.　Herein,　this　study　comprehensively　investigated　the　key　influences　of　ZnO　NPs　on　aerobic　digestion　of　waste　activated　sludge　(WAS)　and　the　potential　mechanisms　involved.　Two　different　entering　pathways,　i.e.,　ZnO　NPs　directly　entered　into　aerobic　sludge　digester　and　ZnO　NPs　initially　entered　into　wastewater　bio-treatment　reactor,　were　tested　to　evaluate　the　different　impacts.　Compared　to　the　control,　ZnO　NPs　initially　entering　into　wastewater　bioreactor　inhibited　WAS　degradation　by　18.2　±　0.1%,　whereas　ZnO　NPs　immediately　entered　into　digester　inhibited　it　by　29.7　±　0.1%.　This　was　accompanied　by　a　similar　decrease　in　inorganic　nitrogen　production　and　oxygen　consumption.　ZnO　NPs　exposure　in　wastewater　bioreactor　changed　WAS　characteristics　in　favor　of　solubilization　in　aerobic　digestion.　Modelling　analysis　indicated　that　ZnO　NPs　inhibited　WAS　hydrolysis,　especially　for　their　direct　entering　into　aerobic　digester.　Correspondingly,　microbial　community　was　shifted　in　the　direction　against　aerobic　digestion　by　the　ZnO　NPs.　Excessive　oxidative　stress　and　Zn2+　release　represented　the　primary　toxicity　factors　for　the　inhibition.　©　2021　Elsevier　B.V.