The presence of seasonal algal blooms with harmful algae-derived organic mattersthreatens the performance of ultrafiltration in drinking water treatment.The hybrid process of in-situozonation with ceramic ultrafiltrationmembrane was proposed for the treatment of algal-rich water with effective removal of algal cells and mitigation of membrane fouling.The mechanism of membrane fouling mitigation with in-situozonation was systematically investigated from the perspective of cake layer,gel layer and membrane pore blocking.The results showed that in-situozonation treatment can effectively mitigate membrane fouling.For instance,transmembrane pressure was reduced by 75.8% with in-situozonation at 1 mg/L.In-situozonationresulted in the formation ofa more porous and thinner cake layerdue to the disintegration of extracellular organic mattersthat attached on the algal surface.Besides,the portion of very high molecular weight biopolymers (around 100 kDa) and the hydrophobic fraction of algae-derived organic matters,the main component intercepted by the ceramic membrane,were reduced with in-situozonation and thus leaded to less severe gel layer fouling.Furthermore,in-situozonation induced the accelerated organics degradation within the ceramic membrane pores,alleviating the membrane pore blocking.