Conditioning for excess sludge and ozonized sludge by ferric salt and polyacrylamide: Orthogonal optimization, rheological characteristics and floc properties

•Effects of PAM types on conditioning of excess and ozonized sludge (OS) were studied.•Ozonization deteriorated dewaterability and flowability of excess sludge.•OS was effectively conditioned by linear PAM with high MW after Fe3+ coagulation.•Fe3+ coagulation enhanced structural strength and thixotropy of conditioned OS.•Rheological analysis bridged physicochemical property of sludge with dewaterability.