Extracellular polymeric substances(EPS)form a stable gel-like structure to combine with water molecules through steric hindrance,making the mechanical dewatering of wastew-ater sludge considerably difficult.Coagulation/flocculation has been widely applied in improving the sludge dewatering performance,while sludge properties(organic fraction and solution chemistry conditions)are highly changeable and have important effects on sludge flocculation process.In this work,the alkalinity effects on sludge conditioning with hydroxy-aluminum were comprehensively investigated,and the interaction mechanisms between EPS and hydroxy-aluminum with different speciation were unraveled.The results showed that the effectiveness of hydroxy-aluminum conditioning gradually deteriorated with increase in alkalinity.Meanwhile,the polymeric hydroxy-aluminum(Al13)and highly polymerized hydroxy-aluminum(Al30)were hydrolysed and converted into amorphous alu-minum hydroxide(Al(OH)3),which changed the flocculation mechanism from charge neu-tralization and complexing adsorption to hydrogen bond interaction.Additionally,both Al13 and Al30 showed higher binding capacity for proteins and polysaccharides in EPS than monomeric aluminum and Al(OH)3.Al13 and Al30 coagulation changed the secondary struc-ture of proteins in EPS,which caused a gelation reaction to increase molecular hydrophobic-ity of proteins and consequently sludge dewaterability.This study provided a guidance for optimizing the hydroxy-aluminum flocculation conditioning of sludge with high solution alkalinity.