To alleviate ultrafiltration (UF) membrane fouling,the pre-coagulation of poly-aluminum chloride (PACl) with the aid of chitosan (CTS) was conducted for synthetic humic acidkaolin water treatment.Pre-coagulation of three molecular weights (MW) CTSs (50-190 kDa (CTSL),190-310 kDa (CTSM) and 310-375 kDa (CTSH)) was optimized with slow-mixing speeds of 30,60 and 90 r/min,respectively.The removal efficiency and floc properties as well as membrane fouling were analyzed,and were compared to results obtained by conventional coagulation with PACl.Results showed that variations in floc properties could be ascribed to the coagulation mechanisms of CTSL/CTSM/CTSH at different slow-mixing speeds,resulting in reduced UF membrane fouling.Specifically,at the low speed of 30 r/min,all three CTS types produced flocs with similar properties,while CTSL resulted in the lowest removal efficiency and aggravated irreversible fouling.At the appropriate speed of 60 r/min,CTSM generated the most compact flocs with the combined effects of bridging and path mechanisms.The compact cake layer formed could alleviate irreversible fouling,which was beneficial for prolonging the operation of the UF membrane.At the high speed of 90 r/min,CTSH formed fragile flocs and aggravated irreversible membrane fouling.We considered membrane fouling to be affected by floc properties and the resultant removal efficiency,which was goved by the MW of the CTS used and the slow-mixing speed applied as well.