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　acid-kaolin　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　governed　by　the　MW　of　the　CTS　used　and　the　slow-mixing　speed　applied　as　well.　(c)　2019　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.