Although　the　treatment　of　natural　surface　waters　by　coagulation　has　been　investigated　extensively,　the　detailed　interaction　between　natural　organic　matter　and　alum　is　still　not　adequately　understood　or　predictable,　owing　to　the　complexity　of　different　components　of　the　organic　matrix　and　the　conditions　during　coagulation.　In　this　paper,　we　present　the　results　of　a　novel　approach　to　the　study　of　the　topic,　which　involved　the　progressive　separation　of　organic　components　according　to　size,　followed　by　coagulation　of　the　filtrate　solution,　in　order　to　expose　the　influence　of　particular　organic　fractions.　Using　two　natural　water　sources,　representative　of　lake　and　river　waters,　solutions　of　different　organic　content　were　obtained　by　progressively　filtering　the　source　waters　using　membranes　of　decreasing　pore　size;　viz.　microfiltration　(MF),　ultrafiltration　(UF),　and　two　grades　of　nanofiltration　(NF).　While　MF　had　little　impact　on　the　range　of　organics　present,　UF　was　able　to　separate　biopolymers　(MW　＞　100　kDa),　and　NF　had　a　substantially　impact　on　the　separation　of　medium-high　MW　(1-10　kDa)　substances.　The　results　of　the　coagulation　tests　showed　that　the　size　of　flocs　was　substantially　greater　when　biopolymers　were　present,　suggesting　their　beneficial　role　in　bridging　precipitated　Al(OH)(3)　nanoparticles.　For　the　smaller　organic　fractions　(＜10　kDa),　the　results　showed　a　trend　of　increasing　floc　size　with　decreasing　organic　MW　and　concentration,　but　the　trend　was　minor　and　may　be　explained　by　charge　effects.　Very　similar　results　were　found　with　both　water　sources,　which　support　the　main　finding　that　biopolymers　have　an　important　influence　on　floc　formation.　(C)　2017　The　Authors.　Published　by　Elsevier　Ltd.