Biological　activated　carbon　(BAC)　prefiltration　is　a　common　method　to　control　fouling　in　ultrafiltration　(UF)　processes.　BAC　filter　depth　is　a　critical　parameter　for　both　design　and　operation　of　a　prefiltration　system.　Here,　the　effect　of　filter　depth　on　the　performance　and　mechanism　of　contaminant　removal　and　UF　fouling　characteristics　was　investigated.　The　results　indicated　that　contaminants　were　mostly　removed　by　the　upper　filter　(0-10　cm),　including　organics,　particles,　and　protein-like　substances,　while　the　deeper　filter　(10-50　cm)　removed　additional　organics,　which　were　mostly　humic-like　substances.　The　upper　layer　intercepted　particles,　which　not　only　reduced　the　number　of　particles,　but　also　significantly　reduced　the　average　particle　size.　The　fouling　control　capability　of　BAC　prefiltration　was　mostly　attributed　to　the　upper　filter,　whose　function　was　greater　than　that　of　the　deeper　filter.　However,　the　primary　fouling　mechanism　did　not　vary　with　filter　depth.　In　summary,　during　the　combined　process　of　BAC　prefiltration　and　UF,　when　membrane　fouling　control　is　the　main　objective,　the　filter　layer　could　be　set　as　a　thin　layer　(such　as　10　cm).　However,　when　the　combined　process　needs　to　reduce　the　concentration　of　organic　matter　at　the　same　time,　a　thicker　layer　is　indispensable　to　achieve　the　desired　performance.