Drainage system/apparatus for treating runoff

A drainage system comprises a gutter 12 connected to a drainage pipe 14 via a pipe inlet 22. The pipe is provided with a vent 36 such that when liquid in the gutter 12 blocks the pipe inlet 22 liquid flowing through the pipe 14 draws air through the vent, helping to prevent glugging and reducing the effects of vibration due to water hammer. A duct 38 having an inlet 40 above may extend from the vent, drawing air from above the pipe inlet. The duct inlet has an aperture 41 that is in a plane inclined to the horizontal such that the flow of air through the duct is progressively slowed as the level of water within the gutter rises. The collector may include an overflow 44. The drainage pipe may include a tapered portion 28 and may have an energy dissipater. Also claimed is an apparatus for treating run-off and including a treatment vessel.