fig2 shows a separator which comprises a gully chamber 2 having a cylindrical surrounding wall 4 and top and bottom end walls not shown for clarity an inlet duct 6 and an outlet duct 8 open into the surrounding wall 4 an inlet may also be provided in the top wall in a region which is close to the surrounding wall 4 and at a circumferential position which is near to the inlet duct 6 such an inlet may be positioned in a roadway passing over the separator and covered by a grating a separator module 10 according to an embodiment of the invention is installed in the gully chamber 2 as shown in fig3 and 4 the separator module 10 comprises a partition arrangement having a partition plate 12 and a separate ledger component 14 the partition plate 12 divides the chamber 2 into an upper region above the partition plate 12 and a lower region below the partition plate 12 the partition plate 12 comprises an inlet chute 16 and an outlet chute 18 the inlet chute 16 comprises an opening 20 which provides an inlet to the lower region from the upper region similarly the outlet chute 18 comprises an opening not shown which provides an outlet from the lower region to the upper region the inlet chute 16 has an arcuate shape which is contoured to conform to the curvature of the inner surface of the surrounding wall 4 the inlet opening 20 is.