Abstract A hydraulic laboratory pilot project was conducted in conjunction with mathematical modeling to refine and demonstrate the swirl flow regulator/solids-liquid separator. The device, of simple annular shape, requires no moving parts. It provides flow regulation with a central circular weir, and simultaneously treats combined storm and sanitary wastewaters with a swirl action, thereby achieving liquid-solids separation. The low-flow concentrate is diverted by a bottom orifice to the sanitary sewerage system for subsequent treatment at the municipal works. The relatively clear liquid either overflows the weir into a central downshaft and receives further treatment, or is discharged to the stream. The device is capable of functioning efficiently over a wide range of combined sewer overflow rates, and can effectively separate suspended matter at a fraction of the detention time required for conventional sedimentation or flotation. For these reasons, serious thought is being given to the use of swirl units as wet weather treatment plant systems.