An essential component in any surface water management system (SWMS) for non potable re-use is the treatment of the surface water. Hard surfaces exposed to traffic will carry a greater range of diffuse pollution than water collected from a roof. There are greater concentrations of sediment particles, hydrocarbons (oils) from vehicles and dissolved heavy metals, predominantly copper and zinc, from exhausts and brakes that runoff from these areas. This runoff needs to be treated before it can be re-used.
BS8515 does not preclude the use of surface water for re-use but it does make reference that this run off will require a higher level of treatment than roof water before it can be re-used. The potential use of surface water can significantly improve water yield and therefore efficiency of any re-use system.