A sewer designed to carry foul sewage and surface runoff in the same pipe.  They transport both wastewater from homes and industry but also carry rain derived surface water from gutters, drains and some highways.  Combined systems are generally not permitted for new developments now due to problems with them becoming overloaded with rain during storms.  Many older drainage systems in the UK are, however, combined and can spill dilute sewerage into watercourses during high flows.  Often for new developments, it is a condition of planning that foul flows can only be taken to existing combined systems if the equivalent amount of surface water can be removed from the system.