Foul versus surface water, gullies, soakaways, drain sizing and falls, septic tanks, and where a drain gives you access to clear a blockage.
Foul water and surface water
Foul water is the used, contaminated water from sanitary appliances - WCs, basins, baths, sinks. Surface water is clean rainwater run-off from roofs and paving. A separate system keeps the two apart in different pipes: surface water goes, in order of preference, to a soakaway or other infiltration system, then a watercourse, and only then a sewer.
Gullies and soakaways
A gully is a trapped fitting at ground level, occasionally basement level, that takes water into the drain below. A branch pipe carrying only wastewater can discharge to a gully, ending between its grating and the top of the water seal; a branch carrying WC discharge cannot. Like any trap, a gully's seal can be lost - a gully that still drains but smells has usually lost its seal. A soakaway is a different fitting again: it lets surface water disperse into the surrounding ground on site, rather than carrying it away as a surface-water drain does.
Drain or sewer?
A drain serves one building, or buildings and yards within the same curtilage. A pipe serving buildings in more than one curtilage is a sewer.
Sizing and falling the underground pipe
A drain carrying only wastewater needs an internal diameter of at least 75 mm; one carrying WC or trade effluent needs at least 100 mm. The minimum fall depends on the flow: 1 in 40 for a 75 or 100 mm drain at a peak flow under 1 litre a second, 1 in 80 for a 100 mm drain with at least one WC connected (or any peak flow over 1 litre a second), and 1 in 150 for a 150 mm drain serving at least five WCs.
Septic tanks and cesspools
Where there is no sewer connection, foul water can go to a septic tank (with a drainage field, mound or constructed wetland to treat the settled effluent) or a cesspool (a sealed tank with no outlet and no treatment: it is ventilated, but its contents are simply held until a tanker empties them). Both should sit at least 7 m from any habitable part of a building, preferably downslope, and within 30 m of vehicle access for emptying. A septic tank's drainage field needs its own clearance: at least 15 m from a building, 10 m from a watercourse and 50 m from a groundwater supply.
Access along the drain
Inspection chambers and rodding eyes are sited at changes of direction, at junctions and along long straight runs, so a blockage can be found and cleared without digging up the run. A full chamber points to a blockage downstream of it; an empty one means the blockage is further upstream.