Three drains in one trench - the potable stormwater drain is on top (in 100mm PVC fro strength, even though 90 mm is permitted). Underneath is the sewer and also the surface/sub-surface drainage. Note :- the grey water from the house (excluding kitchen sullage) is kept separate from other drains in the house until it joins the sewer main line - this allows me to easily re-direct gray-water for recycling at a later date.