We have a settlement tank instead of a septic tank, and we deal with the solids ourselves. The top layer (crust) is taken off and composted, or removed. The middle layer (effluent) goes to the reed beds. The bottom layer (sludge of 'onion gravy' consistency) goes to a separate bed for treatment.
CAT's reed beds are a vertical flow bed of 12 metres by 4 metres, and a horizontal bed of 12 metres by 5 metres, then a pond and a willow trench system.
The diameter of plastic land drain tubing in the vertical flow bed is about 100mm (but this isn't critical). The diameter of the pipe outlets of the vertical reed bed is 110mm, which is a standard sizing for waste pipe. The diameter of pipe for the horizontal bed outflow is 50mm - but this just opens up into the pond. None of the pipe sizes are that important as long as they are big enough for the flow of water, which is usually quite low.