Hi,
We join them together using ordinary solvent weld 1.5" drain pipe and fittings, bought from any plumbers' merchant. The secret is to cut the bend off the outlet elbow, and to then hammer a short piece of 1.5" pipe in from the end which fits onto the tank, until it protrudes a couple of inches from the end you've just cut. It's an interference fit, so doesn't need any solvent, PTFE tape or anything. Having done that, you simply carry on in 1.5" pipe, using solvent, tees, elbows, etc.
Hope this helps.
Andrew