not knowing where the OPs home is situated; most moobahns have deep drains and formed shallow gutters to take off any runoff from rainwater, if that's the case you can put your own drain box inside your wall, run all your grey water pipes into that, then one outlet into the street drain. If he is more remote then a second 3 ring drainage well would be ample; however i would not be mixing a septic tank overflow with grey water. All the Thai builders i know and have dealt with do this. I can give you my builders' contact details if you or OP wishes.