If you have the space you could try a soakaway system. In favourable ground conditions they will work but in clay soils or similar they will eventually fail. Mine failed and I now have a catchment pit where a submersible solids pump kicks in automatically to pump to a herringbone system on the Forrest floor. I'm on my second pump now but the first one lasted around 20 years with no maintenance.
I think you have the right idea in going for the treatment plant, if you asked the supplier maybe he could put you in contact with previous customers to get their honest opinions on the system?
