Normally when we apply for a permission for a client where these issues are raised, we advise the client to retain a competent civil and structural engineer who deals directly with the council on his behalf,
If you're trying to deal with thsi as a self-builder you need an experienced firm and biocycle are one I would recommend. They aer one of the dearest, but they offer a good service. Skimping on somethign as essential as a wastewater treatment system will come back to haunt you.
Alternatively, if you have the site for it, you might consider a reed bed system, but these are quite expensive to set up, install and keep working correctly I understand.
No doubt someone else on AAM will correct me on this if I am in error.
ONQ.