Why not just blank off the overflow from the septic tank and convert it to a cess pit (sealed tank that needs regular emptying) , as I understand it perfectly compliant?
The septic tank would be far too small to use as a cess pit and it would probably require emptying every week. Don't forget it's not only sewage that is disposed, also all the waste water from domestic use etc.
To retrofit a cess pit would be an extremely costly exercise and the size required would be prohibitive to install. The required capacity would be in the region of 60 cubic metres and with the house at full occupancy would provide only 45 days of storage. These figures are taken from official guidelines.
I'm going down the route of a sewage treatment plant, it's just proving more difficult than I expected to find impartial information on reliability and running costs.
