Surely the solution is to install small bore sewer systems in those affected communities, with a small packaged treatment plant or pumping main connection to an existing plant.
Small bore sewers collect the liquid effluent from the septic tank, and so do not require the stringent self-cleansing, peak flow accommodation and accessibility characteristics of a conventional sewer network.
The pipelines are small (100mm diameter), shallow, curvilinear and laid to variable inflective gradients with occasional cleanouts in place of manholes – all of which reduces the construction cost massively. The septic tanks provide settlement/primary treatment as well as peak
attenuation, so the biological and volumetric loads on any public treatment plant are significantly reduced.
Presumably the water authorities could offer a reduced tariff to the beneficiaries of such schemes, to reflect the limited treatment needed and the continued requirement on the occupier to evacuate the septic tank occasionally, but I suspect the economics would be drastically improved compared to the conventional sewer option.
JOHN LOWSBY (F), firstname.lastname@example.org