History, row of industrial units, established 30 years plus, all using gravity sewer. Vented at end run off last manhole.
Other units connected via stub stack with air admittance valve. No problems until:-
Permission given for pumped foul to discharge into last manhole,
Pressure from pump together with rate of discharge meant vent pipe was no longer effective for intermediate units.
Owner now facing cost and disruption of extending stub stacks to become vent pipes.
With external drainage adopted by Anglian Water, chance of recovering cost, remote.