What you are joining
A plant serving several properties sits somewhere — on one owner's land, on common land, or on a plot held by a management company — and the rights to use it, to maintain it and to get to it come from the deeds rather than from being nearby.
The first questions are therefore title questions. Does your title grant the right to discharge into the plant? Does it grant access to it for maintenance? And what obligation does it impose to contribute?
The annual charge
Shared plants are managed, and management costs money: emptying, servicing, power if the plant is powered, repairs, and whatever administration the arrangement involves. The charge recurs and it can rise sharply when the plant needs significant work.
Ask for the last few years' accounts or invoices, not just the current figure. A plant approaching the end of its life is a capital cost waiting to be shared out, and the sums involved for a replacement are large.
Ask whether there is any reserve, and whether the obligation to contribute is enforceable against every property — the positive covenant problem that affects shared private roads applies here in exactly the same way.
The questions about the people
Who actually does the managing, and are they a company, a committee or a neighbour who has always dealt with it? A plant run by one willing individual is working well right up until they move.
What happens if somebody does not pay. What happens if the plant fails. And whether there have been disputes, which the seller must answer honestly if asked.
Also establish compliance, exactly as for an individual system: a shared plant is still a discharge, and it is still regulated.