Yes to your question - What's the minimum limit of people in a community to get a 10% return? If we take the Sneek example, some 32 families (100-120 people?) now forms the lower limit.

But also: Is there a point (scale) that the return gets larger than 10 percent? If we have 500 people, do we get a larger return, say 15%, or will it always be 10%? I've learned in Sweden that a follow-up for some 500 houses in Sneek with the same sewage system is practically go, so we might actually get an answer on that question - in a few years.