Ken, there may be some localized gravity flow in the OP sewage treatment system, but darn little. Nor do I believe all the new sections use gravity flow.

If this is of some interest, for whatever reason, contact Worcester County Public Works for facts.

The OP system was vacuum by design, as far as I recall. I suspect it is due to the large area covered by the OP subdivision originally and the distance from all areas to the plant.

For the plant to be gravity flow, think of the depth the pipes would be at the plant to provide what degree of slope would be required for gravity flow. I expect rather DEEP and very expensive The vacuum system allows all the pipes to be relatively near the surface.

The only factual source for any info on plant design is Worcester County.