[SERKIT]

But look at this photo from the air. You can make out letters written on it (I can make out an "N"). And what would be a walkway out to the drive platform on a sewage treatment tank appears to be a Masonic compass with the vertex at the very center of the circle.

Previous posts show that the little rectangle (white with ragged black outlines) are benches. This site looks ceremonial, otherwise, why would there be benches around a smelly sewage treatment plant?

[RoosterRedux]

Right. And why would the sewage plant be place right next to the primary road between the main house and the temple.

Furthermore, I don't think the population of the island would require a large sewage systems. Septic systems would be sufficient.