lol...although, that is another great explanation.
take my word on this, I manage a concrete batch plant in Alaska, we make an identical looking concrete septic tank, there must be an inlet and outlet one on each end, on average these tanks weigh around 8,000 lbs. give or take, those look like 1500 gallon tanks.
They would be massively heavy if solid, would require a 100 ton crane to lift them if they were.