When I was a kid we went on a field trip to the sewage plant. They had massive tanks (way bigger than swimming pools) full of sewage, and they pumped in large amounts of oxygen in to aerate it all. The worker told us that if anyone fell in there was very little chance of saving them because there was way too much air in the water to float, and you would sink right to the bottom and of course the sewage made it impossible for anyone to see through to find you quickly.