you guys are missing the point of modern sewerage treatment. it isnt just flushed into the sea, or a river. its treated comprehensively with chemicals and filters n stuff. a septic tank works by it sitting there for ages and then it leaches into the ground. if you just pump it straight out of the tank, you might as well just connect a hose straight from your crapper to this 'sprinkler' thing...
Danny, I work with it almost every day.
Depends on the system. Sure, some use chemicals to treat, filter presses to dewater the sludge, etc, etc... Most of what is see is purely biological. Cheaper to run.
Most biological systems use little, if any chemicals. The bugs do all the work. Bacteria is your friend.
No filters. No chemicals. Just natures littlest helpers. As for retention time in the holding ponds, I'd say less than a week. Theres typically at least 2 ponds, one aerobic, one anaerobic. The aerobic one has aerators, or a fountain. Two different bugs, more effective treatment.
I watch ducks swimming in the ponds. I also watch hunters shoot the ducks when they take off. Yummy.