What is this machine called that spins its propeller underwater in a treatment plant or refinery?

Question

We have these weird machines that have an accessible platform on the top and a propeller at the bottom that stirs the water in a water treatment plant or refinery.

What does it do?

What is it called? 

Update:

Here's a birds eye view of the machines. 

https://www.google.com/maps/place/Seaholme+Station...

Update 2:

Here are the coordinates.

-37.85499737456777, 144.85125564002527

Answer 1

They are agitators to mix the flocking agent into the mix.

Answer 2

In a treatment plant, they are simply called mixers.  Not sure as to the purpose behind these, I don't see any chemical feed lines going in, so the next logical purpose is to get air into the water.  This could be for oxygen for biological treatment or to help cool the water. The deliberately long path in the pond indicates either settling time or reaction time is needed, maybe both.

Answer 3

Your link is to a train station.  What are you referring to?