The floating leaves of Azolla turn from green to red as this plant absorbs nitrogen from the water. This is a beautiful thing both for the eye and the environment. Waste water usually includes a relatively high amount of nitrogen which is a bad thing for any body of standing water because it provokes an effect commonly referred to as hypertrophication.
Have you ever seen a lake that was all green and slimy? That’s what we are talking about. Simply speaking, there is less and less oxygen dissolved in this water, which makes it a difficult living place for all life forms that require oxygen… and a paradise for those life forms that do not require oxygen: Anaerobic bacteria. They flourish, and by doing so they release various toxins that are not good at all for all other creatures.
And this is how our Power Plant named Azolla works, absorbing nitrogen from the water. By the way, this makes the rotting parts of this plant a great fertilizer.