First, the use of Glyphosate ...

First, the use of Glyphosate or 2,4,D which are both Carcinogens, should be stopped.  Anyone using that water down stream will be subjected to Cancer.  From my experience, the best approach would be to limit the discharge of Nitrogen from the upstream WWTP's.  What is going on is that you are feeding the Hyacinths just what they need to grow.  If you drastically reduce the Nitrogen and other fertilizer run off, this will also help eliminate this rapid growth.  You have very high BOD and High Nitrogen.  This can be reduced and or eliminated by a high production of Aerobic microbes which will take down the Nitrogen and the BOD.  There is a company who can put in a series of BioCleaner units which would do just what I am suggesting.