Sensor for water quality. Reuse water

Nutrient Sensor Challenge aims to coax market for next generation instruments

which is an EPA initiate to develop real-time sensors under $8K!!
http://www.epa.gov/innovation/examples-epa-prize-competitions
On the other end of the spectrum for low key community orientated “learning” projects
https://publiclab.org/wiki/open-water
My 2cents for reclaimed water is to look first at your local jurisdiction legal requirements you need to meet, then break it into components. Practically speaking its dependent on the source of the reclaimed water, and what your base load of pollutants might be
1) for volume of irrigation water it may be relatively easy to sense
2) for other nutrients you may start with determining best practices, manual sampling, and on to continuous monitoring.
I don’t think there is a one solution fits all easy answer.