Hi @Leah23 ,
The return depends on how you are currently regulating the process.
How does your disk diffuser system stop? On the basis of redox potential or a time-out?
Normally, the O2 sensor is used to achieve more precise control over when to stop aeration (once dissolved O2 concentration is no longer increasing). The savings incurred during this time lapse are what ensure a return on investment.
On the other hand, the ORP sensor gives you more precise control over when aeration resumes (once you have confirmed the end of the denitrification phase).