Oxidation-reduction potential (ORP) measurements were made in conjunction with monitoring of other process variables in biological nutrient removal systems to determine the suitability of using this parameter to monitor and characterize conditions in un-aerated bio-reactors. Results obtained over the course of a three year investigation indicate that ORP monitoring can provide a very meaningful indication of process and bio-reactor performance. Easy to make, ORP determinations are reliable and can be used to distinguish between different levels of respiratory activity and also to identify the onset on non-respiratory conditions. This has practical advantages in that it provides a rational basis for real time control and optimization of process nitrogen removal. In more highly anaerobic environments ORP was found to correlate with observed phosphorus release, thus providing a real time indication of this aspect of process performance. This investigation has concluded that ORP is a unique and very useful practical tool for process monitoring, control and optimization which holds great promise as a process control parameter in these and future applications, in particular as a sensor for continuous process monitoring and automated process control.

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