The process of benchmarking in a drinking water utility plant is a continous process.
It can be done on overall performance indicators, or on performance scores such as production, cost estimates, energy efficiency, etc. etc. AWWA have some good information on the subject, just look on internet.
Optimize treatment based on real time testing for performance indicators.Turbidity , pH , Alkalinity , Nitrite , Total Ammonium and Chlorine degradation will give you a baseline performance indication of normalized seasonal reactions to treatment, temperature and chemical adjustments.
This will give a point of reference for benchmarking i.e. cost of production quantified seasonally