The Accuracy and precision of ORP monitoring in bio-nutrient removal processes

It is important to have complete denitrification in the anoxic zone to provide good conditions for phosphorus removal in a bio-nutrient removal process. The objective of this research was to examine the possibility of using oxidation reduction potential (ORP) as a means of assessing the completeness of denitrification in the anoxic zone. First, the reliability and sensitivity of ten probes were tested and evaluated. Next, ORP values in complete denitrification conditions from biological systems with different initial NO concentrations and denitrification rates were collected and the usefulness ofthese redox values was assessed. Batch testing was used in this research. The Remote Data Acquisition and Control System, an automated data log-in system, was used to collect redox values. ORP probes were tested in quinhydrone buffer solutions. The biological systems in the anoxic testswere proportional combinations of mixed liquors from the anaerobic zone, aerobic zone, and the return sludge line of the UBC bio-nutrient removal process, so that different initial NO concentrations were obtained. Sodium acetate (NaAc) was added to the systems of carbon addition tests in order to achieve different denitrification rates. The probe test results indicated that measured ORP values were linearly related to standard ORP values of the tested quinhydrone solutions. Adjustment factors were developed based on this relationship for each probe. In the ORP vs time curves of biological batch tests, knees characteristic of the minimum first derivative were observed. At these knees, the NO removal efficiencies were found to be 92.6 % on average in anoxic tests, and to be 96.2 % in carbon addition tests. Redox values at the knees had an average of -28 my (adjusted value), with a standard deviation of +8 my (adjusted value). When redox values reached -44 my (adjustedvalue), the lower limit of the 95% confidence interval, the biological system was still a good environment for phosphorus removal in the process. Adjusting measured redox values is not necessary in order to achieve the conclusion about using ORP values to control a denitrification process. The -42 my, the lower limit of the 95% confidence interval from measured redox values, is suggested to be used as a denitrification control guideline. However, probe testing is still considered necessary. It was concluded that ORP monitoring can be used as a control tool in a denitrification process. Probe test procedures, including testing solutions, adjustment factors development, and testing frequency are recommended in the research.Applied Science, Faculty ofCivil Engineering, Department ofGraduat