Operationally stable pH measurement in wastewater treatment using a photocatalytic self-cleaning electrode: Extended field evidence and uncertainty-oriented performance indicators

Abstract

This study presents a 29-day field deployment of a HORIBA photocatalytic self-cleaning pH electrode for continuous wastewater monitoring in a full-scale treatment facility discharging into Laguna Lake, Philippines. The system generated 41,729 valid one-minute pH measurements between 11 December 2025 and 9 January 2026 with near-continuous uptime. The work focuses on uncertainty-oriented operational indicators derived from grouped observations, rather than on a complete formal Guide to the Expression of Uncertainty in Measurement (GUM) uncertainty budget. Statistical indicators included agreement with laboratory reference measurements, rolling median absolute deviation (MAD), grouped daily statistics, trend analysis, and distributional behavior. Laboratory reference measurements were obtained using a benchtop HORIBA pH meter, calibrated with buffer standards traceable to the National Institute of Standards and Technology (NIST). The results indicated low short-term variability, operationally stable variance behavior, preserved sensitivity to transient process events, and no evidence of uncontrolled sensor degradation during deployment. Type A and Type B uncertainty contributions were discussed qualitatively within a grouped-observation framework consistent with JCGM 100:2008 Appendix H.5. The observed temporal behavior was more consistent with gradual process evolution than with fouling-induced sensor instability. Future investigations include deployment in multiple scenarios for inter-electrode reproducibility, parallel experiments with controlled environments, and extended and multi-season temporal studies.