Time-temperature degradation of fatty acids in sewer fat, oil, and grease deposits: a framework for sample storage and handling.

Fat, oil, and grease (FOG) deposits present significant challenges in wastewater systems, yet the stability of their fatty acid (FA) components prior to analysis remains poorly defined. Given the lack of a validated sample-handling and preservation method for wastewater FOG, this study quantified the degradation of FAs in synthetic FOG stored at -20 °C, -10 °C, 4 °C, and 25 °C over 15 months and established a temperature-specific degradation threshold. FA degradation was modelled using zero- and first-order kinetics and temperature-dependent models, including the Williams-Landel-Ferry, reciprocal polynomial, Power-law, and Vogel-Tamman-Fulcher models. Results showed that FAs degraded under all storage conditions, with cumulative losses exceeding 50% at every temperature after 15 months. After one month, degradation ranged from only 1.9% at -20 °C to 15.1% at 25 °C. Kinetic analysis indicated first-order behaviour at -20 °C, 4 °C, and 25 °C, whereas zero-order kinetics were observed at -10 °C, reflecting distinct mechanisms between complete and partial freezing. Temperature-dependent modelling indicated that frozen storage slowed degradation, but FA loss at higher temperatures exceeded model predictions due to concurrent oxidative and hydrolytic reactions. Storage duration at 30% and 50% loss thresholds confirmed that only deep-freezing extended preservation for nearly one year, whereas partial freezing, refrigeration, and ambient storage substantially reduced stability. These findings demonstrate that common storage conditions alter FOG composition prior to analysis and establish a quantitative basis for standardised sample handling and more reliable wastewater research.