Development of a wet digestion method for the analysis of metal soap in fat, oil, and grease deposits using the Taguchi method

Abstract

Conventional methods for evaluating lipid-based metal salts in fats, oils and grease (FOG) deposits often fall short in assessing the efficiency of grease interceptors for food service establishment wastewater, primarily due to their inadequate specificity for fatty acid (FA) metal soaps. To address this issue, this study introduces a wet acid digestion method developed for the accurate analysis of metal soap content in FOG samples. The Taguchi experimental design method was used to determine the optimal conditions for digestion including temperature, contact time, solid/liquid ratio and type of acid mixture with the aim of more accurately quantifying metal salt concentrations. This work focused on identifying the most effective acid combinations for digesting FOG to enable analysis of commonly found metals. Extensive comparisons revealed that HNO3:H2SO4:H2O2 (1:2:1) and HNO3:H2O2 (1:3) were particularly effective at 180°C for 16 h due to their significant oxidation potential. The HNO3:H2O2 (1:3) mixture yielded the highest concentrations of Ca, Mg, Na, and K of 296.38±0.66, 54.95±1.01, 247.65±2.27, and 46.09±3.44 mg/g, respectively, aligning with the stringent optimisation principles of the Taguchi method. A one-way ANOVA test validated significant variances in metal contents (p < 0.005), and a t-test indicated a notably higher concentration of Ca compared to other metals. The study presents a sophisticated approach to evaluating the FA metal soap content of FOG ensuring enhanced accuracy and precision compared to established methods.