Adsorption-style rate kinetics best describes long chain fatty acid inhibition of methanogenesis

Fat-rich wastes can increase viability of food waste anaerobic digestion and divert waste from landfill, but inhibition by long chain fatty acids (LCFAs) is a well-known concern. Inhibition models can estimate the associated risk but their suitability for LCFAs is yet to be clarified. The current work uniquely compared LCFA inhibition model functions using data from rapid 3-day specific methanogenic activity tests with four distinct inocula sampled from full-scale digesters. Oleic acid was tested as model LCFA at 0.25-4 g·L^-1. Microbial community composition was analysed by short-read DNA sequencing. LCFA inhibition resilience differed, with a half-inhibition parameter KI₅₀ ranging from 0.3 g·L^-1 oleic acid (1 mmolar) for the least resilient inoculum to 1.4 g·L^-1 oleic acid (5 mmolar) for the most resilient inoculum. Microbial community differed significantly between the inocula; however, no significant correlations were found between inhibition resilience and key community members, or whole, bacterial, or archaeal communities. The results suggest LCFA is a broad-spectrum inhibitor and inhibition resilience is derived from whole-community redundancy influenced by LCFA pre-exposure. Only an adsorption-style inhibition model gave statistically identifiable parameter values for all tested inocula and simultaneously fit data for an inoculum sampled at two distinct biomass concentrations. The results suggest a general applicability of an adsorption-style LCFA inhibition model and support an adsorption mechanism of LCFA inhibition. Accordingly, an adsorption-style inhibition function is recommended for future use with LCFA. The approach in this study can quantify and manage LCFA inhibition risk at operational facilities to encourage co-digestion of fat-rich wastes and increase digestion viability.