- Anaerobic digestion is a microbially mediated process by which organic matter is decomposed in the absence of oxygen. The end products of anaerobic digestion include treated sludge and biogas, a mixture of methane and carbon dioxide, which can be recovered and used for energy generation. The majority of wastewater treatment plants currently flare their produced biogas because they do not produce enough to make recapture economically viable. Co-digestion has become a popular option to increase biogas production, thus increasing recapture potential. Grease trap waste, or FOG (fats, oils, and greases), is a co-substrate that has shown the highest energy potential. However, process optimization is lacking in terms of reaching maximum potential methane production rates, partially due to a lack of knowledge on the influence of different loading regimes. To address the effects of stepwise and shock loading regimes on the anaerobic co-digestion of FOG, experiments were conducted and reactor operation monitored. The step-wise loading study was conducted at organic loading rates of 1.3, 1.6, 1.9, and 2.7 g VS/L/d, while the shock loading study consisted of recovery periods and three different shock sizes of 1.1, 2.5, 3.2, and 10.7 g VS/L/d, respectively. While stepwise and shock loading studies have been conducted before, to the best of the author’s knowledge, none have compared the methane yields of shock versus stepwise loading directly. Overall, the shock loading regime resulted in greater methane yields, even at higher loading rates, compared to the stepwise loading study.