A co-digestion investigation was conducted using small-scale digesters in Costa Rica to optimize their ability to treat animal wastewater and produce renewable energy. Increases in methane production were quantified when swine manure was co-digested with used cooking grease in plug-flow digesters that operated at ambient temperate without mixing. The co-digestion experiments were conducted on 12 field-scale digesters (250 L each) using three replications of four treatment groups: the control (T0), which contained only swine manure and no waste oil, and 12.5, T5, and T10, which contained 2.5%, 5%, and 10% used cooking grease (by volume) combined with swine manure. The T2.5 treatment had the greatest methane (CH(4)) production (45 L/day), a 124% increase from the control, with a total biogas production of 67.3 L/day and 66.9% CH(4) in the produced biogas. Increasing the grease concentration beyond T2.5 produced biogas with a lower percentage of CH(4), and thus, did not result in any additional benefits. A batch study showed that methane production could be sustained for three months in digesters that co-digested swine manure and used cooking grease without daily inputs. The investigation proved that adding small amounts of grease to the influent is a simple way to double energy production without affecting other digester benefits. (C) 2010 Elsevier Ltd. All rights reserved.