Methane production in low-cost, unheated, plug-flow digesters treating swine manure and used cooking grease

Abstract

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 (250L each) using three replications of four treatment groups: the control (T0), which contained only swine manure and no waste oil, and T2.5, 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 (CH4) production (45L/day), a 124% increase from the control, with a total biogas production of 67.3L/day and 66.9% CH4 in the produced biogas. Increasing the grease concentration beyond T2.5 produced biogas with a lower percentage of CH4, 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.