INCREASING THE SUSTAINABILITY OF PSYCHROPHILIC SMALL-SCALE ANAEROBIC DIGESTERS

Abstract

The research was aimed at increasing the energy production efficiency of small-scale anaerobic digesters in temperate climates while quantifying their environmental impacts. Biochemical methane potential tests were used to quantify methane (CH4) production from separated and unseparated manure during psychrophilic digestion, and compare CH4 production when pre-incubated alternative inocula (wetland sediment (WS), landfill leachate (LL), mesophilic digestate (MD)) were used. Methanogenic and Archaeal communities were analyzed using T-RFLP and qPCR.

At 24 ºC, unseparated manure produced significantly higher (40%) quantity of CH4 than separated manure due to higher volatile solids (VS) content, but differences were insignificant at digestion times of ≤16 days. At lower digestion times, farmers could digest liquid, separated manure without sacrificing CH4 production, but at longer digestion times, the VS in unseparated manure has the time necessary for CH4 conversion.

The alternative inocula studies showed that LL inoculum after incubation for 91 days at 25 ºC produced significantly higher quantity (≥20%) of CH4 than MD and WS during digestion at the same temperature, and was not significantly different in CH4 quantity than MD that was incubated and digested at 35 ºC (202 ± 4 L/kg VS). Methanosarcinaceae was dominant in the LL reactor, while the other reactors were abundant in Methanosaetaceae, indicating that inoculum rich in Methanosarcinaceae may be beneficial for starting digestion at lower mesophilic temperature ranges. Longer incubation time generally reduced the inoculum amount needed for batch digestion and prevention of volatile fatty acids accumulation. In batch systems with long digestion time (90 days), MD inoculum from well-established digesters, 35% inoculum to substrate ratio, and 35 ºC operation temperature are recommended for highest CH4 production per unit of digester volume.

Additionally, life cycle assessments (LCA) were conducted to compare the sustainability of an unheated Chinese fixed-dome digester with a heated and insulated small-scale plug-flow digester in the US. The LCA showed that the US plug-flow digester was more sustainable than the Chinese fixed-dome system only in climate change category, but contributed negatively towards 17 impact categories. Digester heating and heating infrastructure were the main contributors towards the negative impacts observed in the US plug-flow digester.