Underground Anaerobic Digester to Solve the Energy Balance Problem in Temperate Regions: A Pilot Study

Abstract

Abstract. Ambient temperature fluctuations that often yield a negative net energy gain are a problem associated with anaerobic digesters in temperate regions. To address this problem, the performance of an underground anaerobic digester (UGAD) was investigated. Because the soil temperature 3.0 m below the surface only varies in a range of ±11.7°C over the seasons, it is possible to achieve a net energy gain. As such, a 20 m3 UGAD was built and operated under mesophilic conditions from October 2010 to July 2011. The net energy balance for the maintenance of the mesophilic environment, methane production, organic and pathogen removal rate, and free ammonia (FA) concentration were examined. During the winter, when the average ambient temperature was -2.1oC, the daily net energy gain was 66.4 MJ. In addition, throughout the 268-day observation, the average methane production and specific yield at STP (273 K) were 7.68 m3 day-1 and 333 L-CH4 kg-1 CODremoved, respectively. The total chemical oxygen demand, 5-day biological oxygen demand, , and removal efficiency were 64.65%, 85.82%, 99.79%, and 99.82%, respectively. The combination of long HRT (27 days) and FA concentration may be the reason for the high pathogen removal efficiency. In addition, FA concentrations ranging from 146.1 to 653.5 mg-N L -1 did not disrupt the anaerobic digestion processes. This study suggests that UGAD can be used as an alternative system for the anaerobic treatment of swine slurry in a temperate region with high organic and pathogen removal efficiency and net energy gain, even during the winter.