Spruce bark beetles as a biological waste that can be processed in a biogas plant

Abstract

Biogas plays an important role in reducing the amount of biowaste that is converted to methane which is usable by multiple pathways in the energy sector. Bark beetles are among the most important pests of spruce forests in Europe. Principal measures of managing bark beetle populations include trapping them with pheromone traps. Biomass of bark beetles collected by the traps constitutes a waste of forest resources that could potentially be processed in biogas plants. The aim of this study was to provide information about the trapping of bark beetles by pheromone traps in Tatra National Park during 2005-2018 and to verify the energy potential of a combination of liquid digestate and bark beetle biomass collected by pheromone traps. The bark beetle biomass is a seasonal small-volume nitrogen-rich material which could be processed at agricultural biogas plants. The biochemical methane potential batch test performed at 40 degrees C showed a high degree of digestion of milled beetles within 40 days, with a methane production of 0.242 Nm(3) per kilogram of total solids (TS). The single-stage low-solids mesophilic fed-batch anaerobic digestion of untreated beetles was carried out in a lab-scale rotary drum reactor. Within 52 days, the average organic loading rate was 1.019 kg VS m(-3)day(-)(1), where VS stands for volatile solids. The specific biogas and methane production reached 0.218 Nm(3)kg(-1) TS and 0.140 Nm(3)kg(-1) TS, respectively. The cuticles of the beetle bodies were not obviously disturbed by the fermentation process. The digestate was quite rich in nitrogen, but the other element contents did not differ too much from fertilizer made from manure or phytomass.