ENVIRONMENTAL AND ENERGY ASSESSMENT OF THE MANAGEMENT OF DIGESTATE FROM BIOGAS PRODUCTION

Abstract

Producing biogas from agricultural and industrial waste, one has to deal not only with energy production, but also with the nutrients contained in the digestate, from which nutritious fertilizers are obtained. Storage, processing and transportation of digestate to places of use are related to energy costs and greenhouse gas (GHG) emissions. This work has conducted a study that determined the energy costs and GHG emissions in the digestate handling processes of a biogas power plant: homogenization, pumping and transportation. Energy costs were determined by assessing the power of the mixers and pumps operating in the digestate storage and their operating time. During transportation, digestate mixers are constantly operating in the lagoons, which ensure a homogeneous mixture of the liquid fraction. Pumps are used to transfer the digestate from the lagoon to the tank, and their operating time corresponds to one loading operation, depending on the capacity of the vehicle. For transportation from the biogas plant storage to customer storage, the energy consumption was calculated by evaluating the fuel consumption of vehicles during the transportation cycle. The environmental assessment of digestate handling and transportation was carried out by determining the induced carbon dioxide (CO₂) emissions that occur in technological processes using electricity and fuel. The studied biogas plant generates about 33 thousand tons of digestate per year, which is stored in a sealed lagoon before transportation. The digestate is removed from the lagoon during the fertilization season. Three scenarios are selected in this work, which differ in the distances of digestate transportation - 8, 22 and 100 km. The liquid digestate fraction is transported from the biogas plant lagoon to customer storage by trucks with 24-ton tanks equipped with pumping systems. The study found that the highest energy consumption is incurred for digestate transportation – from 2.18 kWh/t to 27.0 kWh/t, depending on the transportation distance. Digestate logistics processes also have the most significant impact on carbon dioxide emissions. CO₂ emissions caused by digestate vehicles per tonne of digestate are most significant when transporting digestate over the longest distance, reaching 7.34 kgCO₂/t. Transportation over a distance of 22 km causes 1.62 kgCO₂/t of CO₂ emissions, and over a distance of 8 km – only 0.59 kgCO₂/t. However, when transporting digestate over the shortest route, higher CO₂ emissions are caused by the operation of digestate mixers (about 57.1% of total CO₂ emissions), somewhat less (about 37.3%) by transportation and about 5.6% by pumping.