Net Energy Performance of Full-Scale Mesophilic Anaerobic Digesters under Variable Climate and Mixing Conditions

Abstract

Anaerobic digestion (AD) is a popular green technology that can transform organic waste into methane-rich biogas. However, the energy requirements for heating and mixing significantly influence the net energy balance of full-scale digesters. This study presents a consistent, computation-based energy balance to evaluate the operational conditions of a mesophilic continuous stirred tank reactor (CSTR) in terms of being energy positive or negative. A 2000 m³ digester treating 15% total solids (TS) manure and food waste at 37°C with a hydraulic retention time of 25 days was simulated under various ambient temperatures, insulation levels, mixing rates, and methane yields. We assigned a conservative yield of 0.22 m³ CH₄ kg⁻¹ VS to estimate the amount of methane produced. Finally, we considered 3.5 kWh m⁻³ CH₄ for conversion to electricity production, assuming the technologies operate effectively on average. We applied a U–A heat exchanger (HX) and attempted to determine the heat loss and mixing energy, which ranged from 1 to 3 W m⁻³. Over 6,000 kWh of net energy per day were available when the temperature was approximately 15°C, while less than 100 kWh/d of biogas were produced when the temperature was below 5°C. On the other hand, cold climates, poor insulation, thick adjacent feed, and low methane yield (≤0.12 m³ CH₄ kg⁻¹ VS, where negative values were even obtained at −17%) typically resulted in a deficit in electricity production from anaerobic digestion, which could lead to an energy-negative digester. A sensitivity analysis reveals that the dominant influencing parameters include ambient temperature, reactor U-value, and mixing power. The conclusion is that improved insulation, more effective mixing, and recovery of combined heat and power (CHP) waste heat are necessary.