Intermittent mixing facilitates energy recovery and low carbon emissions from high-solids anaerobic co-digestion of food waste and sewage sludge

Abstract

Mixing inside high-solids anaerobic co-digestion (HS-AcoD) is essential for process feasibility and economic sustainability. This study developed a proper energy assessment for a high solids anaerobic digestion-combined heat and power (AD-CHP) system to clarify the impact of mixing on methane production, energy recovery and carbon emission reduction during HS-AcoD of food waste (FW) and sewage sludge (SS). Results indicated that intermittent mixing enhanced methane production and shortened the lag phase compared with unmixing and continuous mixing. The modified Gompertz model yielded better fitting than the logistic and transfer function models via kinetic analysis. In the case of a scaled-up AD-CHP system, intermittent mixing with 15 min/h boosted energy output (2.14 × 103 kWh/tonne VS) at 21 d. Compared with continuous mixing, 15 min/h intermittent mixing at 13 d improved the energy recovery ratio from 31% to 45% and carbon emissions reduction from 0.25 t CO2/t VS to 0.35 t CO2/t VS. For a high availability of FW and SS in China, the AD-CHP system with intermittent mixing would have higher net energy output (128.4 × 109 kWh) and carbon emission reduction (15.3 million tonnes) by the full utilization of these biomasses. These results are expected to provide theoretical support for the high solids AD-CHP system in determining the optimal mixing strategy with maximum energy production for FW and SS disposal.