Energy and economic assessment of hydrothermal-treatment-coupled anaerobic digestion

Hydrothermal treatment enhances biomethane (renewable energy source) production from the anaerobic digestion of municipal sewage sludge but requires substantial energy (heat). A complete energy and economic analysis for a hydrothermal-treatment-coupled anaerobic digestion system is missing. This study comprehensively analyzes the energy and economic aspects of three hydrothermal-treatment-coupled anaerobic digestion systems and a control for municipal sludge treatment: A control (anaerobic digestion only), a pre-stage system, an inter-stage system, and a post-stage/recycle system. The large energy consumption of hydrothermal treatment challenges the overall energy and economic neutrality in the systems. Hydrothermal treatment is economically beneficial only if it is conducted at a low or moderate temperature (100 to 130 °C), when the raw sludge has a high content of complex organic matter (especially particulate organic matter), and when the control has a low biogas yield. To reduce the energy burden of hydrothermal treatment and make the systems more economically beneficial, this study proposes to use direct sunlight as a “free,” renewable energy source to conduct hydrothermal treatment. In addition, the energy consumption of hydrothermal treatment can be reduced by separating the solids and liquid fractions in sludge using centrifugation and applying hydrothermal treatment to only the solids. Recovering carbon dioxide from the biogas has significant environmental and economic benefits. Among the alternatives, the post-stage/recycle system is more beneficial from an economic viewpoint and should be considered in future studies. Conversely, the inter-stage system should be avoided because it is more complex to maintain, requires a larger footprint, and has less significant economic benefits.