Comparative assessment of dewatered sewage sludge combustion and biogas production: Environmental, economic and energy considerations

The escalating production of sewage sludge necessitates sustainable management strategies that balance energy recovery, environmental impact, and economic viability. This study presents a comprehensive techno-economic and environmental comparison of two sewage sludge-to-energy pathways: direct combustion and biogas production through anaerobic digestion. A computational model was developed to evaluate the complete sludge management chain, from wastewater treatment plant discharge to final disposal, accounting for auxiliary fuel requirements in combustion and digestate disposal costs in biogas production. Six sewage sludge (9.57-16.51 MJ/kg) were analyzed using data from Turkish wastewater treatment plants. Results demonstrated that high-calorific sludges required 23.8% less supplementary fuel than low-calorific samples, with installed capacities ranging from 0.84 to 1.58 MW for combustion systems and 0.66 to 0.79 MW for biogas systems. Only the highest calorific sludge achieved an emission factor (439.88 kg CO₂/MWh) lower than Turkey's electricity grid (442 kg CO₂/MWh), while biogas systems exhibited consistent emission factors (780-800 kg CO₂/MWh) across all scenarios. Economic analysis revealed that neither pathway is viable without government incentives at current market prices (67.81 USD/MWh). Total annual emissions from incineration (8,300 tons CO₂/year for low-calorific sludge) substantially exceeded those from biogas production (4,000-4,500 tons CO₂/year).