Anaerobic digestion of thermal hydrolysis pretreated sludge: Process performance, metagenomic analysis, techno-economic and life cycle assessment

Abstract

This study assessed the potential of thermal hydrolysis process (THP) combined with anaerobic digestion (AD) for high solids sewage sludge treatment across various hydraulic retention times (HRTs). Optimal performance was achieved at a 10-day HRT (6 kg VS/m³·day), yielding 408 L CH₄/kg VS added and 54 % volatile solids (VS) removal under THP conditions of 160 °C, 30 min, and 6 bar pressure. Microbial analysis revealed predominant acetoclastic and hydrogenotrophic methanogens. Four scenarios were designed and analyzed for environmental and economic performance: Scenario 1 (conventional AD-CHP), Scenario 2 (conventional AD-BioCNG), Scenario 3 (THP AD-BioCNG), and Scenario 4 (THP AD-CHP). The results showed that scenarios with CHP integration achieved better environmental performance by generating sufficient energy to meet demand, with energy consumption as a key factor. Notably, scenario 4 had the lowest global warming potential (GWP) at −0.0185 kg CO₂-eq, outperforming conventional AD (Scenario 1) with CHP, which had a GWP of −0.00232 kg CO₂-eq. However, profitability analysis showed that Scenario 3 was the most economically viable, with a net present value (NPV) of $4.3 million, an internal rate of return (IRR) of 10.21 %, and a 17-year payback period. Although it had higher capital ($58 million) and operational costs ($12.5 million/year) than Scenario 4 ($45 million and $8.6 million/year), its greater biomethane yield resulted in higher revenue ($20.7 million/year), making it the most profitable option. While Scenario 4 offered the best environmental benefits, Scenario 3 emerged as the most financially sustainable choice. These findings highlight the environmental and economic advantage of utilizing THP-AD process over conventional AD, suggesting that THP-AD optimizes methane production, solids reduction, and environmental impact, making the Bio CNG pathway a sustainable and economically viable option.