Waste to wealth transformation: maximising resource and energy recovery through hydrothermal treatment of sewage sludge - a technoeconomic evaluation

Abstract

This study provides a comprehensive technoeconomic evaluation of integrating hydrothermal treatment (HT) with anaerobic digestion (AD) and struvite precipitation to enhance resource recovery from sewage sludge, offering an innovative and sustainable solution for wastewater treatment plants (WWTPs). Under optimal conditions (220 C, 20 min), HT of waste-activated sludge (WAS) significantly increased soluble chemical oxygen demand (SCOD), resulting in enhanced biomethane yields during AD in comparison to untreated sludge. Comparative analysis of four scenarios, standalone AD, AD with HT, HT with toxin adsorption, and HT with both toxin adsorption and struvite precipitation, highlighted notable improvements in resource efficiency and economic viability. The optimised HT-AD configuration achieved up to 30 % higher methane yields and 25 % lower production costs. Incorporating struvite precipitation further reduced the minimum biomethane selling price from $2883.85/ton to $337.49/ton. Economic analysis, using net present value (NPV), internal rate of return (IRR), and payback period as key indicators, showed the ultimate valorisation configuration (HT with toxin removal and struvite precipitation) achieving an NPV of approximately $2.5 billion over a 30-year plant lifespan. Sensitivity analysis underscored that while struvite precipitation costs influence profitability, overall production costs in the optimised configuration were resilient to market fluctuations. Alternative magnesium sources were identified as critical to reducing operating costs. This work offers practical insights and policy recommendations for resource-constrained regions on adopting HT-AD systems to support sustainable sludge management. Aligning with circular economy principles, this study advocates for waste-to-resource strategies that advance environmental sustainability while lowering operational costs in WWTPs.