Wastewater Sludge as Energy Resource: Technoeconomic and Environmental Implications

The present study represents a review of the technoeconomic and environmental efficiency of wastewater sludge treatment techniques. The global production of municipal sewage sludge has reached approximately 45 Mt/year (dry weight), with annual generation in regions such as the United States (US), European Union (EU), and China ranging from 18 to 33 Mt/year. Conventional disposal methods—landfilling, agricultural application, and incineration—pose environmental hazards and are increasingly deemed unsustainable. In response, advanced thermochemical [pyrolysis, gasification, hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL)] and biochemical (anaerobic digestion, AD) processes are being explored for sludge valorization. AD is the most mature and economical route, delivering internal rates of return (IRR) around 9%–11%, payback periods between 3.6 and 12.9 years, and costs of ∼EUR 66/t wet sludge; when combined with thermal hydrolysis pretreatment, energy efficiency improves by ∼14% and IRR reaches 10.6%. The integration of AD with high-pressure bio-CNG upgrading systems demonstrates IRR up to 11.2% and a payback period under 13 years. HTC yields hydrochar with heating values between 10 and 15 MJ/kg, requiring low drying energy (≈5% electrical, ≈20% thermal of biomass energy content) and can recuperate >60% of input energy via combined heat and CH4 generation. HTC–AD systems show attractive economics: total treatment costs range from €94 to €135/t wet sludge, with Finnish setups breaking even at €38/t. HTL is especially promising: pilot systems operating at 300–350°C achieve biocrude yields up to ∼35 MJ/kg, process energy return ratios >3, and positive energy balance. However, small–scale HTL currently results in minimum fuel selling prices of EUR 21.5–64.7/GJ—considerably higher than AD-derived biogas (≈EUR 6–12/GJ). In summary, AD and HTC–AD offer proven technoeconomic viability. HTL and pyrolysis/gasification present strong energy recovery potential and circular–economy alignment but are constrained by high capital and operating costs, moisture handling, and product consistency issues. Strategic integration of these technologies—with supportive policy and product-market frameworks—is essential to unlock sludge’s full potential as a renewable energy resource. Future research should aim to integrate these methods into a zero-waste process and conduct comprehensive assessments to ensure successful implementation on a commercial scale.