Rethinking sewage sludge valorisation options and pathways via thermochemical output upgrading: a product-oriented analysis

Abstract

The recycling of digested sewage sludge (biosolids) to land is common, offering soil and nutrient benefits. However, regulatory uncertainty, emerging contaminants, and limited land availability call for alternative strategies. Thermochemical options, including pyrolysis and gasification, enable resource and energy recovery from sludge, producing outputs like biochar, syngas, and bio-oil. This study evaluates the potential of sludge pyrolysis alongside gasification by reviewing conceptual plant designs and assessing economic and technical feasibility. Pyrolysis gas, or syngas, upgrading is capital intensive but can yield high-value outputs. Alternatively, direct combustion of pyrolysis gas and syngas for heat or power offers lower post-treatment needs and economic viability, especially on large scales if quality standards are met. The research also examines sustainability and operational feasibility, noting key challenges such as sludge drying energy demands, feedstock variability, and regulatory uncertainty around land application of biochar. The findings support sustainable sludge management by identifying promising valorisation pathways and informing circular economy approaches in wastewater treatment. Further research is needed to optimise processes, improve output upgrading, and develop supportive regulatory frameworks.