Energy and resources recovery from excess sewage sludge: A holistic analysis of opportunities and strategies

Wastewater treatment processes produce large amounts of excess biological sewage sludge, which disposal re-quires large amounts of energy and expenditure, and is not devoid of adverse environmental effects. Economic and sustainability considerations suggest that embedded resources in sludge (energy, nutrients, raw materials and process by-products) should be properly exploited to create and consolidate a virtuous wastewater-based Circular Economy cycle. This paper presents a review of sludge-to-energy and sludge-to-resources recovery routes with emphasis on recent developments, as well on the benefits and limitations of applicable technologies for ensuring sustainability and environmental efficiency through a critical analysis of current literature. In particular, improvement of anaerobic digestion final products exploitation, thermochemical technologies for sludge-embedded resources recovery, sludge biorefinery and nutrients recovery are discussed. In most cases, however, the technology readiness level (TRL) of the described technologies is still low for generalized adoption, hence, notwithstanding the excellent development prospects, it is probably still too early to predict the real impact of the many bioproducts recoverable from biological sludge on an actual global circular bioeconomy.