Sewage Sludge Biorefinery for Circular Economy

Abstract

Sewage sludge processing and disposal have a significant weight on the energy and economic balances of wastewater treatment operations and contribute substantially to greenhouse gas emissions related to wastewater processing. Despite this, sewage sludge contains substantial recoverable resources in the form of energy and useful molecules. The current challenge, other than reducing the environmental and economic impacts of its disposal, is to recover energy and materials from this waste stream, implementing a biosolid-centered circular economy with the greatest possible added value. A number of options along these lines exist, and others are being investigated, ranging from biological processes, thermochemical technologies, bioelectrochemical processing, biorefineries and others. Recoverable resources comprise biogas from sludge fermentation, liquid and solid end products (e.g., biodiesel and biochar) and valuable nutrients (N and P). This paper presents a state of the art of biorefinery, with emphasis on recent developments in non-conventional resource recovery from EBSS streams for sludge-based circular economy implementation. Expectations and limitations, including technological readiness, of these technologies are discussed.

The Use of Solidified Carbon Dioxide in the Aerobic Granular Sludge Pre-Treatment before Thermophilic Anaerobic Digestion

“…The most common technology for the recovery of energy and valuable materials from sewage sludge is anaerobic digestion (AD) [7]. It is a well-known and optimized method, as indicated by a vast number of closed digesters exploited in the technical scale.…”

Assessment of EU Bio-Based Economy Sectors Based on Environmental, Socioeconomic, and Technical Indicators

“…However, they currently face challenges in making the leap to the industrial level due to the need for further development to address issues such as high processing costs, low substrate conversion efficiency, quality improvement, high energy demand, poor technology transfer from academia to industrial application, and the resolution of certain legislative issues [67,93,96]. Although said needs apply to all waste types, according to the literature, those that seem to require more effort to achieve industrial application are sludges [64,81,82]. And finally, despite the aforementioned challenges, several promising technologies are being developed to transform waste streams in secondary raw materials targeting higher-value products.…”

Urban Wastewater Mining for Circular Resource Recovery: Approaches and Technology Analysis

“…Additional resources are generated within the UWW processing cycle itself: microbiological activity transforms the original organic compounds into biomass and fuels (e.g., biogas) and generates secondary recoverable resources embedded in biomass such as lipids, polyhydroxyalkanoates (PHAs), proteins, and other products [56]. Economic and sustainability considerations suggest that resources embedded in sludge (energy, nutrients, raw materials, and process by-products) should also be properly exploited to consolidate a virtuous wastewater-based Circular Economy cycle [57].…”

“…Recent approaches in sludge biorefineries have aimed at the direct recovery of specific biomass components, such as organic acids and/or alcohols, as well as other industrially relevant molecules (such as polyhydroxyalcanoates (PHAs), EPSs, VFAs, proteins, biopesticides, enzymes, and solvents) and biofuels considered more industrially valuable than biogas normally obtained from ASD. A state-of-the-art review of biorefinery possibilities was recently presented by Cecconet and Capodaglio [56]. Appropriately sequenced biorefinery processes can maximize the recovery of value-added products and facilitate the subsequent processing steps.…”