Exploring the potential for recycling sewage sludge in biodiesel using deep eutectic solvents: A proposal of circular economy based on life cycle assessment approach

This study presents an innovative cleaner production strategy for the energetic valorization of greasy wastewater sludge (WS) through biodiesel synthesis, using choline chloride (ChCl)-based deep eutectic solvents (DES) with p-toluenesulfonic acid (PTSA) and oxalic acid (OA) as green catalysts. Biodiesel production was evaluated using two approaches: (i) a one-step cleaner production process with different ChCl:PTSA and ChCl:OA ratios, and (ii) a conventional two-step method (esterification-transesterification) with H₂SO₄ and CaO as catalysts. The environmental impact of both production methods was assessed via life cycle assessment (LCA). Yields of up to 53 % were achieved in extracting grease from WS. The conventional two-step process showed biodiesel production yields of 88 % at 9 h, while the one-step process showed yields of up to 98 % with ChCl:PTSA and 59 % with ChCl:OA at molar ratios of (0.1 and 0.2) DES:FFA, and (4 and 6) hours, respectively. ChCl:PTSA emerged as the superior catalyst, providing higher energy efficiency, reduced reaction times, and up to a 56 % reduction in greenhouse gas (GHG) emissions compared to the two-step method. Its application resulted in a 64 % reduction in GHG emissions compared to conventional diesel, mainly due to reduced energy demands during fat extraction and reaction processes. However, further optimization is necessary to minimize methanol and energy consumption during esterification/transesterification, as these factors account for 50 % and 20 % of total GHG emissions, respectively, when using DES-based biodiesel production. This work highlights the potential of DES as a sustainable and energy-efficient alternative for biodiesel production, paving the way for greener solutions in WS valorization.