Using circular economy principles in the optimisation of sludge-based activated carbon production for the removal of perfluoroalkyl substances

Massive sewage sludge (SS) production from municipal wastewater treatment plants and the presence of numerous pollutant types render the process of SS treatment and disposal costly and complex. Here, resource recovery from SS was maximised via the optimisation of sludge-based activated carbon (SBAC) production for the removal of poly- and perfluoroalkyl substances (PFASs), while considering economic factors and minimising environmental impacts. SBAC production optimisation was realised under different operating conditions (different ZnCl2 impregnation ratios and different pyrolysis activation temperatures and durations). The sorption capacity of the optimised SBAC with respect to the removal of nine commonly detected PFASs, with environmentally relevant concentrations (∽50 μg/L), from simulated wastewater was evaluated. Economic analysis and life-cycle assessment (LCA) were also performed to determine the feasibility of the process and its potential role in the circular economy. Batch adsorption tests confirmed the high efficiency of the optimised SBACs for PFAS removal (93–100 %), highlighting the possibility of converting SS to SBAC. Economically speaking, the optimised SBAC at 1.5 M ZnCl2, 500 °C, and 0.75 h reduced total production cost by 49 %. Further, the cost could be reduced to as little as 1087 US $/metric-ton compared with that corresponding to the original conditions (2.5 M ZnCl2, 500 °C, 2 h; 2144 US $/metric-ton). LCA results also showed that freshwater ecotoxicity, marine ecotoxicity, and human non-carcinogenic toxicity were the most affected environmental impact indicators, showing a 49 % decrease when ZnCl2 impregnation ratio was reduced from 2.5 to 1.5 M. These findings highlighted the optimal conditions for the production of SBAC with high sorption capacity at a reduced cost and with reduced environmental impacts. Thus, they can serve as valuable tools for decision making regarding the selection of the most sustainable and economically feasible process for PFAS removal. [Display omitted] •Sewage sludge treatment is a costly and complex process involving pollutant removal.•ZnCl2 primarily contributes to sludge-based activated carbon (SBAC) production cost.•SBACs can achieve 93–100 % removal of poly- and perfluoroalkyl substances.•SBACs reduce ecotoxicity indicators by 49 %, based on life cycle assessment.•SBACs show potential based on performance, environmental, and economic assessments.