Enhanced removal of small microplastics (1–5 μm) from secondary-treated wastewater using Spent Coffee Grounds biochar: Application of flow cytometry

Microplastics (MPs) pose a significant threat to aquatic ecosystems and global sustainability due to their persistence, widespread presence, and potential to cross biological barriers, transferring through food chains. Wastewater treatment plants are primary pathways for MPs entering aquatic environments, underscoring the need for effective removal technologies before effluent discharge. This study aims to develop a novel, sustainable approach based on Flow Cytometry (FCM) for monitoring biochar-based filtration and rapidly enumerating removed MPs within the 1–5 μm range. While commonly used in medical fields, FCM's application in environmental monitoring, particularly for MPs, is relatively new. Here, FCM was used to assess the effectiveness of biochar derived from spent coffee grounds (SCG) as a low-cost, eco-friendly adsorbent for MP removal from secondary-treated wastewater. Filtration experiments with short contact times revealed that SCG-biochar achieved nearly 100 % removal efficiency for MPs sized 2.0 μm and larger, across an initial concentration range of 8 to 18 mg/L. For more challenging 1.0 μm MPs, at an initial concentration of 1.4 mg/L, biochar exhibited around 49.0 ± 1.2 % removal efficiency when exposed to wastewater, significantly outperforming traditional sand filters. Moreover, SCG-biochar accumulated at least 8.5 mg of 1.0 μm MPs per kg of biochar, equivalent to 1E+9 1.0-μm MP/kg. Notably, < 1.5 % of retained MPs were remobilized during post-filtration flushing. Microscopic analysis further revealed that MPs of various sizes were immobilized deep within the biochar's intricate pore structures, highlighting its potential as a robust and efficient solution for sustainable wastewater treatment, particularly in advanced filtration technologies. • SCG-biochar outperforms sand filters for microplastic removal due to its pore structure. • Near-complete efficiency was achieved for particles sized 2.0–5.0 μm. • SCG-biochar showed 49 % removal efficiency for 1.0 μm MPs, remobilizing < 1.5 % of retained particles. • Biochar filter maintains performance despite natural organic matter and high MP loads. • Flow Cytometry enables rapid and accurate analysis of sub-micron particles.