Pumice stone waste as biofilm carrier in MBBR systems: impacts on wastewater treatment and colour removal in the denim industry

The denim industry generates wastewater with high organic loads and intense colouration due to the extensive use of synthetic dyes. Conventional biological treatment methods are generally inadequate for colour removal, necessitating alternative approaches. This study investigates the reuse of waste pumice stones - discarded during denim processing - as a dual-function material for industrial wastewater treatment. Pumice was utilised both as (i) a biofilm carrier in a Moving Bed Biofilm Reactor (MBBR) to enhance chemical oxygen demand (COD) removal, and (ii) an adsorbent for residual colour reduction. Pumice particles (10-14 mm) were applied at a 30% filling ratio in the MBBR and compared with a suspended-growth control system. The pumice-assisted MBBR achieved approximately 60 mg/L higher COD removal, confirming its efficacy in biofilm support. Complementary batch adsorption experiments demonstrated up to 45% colour removal, with performance varying across wavelengths (436, 525, and 620 nm). These results highlight the potential of repurposed pumice stones as a sustainable and costeffective resource for enhancing industrial wastewater treatment while promoting waste valorisation.