Valorizing Boiler Combustion Residue into Activated Carbon for Tofu Effluent Treatment

Tofu-processing wastewater is known for its high chemical oxygen demand (COD), strong color and odor, and acidic pH, making it unsuitable for direct discharge. This study assesses the effectiveness of a circular adsorptive treatment using activated carbon derived from boiler combustion residues (500–600 °C) and identifies contact time (t) as the critical kinetic variable for four key parameters: COD, color, odor, and pH. Batch tests were conducted with 200 mL of effluent, 25 g of adsorbent, and agitation at 750 rpm for contact times ranging from 0 to 30 minutes. Within 10 minutes, the effluent transformed from yellow-muddy to clear, and from foul-smelling to odorless, with the pH increasing from 4.1 to 7.0; stabilizing around 8.0 by 25–30 minutes. The COD decreased from 3425.25 to 1030.80 mg L⁻¹ at 30 minutes, representing approximately 70% removal, indicating a rapid initial stage followed by slower intraparticle diffusion. The swift improvement in aesthetics and pH neutralization can be attributed to the effective adsorption of chromophores/odorants and the presence of residual alkalinity (e.g., Ca/K/Mg phases) in the ash-derived carbon. While the COD levels remained above the typical limit of 300 mg L⁻¹, the results demonstrate that ash-based activated carbon can quickly address color/odor and pH concerns, with further COD reduction achievable through extended contact time or higher doses. These findings establish a framework for optimizing the performance of low-cost, locally sourced adsorbents and suggest potential integration with pre-treatment or polishing steps in cases where strict COD limits are imposed.