BIOCHAR BASED FILTRATION SYSTEM FOR SEPTIC TANK EFFLUENT: PERFORMANCE EVALUATION AND NUMERICAL MODELLING

Septic tank effluent (STE) contains elevated nutrients, organic matter, and pathogens, posing environmental and public health risks. In this study, STE was treated using oak woodchip biochar in fixed-bed filtration systems. A 4 × 2 factorial design assessed effects of particle size (fine, medium, coarse, mixed) and flow regime (continuous, intermittent) on removal of COD, TOC, TN, NH₄⁺-N, and K⁺. Biochar produced at 700°C exhibited a highly aromatic, alkaline, porous structure with oxygen functional groups, confirmed by SEM–EDS and XPS, with carbon increasing from 49% to 91%. Particle size significantly influenced removal (p < 0.001), while flow regime effects were minimal. Fine biochar achieved the highest reductions (10.07–76.84%). Random Forest models accurately predicted removal efficiencies (R² > 0.97 for COD, TOC; 0.95–0.96 for TN; 0.86–0.89 for NH₄⁺-N; 0.65–0.82 for K⁺). These results support fine and mixed biochar as promising polishing media for decentralized STE management.