Dairy wastewater is a nutrient-rich agro-industrial effluent characterized by elevated chemical oxygen demand (COD), suspended solids, and nitrogen and phosphorus concentrations, requiring low-cost and decentralized treatment options. This study evaluated a laboratory-scale downflow biofiltration system packed with mixed wheat straw and wood chips for multi-parameter treatment of real dairy wastewater. Four identical columns (BTF1-BTF4) were operated using particle sizes of 0.40, 2.36, 4.75, and 9.50 mm at hydraulic retention times (HRTs) of 5, 10, and 24 h. The influent contained 23.70 mg L-1 PO4 3-, 33.15 mg L-1 NO3 -, 413 mg L-1 COD, 1,595 mg L-1 total suspended solids (TSS), and 1,463 mg L-1 total dissolved solids (TDS). At 24 h, BTF1 achieved the highest phosphate and COD removal (91.27% and 76.51%, respectively), whereas BTF2 showed the best nitrate and TSS removal (80.38% and 92.66%, respectively). One-way ANOVA indicated that HRT significantly affected phosphate and COD removal (p < .001), while particle size effects were not statistically significant at alpha = .05. A multiple linear regression model explained 80.2% of the variance in COD removal, with HRT identified as the dominant positive predictor. A composite performance index ranked BTF2 at 24 h as the optimal overall treatment condition. A supplementary extended-operation assessment (up to 72 h) implied initial operational stability parameters being adequate for solids removal and partial organic load attenuation (details below), but also indicated significantly higher TDS levels in effluents and reduced phosphate buffering capacity after prolonged contact, supporting the hypothesis that solute leaching from media into effluents may trigger over time with increasing solid deposition potential. The early acidification followed by recovery of effluent pH toward near-neutral conditions suggested system stabilization that could support longer retention in a practical setting. Mixed lignocellulosic biofiltration can be a technically feasible wastewater treatment approach, with the potential for long-term sustainability in decentralized dairy wastewater management especially in most resource-limited contexts.