Enhancing dairy wastewater treatment: Effects of hydraulic and organic loading rates in vermifiltration systems

• Lower hydraulic loading rates improved COD, TAN, and nitrate removal efficiencies • Optimal organic loading rate of 2.5 kg [COD] m⁻² d⁻¹ boosted worm biomass growth • Higher HLRs enhanced ortho-P reduction but lowered COD and nitrogen removal • Balancing HLR and OLR is key for efficient dairy wastewater vermifiltration Vermifiltration is increasingly recognized as a cost-effective and environmentally sound method for treating concentrated agricultural effluents. This study evaluated how varying hydraulic loading rates (HLRs), and corresponding organic loading rates (OLRs) affect both the treatment efficiency and earthworm biomass dynamics in a vermifiltration system treating dairy wastewater. The experiment was conducted over 60 days using a bench-scale system with a surface area of 0.36 m² and media depth of 45 cm. Four hydraulic loading rates, ranging from 0.5 to 3.0 m³ m⁻² d⁻¹, were tested. These corresponded to organic loading rates between 1.3 and 7.6 kg [COD] m⁻² d⁻¹. System performance was evaluated based on removal efficiencies for COD, TS, TSS, TN, NH₄⁺–N, NO₃⁻–N, TP, and ortho-P. Earthworm biomass gain was also measured as an indicator of biological activity and system health. Removal efficiencies across treatments varied by parameter, with COD ranging from 21–52%, TS from 25–34%, TSS from 43–75%, TN from 33–62%, TAN from 22–53%, and NO₃⁻–N from 21–72%. Optimal removal occurred at the lower HLRs (0.5–1.0 m³ m⁻² d⁻¹) and OLRs (1.3–2.5 kg [COD] m⁻² d⁻¹), likely due to extended hydraulic retention times that enhanced microbial processing and earthworm-mediated degradation. Earthworm biomass peaked at an OLR of approximately 2.5 kg [COD] m⁻² d⁻¹, suggesting that higher organic inputs may surpass the tolerance threshold for earthworm productivity. Conversely, ortho-P removal improved with increasing HLR, indicating that elevated flow rates may enhance phosphorus mineralization. These findings underscore the necessity of optimizing HLR and OLR to balance pollutant removal with biological activity in vermifiltration systems managing dairy effluent.