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

Abstract

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 to 52 %, TS from 25 to 34 %, TSS from 43 to 75 %, TN from 33 to 62 %, TAN from 22 to 53 %, and NO₃⁻–N from 21 to 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.