Performance and Microbial Dynamics of Vermifiltration‐Integrated Constructed Wetland Treating Rice Mill Wastewater Under Elevated COD and Lignin Loading

Higher organic concentrations in rice mill wastewater (RMW) are accompanied by increased lignin concentrations; however, their combined impact on treatment performance remains insufficiently understood. In this study, a vermifiltration-integrated constructed wetland (CW) system was evaluated to elucidate the effects of increasing organic concentrations on stage-specific functional performance. RMW with influent chemical oxygen demand (COD) of 2000 and 4000 mg/L, with corresponding lignin concentrations of 150 and 300 mg/L, representative of typical and peak loading conditions, was treated. Increasing influent strength from 2000 to 4000 mg/L reduced treatment efficiency, with COD and lignin removal efficiencies decreasing by 21.6% and 24.8%, respectively, at a CW length of 30 cm, and by 18.9% and 17.3% at 50 cm. However, increasing influent COD from 2000 to 4000 mg/L resulted in higher absolute removal, with COD removal increasing from 1838 to 2880 mg/L at a CW length of 30 cm and from 1886 to 3060 mg/L at a CW length of 50 cm, and corresponding lignin removal increasing from 118 to 174 mg/L and from 124 to 196 mg/L, respectively. Despite the increase in absolute removal, the decline in removal efficiency indicated increased system stress under higher loading conditions. The lignin inhibition test identified a critical inhibitory concentration of 253 mg/L, providing a mechanistic basis for interpreting system behavior. Although higher COD loadings promoted the proliferation of heterotrophic bacteria and actinomycetes, specific oxygen uptake rate and enzyme assay revealed suppressed microbial activity, with excessive EPS production indicating a protective microbial response under stress-induced conditions. The presence of Sphingomonas, Stenotrophomonas, and Pseudomonas indicated lignin-degradation potential of the system. Overall, the findings demonstrated that lignin concentration, rather than COD alone, governs metabolic inhibition, stressing the necessity of lignin-targeted pretreatment strategies to sustain treatment performance under high-strength RMW conditions.