EVALUATION OF POLLUTANT REMOVAL KINETICS FOR GREYWATER TREATMENT IN HORIZONTAL FREE SURFACE FLOW CONSTRUCTED WETLAND

Amidst a global freshwater shortage, reusing treated greywater is a viable option for supplementing non-potable demands. To ensure effective and sustainable treatment, understanding the kinetics of pollutant removal is essential for optimizing horizontal free surface flow constructed wetlands (HFSF). This study evaluates these kinetics for greywater in a continuous HFSF wetland planted with water hyacinth (Eichhornia crassipes) under hydraulic loading rates (HLRs). A pilot-scale HFSF wetland (12 m × 1 m × 1 m) constructed at the National Water Resources Institute, Kaduna was operated continuously at HLRs of 0.20, 0.25, and 0.30 m day-1. Greywater samples were collected biweekly and analysed for biochemical oxygen demand (BOD5), total phosphorus (TP), total suspended solids (TSS), and ammonium nitrogen (NH4–N). First-order kinetic models (k–C), a modified first-order model with background concentration (k–C*), and a Continuous Stirred Tank Reactor (CSTR) were applied to derive rate constants and assess the model’s performance. First-order rate constants increased with HLR, indicating faster reaction kinetics; however, the overall efficiency of the pollutant removal slightly declined at higher HLRs due to the reduced retention time. TSS removal declined due to resuspension and NH4–N removal was limited by oxygen deficiency at 0.30 m day-1. The models demonstrated relatively better predictive agreement for TP and NH4–N than for BOD5 and TSS, reflecting non-linear processes. The 0.20 m day-1 HLR provided the most sustainable performance through longer retention, effective biodegradation, sedimentation, and nitrification. The derived k values fall within global ranges, validating their use in wetland design.