Kinetic modelling and performance evaluation of vertical subsurface flow constructed wetlands in tropics

Abstract

The design of vertical subsurface flow (VSSF) constructed wetlands (CWs) uses kinetic models to calculate the area based on the kinetic reaction rate constant (k) specific to local environmental conditions and target pollutants. Currently, kinetic modelling does not fully account for the impact of the hydraulic loading rate (HLR), which influences the wetland performance. This study used four experimental VSSF CWs operated at HLRs of 5, 10, 20 and 40 cm/day to investigate the applicability of three first order kinetic models combining plug-flow and continuous stirred tank reactor (CSTR) flow patterns. The target pollutants were BOD5, NH4+ and NO3-. For each pollutant, estimated k values varied between different HLRs and between plug flow and CSTR models. Assessment of uncertainty in kinetic modelling showed that all three models exhibit a similar trend in predicting the concentrations of BOD5 and NH4+ at 5–20 cm/day HLRs. A substantial removal of BOD5 (>88 %) and NH4+ (>70 %) were found for the investigated HLRs, although NO3- removal was not satisfactory. The HLR had a positive impact on mass removal rates (MRRs) for BOD5 and NH4+. Accordingly, 20 cm/day was deemed as the highest viable HLR for designing effective VSSF wetlands for the removal of BOD5 and NH4+. All three models can be employed to design VSSF wetlands at 20 cm/day HLR to treat BOD5 using k values of 0.352 (k-C), 0.380 (k-C*) and 0.996 (CSTR) m/day and to treat NH4+ using k values of 0.170 (k-C), 0.173 (k-C*) and 0.273 (CSTR) m/day.