Biochemical performance modelling of non-vegetated and vegetated vertical subsurface-flow constructed wetlands treating municipal wastewater in hot and dry climate

Abstract

Wastewater treatment and subsequent effluent recycling for non-drinking purposes such as irrigation contributes to the mitigation of the pressure on freshwater resources. In this study, two vertical sub-surface flow constructed wetland (VSSF-CW) pilot plants were operated to treat municipal wastewater and their effluents were reused for irrigation purposes. One of the wetlands was vegetated with Phragmites australis (Cav.) Trin. ex Steud. (common reed) to compare its efficiency of pollutant removals with the non-vegetated system, which had the same design. COMSOL Multiphysics 3.5a was operated for the Activated Sludge Model 2 (ASM2) to predict the chemical oxygen demand (COD) and ammonia-nitrogen (NH4-N) concentrations. The effluent quality of both treatment systems was assessed for several parameters. Computer simulations show a good compliance between the measured and predicted values of COD and NH4-N for the vegetated system. The calibrated model could be effectively used to predict the behaviours of those parameters as a function of time. Moreover, the effluents of both vegetated (VFp) and non-vegetated (VF) VSSF-CW were significantly (p < 0.05) improved compared to influent. Significant (p < 0.05) effects due to the presence of P. australis were observed for removals of total suspended solids (TSS), 5-day biochemical oxygen demand (BOD5), COD, NH4-N and ortho-phosphate-phosphorus (PO4-P). However, significant increases (p < 0.05) were noted for electrical conductivity (EC), total dissolved solids (TDS), nitrate-nitrogen (NO3-N) and sulphate (SO4) of both effluents compared to the raw wastewater. Except for EC, NH4-N and SO4, all water quality parameters complied with irrigation water standards.