A comparative study of pollutant removal stability in hybrid wetland columns

Abstract

This study investigated the effects of two alternative substrates (wood mulch and zeolite) on the performance of three laboratory-scale hybrid wetland systems that had identical system components and configurations. Each system consisted of a vertical flow (VF) wetland column, followed by a horizontal flow (HF) column and a vertical flow column. The substrates employed were wood mulch, gravel and zeolite, and Phragmites australis were planted in each column. The systems received synthetic wastewater, with pollutant loadings in the range of 8.5-38.0 g/(m2·d) total nitrogen (TN) and 4.0-46.4 g/(m2·d) biological oxygen demand (BOD5). Wood mulch and zeolite substrates showed higher efficiencies in terms of removing nitrogenous compounds and biodegradable organics. The supply of organic carbon from the organic mulch substrates enhanced denitrification, while adsorption of influent ammoniacal nitrogen (NH4—N) in zeolite played a major role in the removal of nitrogenous species in the wetland columns. Overall, the average percentage removals of TN and BOD5 reached >66% and >96% respectively, indicating stable performances by the hybrid wetland systems under the experimental loading ranges. Mathematical models were developed, based on the combination of Monod kinetics and continuously-stirred tank reactor (CSTR) flow patterns to describe the degradation of nitrogenous compounds. Predictions by the models closely matched the experimental data, indicating the validity and potential application of Monod kinetics in the modelling and design of treatment wetlands.