Effects of Different Substrates and Hydraulic Retention Time (HRT) on the Removal of Total Nitrogen and Organic Matter in a Sub-Surface Horizontal Flow Constructed Wetland

Abstract

The aim of this study was to investigate the treatment efficiency of constructed wetlands (CWs) planted with phragmites australis and scirpus maritimus. The wetland systems operated under horizontal subsurface flow using three different substrates at varied hydraulic retention times. Each wetland type consisted either of 0.36 m2 alum sludge (AS), gravel (G), or zeolite (Z) as substrate. All systems were subjected to two hydraulic retention times (HRT) of 4 and 3 days respectively. Each wetland bed received 0.012 m3/d to 0.03 m3/d of synthetic wastewater corresponding to a hydraulic loading rate of 0.035 to 0.067 m/d and a COD loading rate of 0.026 kg COD (m2/d)╶ 1 to 0.035 kg COD (m2/d). The constructed wetland using zeolite as the substrate achieved significantly higher COD and TN removal with the 4 day HRT compared to gravel and alum sludge substrates. The average COD removal efficiency whilst using zeolite was approximately 88%, followed by 78% for gravel and 67% for alum sludge. Average removal efficiency for total nitrogen was 96% in zeolite, 43% for gravel and 20% for alum sludge. Changing the HRT to 3 days, showed a slight decreased in the COD removal efficiency to 85 % using zeolite. However, the gravel and alum sludge substrates showed an increase in COD removal reported as 93% and 91% respectively. Similar removal patterns were found for nitrogen at the three days retention time. Nitrogen removal by zeolite and gravel were found to be low and approximately 3% for zeolite and 29% for gravel. Total nitrogen removal efficiency using alum sludge increased slightly to 4%. It was observed that the substrate type and retention times had a major effect on the removal efficiency of organic matter and total nitrogen in CWs.