Pollutant removal employing tidal flow constructed wetlands: Media and feeding strategies

Abstract

Six partially saturated and unsaturated tidal flow constructed wetlands (TFCWs) were studied for the removal of nitrogen, phosphorus and organics from municipal wastewater. The TFCWs were packed with organic (biochar, coal, coco-peat), waste (slag), construction (gravel, concrete block) materials and planted with Phragmites or Vetiver. Daily wastewater feeding cycle was divided into sub-cycles across all TFCWs. Experimental analyses illustrated higher nitrogen and organics removals in organic media based TFCWs (71–85% and 84–96%, respectively), when compared with those of waste, construction material based units (49–69% and 74–95%, respectively); carbon availability from organic materials increased denitrification that was also supported by energy dispersive spectroscopy (EDS) analyses. Construction and waste material based TFCWs achieved better P removals (≥93%); Ca/Al/Fe ingredients of such materials allowed P removal via adsorption. Increment of wastewater contact period inside the media triggered physico-chemical and microbial removal routes. Partially saturated coal packed TFCW was the most efficient unit in terms of N removal percentage; presence of stationary water volume created anoxic environment and reduced input load, thereby improving removal performances. In terms of removal rates (g/m2 d), unsaturated TFCWs showed higher removal (over partially saturated units), which could be linked with greater input load. Mass balance analyses indicated ≤3% N removal through plant uptake; variation of plant species did not influence N removal. Therefore, observed removal kinetics was governed by adsorption and microbial routes. The results of this study suggest that, feeding sub-cycles could improve pollutant removals in TFCWs when packed with specific media.