Pollutant removals employing unsaturated and partially saturated vertical flow wetlands: A comparative study
School of Engineering
This paper provides a comparative evaluation between unsaturated and partially saturated vertical flow (VF) wetlands, that were employed to investigate nitrogen, organics, sulfate and coliform removals from municipal wastewater. The wetlands included organic straw materials as the main media and were planted with Canna indica species. Media saturation proportion in partially saturated systems ranged between 15.0 and 45.0% (with respect to media volume). Performance analyses indicated simultaneous nitrogen (52.7%) and biodegradable organics (47.0%) removals in unsaturated VF wetland. Mean ammoniacal nitrogen (NH4-N) and total nitrogen (TN) removal rates were recorded as 1.30 and 1.26 g/m2 d, respectively. Organic carbon (C) leaching from the straw media supported denitrification of the nitrified products in unsaturated system. In contrast, nitrification was the limiting step in partially saturated VF wetlands due to the development of anoxic/anaerobic conditions inside the media. Nitrogen removal percentages ranged between 29.0 and 42.0% between these wetlands. Decrease of aerobic zones also reduced aerobic organic removals in these systems. Nitrogen and organics removals improved across unsaturated VF wetland, when input hydraulic loadings were 50.0% reduced. In contrast, saturated VF wetlands showed lower nitrogen removal rates when subjected to reduced hydraulic loadings, probably due to uneven distribution of wastewater flow (inside the media). Partially saturated VF system (with 45.0% saturation ratio) showed higher sulfate removals (41.6%); mean sulfate removal rate was 0.75 g/m2 d. Anaerobic conditions of such wetland coupled with C availability from the straw media allowed observed removals. Coliform mortality was higher in VF wetlands that were operated under more aerobic conditions.