NITROGEN REMOVAL FROM RECLAIMED WATER APPLIED TO CONSTRUCTED AND NATURAL WETLAND MICROCOSMS

A series of constructed and natural wetlands are currently being used by Orange County, Florida to polish reclaimed municipal wastewater. The system consists of constructed wetlands with low organic matter sandy soils, alternating with natural wetlands with organic soils. The hydrologic flow pattern is through constructed wetland --> natural wetland --> constructed wetland --> natural wetland. Nitrogen removal potential of these wetlands was measured in (1) soil microcosms of each wetland batch fed with reclaimed water at a hydraulic retention time (HRT) of 21 days (experiment I), and (2) soil microcosms of each wetland sequentially batch fed for an HRT of 5 days per wetland for a total HRT of 20 days for the system (experiment II). The experiments were conducted in a greenhouse maintained at 18-32-degrees-C. Floodwater nitrogen removal, expressed as first-order rate constants, ranged from 0.086 (+/-0.030) to 0.214 (+/-0.020) days-1 for the wetland soils studied. Nitrogen removal was more rapid in wetlands with mineral soils than organic soils. Based on these rates, 50% of the inorganic nitrogen loaded to these wetlands would be removed at an HRT of 3-8 days. The first-order rate constants obtained from experiment I were used to predict the floodwater nitrogen removal in experiment II (sequentially batch fed system) and nitrogen removal at the field site. At ambient nitrogen addition levels, the agreement between predicted and measured values was highly significant (slope = 0.98, R2 = 0.98). The overall nitrogen removal efficiency was in the range of 83-89% in the wetlands evaluated. Nitrification and denitrification appear to be major nitrogen removal mechanisms in the wetlands.