Oxygen Transport through Aquatic Macrophytes: The Role in Wastewater Treatment K. R. Reddy,* E. M. D’Angeloand T. A. DeBusk ABSTRACT Laboratory experiments were conducted to determine the effec- tiveness of three floating and six emergent aquatic macrophytes in improving domestic wastewater quality, based on their capacities for O2 transport into the effluent. Oxygen transport into the rooting zone of theplants created an oxidized microenvironment, thereby stim- ulating Cand Ntransformations criticalto wastewater treatment. Plants were cultured in flasks containing deoxygenated primary and secondary sewage effluent for an 8-dperiod. Oxygen transport by the plants was measured in terms of both O2 consumed by the effluent (biological 02 demand reduction--BODs) and increased effluent dis- solved 02. Two floating plants, pennywort (Hydwcotyle nmbellata L.)and waterhyacinth [Eichkornia crasslpes (Mart.) Solms], and the emergent plants pickerelweed (Pontederla cordata L.) and common arrowhead (Sagittaria latifolia L.), were superior in improving pri- mary sewage effluent quality, by reducing BOD5 upto 88%, NH4- N up to 77%, and increasing dissolved O2 up to 6.1 mg L -I. Nitri- fication rates in pennywort- and water hyacinth-based water treat- ment systems were calculated to bein the range of 12to 47kg NH 4- N ha -1 d -!. Oxygen transport through plants accounted for up to 90% of the totalO2 transported into the effluent. In separate batch experiments, the effectiveness of diffuse mechanical aeration (5 and 50 mL air min -t) and of biological aeration (02 transport by selected plants including pennywort, waterhyacinth, pickerelweed, and com- mon arrowhead) onthe rateof contaminant removal from deoxy- genated primary sewage effluent were compared for a 26-d period. Biological and mechanical aeration effected similar BODs removal. First-order reaction rateconstants for BOD~ removal were from 0.0066 to 0.0079 h -~ and from 0.0041 to 0.0051 h -1 forbiological and mechanical aeration, respectively. Rate constants for NH4-N removal were from 0.0024 to 0.0107 h -~ for theplant treatments. Virtually complete BOD~ removal occurred in biological and me- chanical aeration treatments within 20d. Complete nitrification of NH4-N had occurred within 12d aftermechanical aeration was in- itiated, but subsequent N-loss by denitrification was inhibited. In the biological aeration treatments, negligible effluent (NO 3 + NO2)- Nlevelswere measured, but 65to 100% NH4-N loss occurred both by plant assimilation and by sequential nitrification-denitrification reactions. A QUATIC PLANTS rooted in anaerobic sediments and anoxic waters transport 02 through stems and leaves into their rooting zones. The mechanism ofO2 transport through aquatic plants into the rooting zone has been demonstrated by several researchers (Armstrong, 1964, 1967; Conway, 1937; Dacey, 1980; Grosse and Mevi-Schutz, 1987; Moorhead and Reddy, 1988; Teal and Kanwisher, 1966). Oxygen is used by plant roots for aerobic respiration. Indeed, tolerance to flooding is determined primarily by the capacities of plants to aerate their root systems (Armstrong, 1978). Moreover, rhizosphere oxidation detoxifies H2S K.R. Reddy and E.M. D’Angelo, Soil Science Dep., I06Newell Hall, Univ. of Florida, Inst. of Food and Agile. Sci., Gainesville, FL 32611-0313; and T.A. DeBusk, Reedy Creek Energy Services, Inc., P.O. Box 10000, Lake Buena Vista,FL 32830. Florida Agric. Exp. Stn. Journal Series R-00084. Received 1 Feb.1989. *Corresponding author. Published in J. Environ. Qual. 19:261-267 (1989). along with reduced forms of Fe and Mn(Armstrong, 1971; Gambrell and Patrick, 1978; Ponnamperuma, 1965) and activates biogeochemical processes includ- ing oxidation of soluble organic compounds and ni- trification of NH4-N (Goodand Patrick, 1987; Reddy et al., 1989). Thus, rhizosphere oxidation has impor- tant implications in wastewater treatment. In the past decade, use of the aquatic macrophytes cultured in shallow ponds and artificial wetlands for wastewater treatment has proved economically feasi- ble for small communities (Crites and Mingee, 1987; Duffer, 1982; Reedet al., 1988). To improvewaste- water treatment efficiency and to reduce production of odors and mosquitoes (Culex spp. and Anopheles spp.), mechanical aeration was suggested for ponds containing floating aquatic macrophytes (Reedet al., 1988), since 02 transport through the plants was thought to be insufficient to support maximum root and microbial respiration. However,a recent study (DeBusk et al., 1989) has shown no consistent water quality improvement in waterhyacinth ponds having mechanical aeration. Weber and Tchobanoglous (1986) and DeBusk et al. (1989) also did not recom- mend mechanical aeration, because of its energy re- quirements. The success of using aquatic plants for secondary treatment of primary sewage effluent is dependenton the capacity of plants to transport O2 into the root zone, with subsequent utilization of the excess 02 dur- ing microbial respiration. Selection of suitable aquatic macrophytes plays a critical role in optimizing these systems for maximum contaminant removal. Numerous studies have evaluated the potential nu- trient removalvia plant assimilation for wastewater treatment (Reddy, 1983, 1984; Reddy and DeBusk, 1985), but the role of 02 transport by several aquatic macrophytes commonly used for wastewater treat- ment has yet to be studied intensively. To evaluate this mechanism, laboratory experiments were con- ducted with the following objectives: (i) to determine the 02 transport capacity of aquatic macrophytes as determined by wastewater BOD5 (biological 02 de- mand) reduction, and (ii) to compare the efficiency biological aeration (O2 transport through plants) with mechanicaldiffuse aeration in reducing BOD5 of pri- marysewage effluent. MATERIALS AND METHODS Aquatic plants wereobtained fromthe St. Johns River marsh and werecultured in 10% modified Hoagland nutrient solutions until used in the experiments (Reddy andDeBusk, 1985). Atotal of nine plant treatments (three floating and six emergent macrophytes) wereevaluated(see Table1 for list of plants). Many of these plants are currentlyused in aquatic plant-based water treatment systems(Tchobanog- lous, 1987;Wolverton, 1987). Primary sewage ettluent was obtained from the Reedy CreekImprovement District sew- age treatment facility locatedat Lake Buena Vista, FL. Sta- tistical significance of the data was evaluated using the gen- eral linear model (SAS Inst., 1985). 261