Wetlands and Aquatic Processes Particulates, Not Plants, Dominate Nitrogen Processing in a Septage-Treating Aerated Pond System M. Robert Hamersley,* Brian L. Howes, and David S. White ABSTRACT to NO x via oxygen (O 2 ) loss from roots (Reddy et al., 1989). Plants are sources of organic carbon that can In pond and wetland systems for wastewater treatment, plants are support the denitrification of NO x to nitrogen gas (N 2 ) often thought to enhance the removal of ammonium and nitrogen through the activities of root-associated bacteria. In this study, we by providing carbon substrate and/or creating anaerobic examined the role of plant roots in an aerated pond system with microsites (Gersberg et al., 1986; Hamersley and Howes, floating plants designed to treat high-strength septage wastewater. 2002). Microenvironments surrounding submerged roots We performed both laboratory and full-scale experiments to test the and leaves may also indirectly affect N cycling by sup- effect of different plant root to septage ratios on nitrification and porting the activities of nitrifying and/or denitrifying denitrification, and measured the abundances of nitrifying bacteria bacteria. Plant roots create fixed attachment sites for associated with roots and septage particulates. Root-associated nitrify- N-processing bacteria, potentially increasing populations ing bacteria did not play a significant role in ammonium and total (Kaplan, 1983). nitrogen removal. Investigations of nitrifier populations showed that Direct N uptake by plants is proportional to biomass only 10% were associated with water hyacinth [Eichhornia crassipes (Mart.) Solms] roots (at standard facility plant densities equivalent production rate and plant N content. Water hyacinth to 2.2 wet g roots L -1 septage); instead, nitrifiers were found almost has frequently been used for wastewater N removal entirely (90%) associated with suspended septage particulates. The because of its growth rates (42 dry g m -2 d -1 ) and N role of root-associated nitrifiers in nitrification was examined in labo- uptake (19.7 mg N m -2 h -1 ) (Reddy and Sutton, 1984). ratory batch experiments where high plant root concentrations (7.4 Nitrogen removal by plant uptake requires regular bio- wet g L -1 , representing a 38% net increase in total nitrifier populations mass harvest, or else biomass N storage reaches steady over plant-free controls) yielded a corresponding increase (55%) in state, where uptake is balanced by the return of N into the non-substrate-limited nitrification rate (V max ). However, within the wastewater through senescence, leaching, and decay. the full-scale septage-treating pond system, nitrification and denitrifi- The benefits of N removal through plant uptake are cation rates remained unchanged when plant root concentrations were increased to 7.1 g roots L -1 (achieved by increasing the surface area also limited by the potentially costly composting and available for plants while maintaining the same tank volume). Under drying procedures required for biomass disposal (Bag- normal facility operating conditions, nitrification was limited by am- nall et al., 1987; DeBusk and Reddy, 1987). Regardless, monium concentration, not nitrifier availability. Maximizing plant root plant uptake is only a partial solution to the problem concentrations was found to be an inefficient mechanism for increasing of N disposal, since N is merely transferred from inor- nitrification in organic particulate-rich wastewaters such as septage. ganic to organic forms, which can subsequently re-release inorganic N to the environment on decay. Although plant uptake can remove NH 4 from waste- N itrogen (N) removal from wastewater is mandated water, oxidation to NO x by nitrifying bacteria is the in most jurisdictions because of the toxicity of am- dominant removal mechanism in most biological treat- monium (NH 4 ) to aquatic fauna and the contribution ment systems. The contribution of O 2 lost through wet- of N compounds (including nitrite + nitrate [NO x ]) to land plant roots (0.02–9.6 g m -2 d -1 ) to wastewater aera- the eutrophication of aquatic ecosystems. In pond and tion may be small relative to O 2 diffusion from the gravel bed wastewater treatment systems engineered atmosphere (Moorhead and Reddy, 1988; Howes and for N removal, aquatic macrophytes are thought to con- Teal, 1994), and unaerated constructed wetlands typi- tribute to N removal through a variety of mechanisms cally have low dissolved oxygen (DO) concentrations (Orth and Sapkota, 1988; Peterson and Teal, 1996; (approximately 0.5 mg L -1 ). As a result, long retention Weisner et al., 1994). Early research on plant roles in times are required to reduce NH 4 concentrations to lev- wastewater N removal focused on N uptake through els that allow discharge to aquatic environments (Dinges biomass production (Reddy and Sutton, 1984). Aquatic and Doersam, 1987; Reed and Brown, 1995). Although plants may also promote microbial nitrification of NH 4 O 2 translocation is critical to aquatic plant survival, in practice it does not seem to contribute significantly to M.R. Hamersley, Department of Biology, Woods Hole Oceano- nitrification within wetland treatment systems (Watson graphic Institution, Woods Hole, MA 02143. B.L. Howes, D.S. White, et al., 1990; Reed and Brown, 1995). The high O 2 de- and M.R. Hamersley (current address), School for Marine Science and Technology, University of Massachusetts, 706 Rodney French Boulevard, New Bedford, MA 02744-1221. Received 12 Nov. 2002. Abbreviations: DO, dissolved oxygen; DON, dissolved organic nitro- *Corresponding author (rhamersley@umassd.edu). gen; K M , half-saturation (Michaelis) constant; MPN, most probable number; ON, organic nitrogen; PON, particulate organic nitrogen; Published in J. Environ. Qual. 32:1895–1904 (2003).  ASA, CSSA, SSSA TN, total nitrogen; TSS, total suspended solids; V max , maximum (non- substrate-limited) nitrification rate. 677 S. Segoe Rd., Madison, WI 53711 USA 1895