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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Particulates, Not Plants, Dominate Nitrogen Processing in a Septage-Treating Aerated Pond System

^a Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02143
^b School for Marine Science and Technology, University of Massachusetts, 706 Rodney French Boulevard, New Bedford, MA 02744-1221

^* Corresponding author (rhamersley{at}umassd.edu).

Received for publication November 12, 2002. In pond and wetland systems for wastewater treatment, plants are often thought to enhance the removal of ammonium and nitrogen through the activities of root-associated bacteria. In this study, we examined the role of plant roots in an aerated pond system with floating plants designed to treat high-strength septage wastewater. We performed both laboratory and full-scale experiments to test the effect of different plant root to septage ratios on nitrification and denitrification, and measured the abundances of nitrifying bacteria associated with roots and septage particulates. Root-associated nitrifying bacteria did not play a significant role in ammonium and total nitrogen removal. Investigations of nitrifier populations showed that only 10% were associated with water hyacinth [Eichhornia crassipes (Mart.) Solms] roots (at standard facility plant densities equivalent to 2.2 wet g roots L^-1 septage); instead, nitrifiers were found almost entirely (90%) associated with suspended septage particulates. The role of root-associated nitrifiers in nitrification was examined in laboratory batch experiments where high plant root concentrations (7.4 wet g L^-1, representing a 38% net increase in total nitrifier populations over plant-free controls) yielded a corresponding increase (55%) in the non-substrate-limited nitrification rate (V_max). However, within the full-scale septage-treating pond system, nitrification and denitrification rates remained unchanged when plant root concentrations were increased to 7.1 g roots L^-1 (achieved by increasing the surface area available for plants while maintaining the same tank volume). Under normal facility operating conditions, nitrification was limited by ammonium concentration, not nitrifier availability. Maximizing plant root concentrations was found to be an inefficient mechanism for increasing nitrification in organic particulate-rich wastewaters such as septage.

Abbreviations: DO, dissolved oxygen • DON, dissolved organic nitrogen • K_M, half-saturation (Michaelis) constant • MPN, most probable number • ON, organic nitrogen • PON, particulate organic nitrogen • TN, total nitrogen • TSS, total suspended solids • V_max, maximum (non-substrate-limited) nitrification rate

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JEQ 2003 32: 1577-1582. [Full Text]