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Assessing the Bioavailability of Effluent Organic Nitrogen (EON) Using a Suite of Water Quality-Based Assays

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Effluent from wastewater treatment plants includes inorganic and organic nitrogen (N). We term the organic N in effluent, EON. Quantitative data on EON bioavailability in waters down stream of wastewater treatment plant discharge points is lacking and there is currently no standard method to accurately assess EON bioavailability to any system. Any method that is developed to determine EON bioavailability must take into account a number of variables. It must account for uptake in the proximate receiving waters (typically freshwater) as well as the estuarine and saline waters down stream. It must also be sensitive to changing microbial ecology and environmental conditions along the estuarine gradient, including the generation of photodegradation products and changes in salinity.

Here we review results of recent bioavailability, photochemical release and salinity release assays used to quantify the percentage of EON that is bioavailable along an estuarine gradient. The EON used in these experiments came from multiple wastewater treatment plants. If a sizable fraction of EON is determined to be non-bioavailable by robust assessment methods such as those proposed here, then perhaps the cost of N removal by utilities can be substantially reduced or those facilities that have already invested in enhanced nitrogen removal upgrades will be able to justify selling credits in regions where nutrient trading is allowed.

Results from the assays conducted to date indicate that: 1) EON bioavailability can be quantified in relatively short-term (< 7 days) bioassays; 2) that a measurable fraction of EON was utilized by microbial communities along an estuarine gradient at a range of salinities, but that complete EON uptake was not observed; 3) that exposure of filtered EON to UV radiation resulted in the release of labile ammonium and dissolved primary amines; and 4) that exposure of EON to increased salinities resulted in the release of ammonium (Bronk et al. and Filippino et al. in preparation). Collectively these results indicate that EON can be a source of bioavailable N to estuarine microbial communities and thereby contributes to coastal eutrophication, but that not all EON is bioavailable as assumed by regulatory agencies.
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Keywords: Effluent; bioassay; organic nitrogen; photochemistry; salinity

Document Type: Research Article

Publication date: 01 January 2009

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