Nitrogen Removal from Stormwater: A Pilot Plant Research Project

A research pilot plant has been operated for over 150 days to investigate nitrogen removal from urban stormwater. Parallel zeolite and sand filters were operated at 0.125 gal/ft²-day on storm pond water amended with ammonia nitrogen (NH₃-N). The zeolite filter contained chabazite 500H with an ion exchange capacity of 2.5 meq./gram. NH₃-N reduction was much higher in the zeolite filter for the first 28 days, presumably due to ion exchange. Adjustments to influent dissolved oxygen (DO) and NH₃-N levels led to improved performance by both filters over the next 64 days. During a subsequent 14 day period, zeolite and sand filters reduced 0.781 average influent NH₃-N by 93.3 and 86.7%, respectively. Nitrite and nitrate concentrations increased through both filters, indicating that nitrification had been established. Further analysis indicated that pond water contained organic nitrogen (ca 1 mg/L) that was not substantially removed though zeolite or sand filters. The organic nitrogen was predominantly dissolved organic nitrogen (DON) that was not retained by a 0.45 micrometer filter. To provide a preliminary assessment of non-steady performance, a flowrate perturbation was performed to test columns containing zeolite and sand media. NH₃-N levels rose appreciably in sand column effluent, while the column containing zeolite media was highly effective in preventing NH₃-N breakthrough.