ASSESSING THE EFFECTIVENESS OF HETEROTROPHIC AND AUTOTROPHIC DENITRIFICATION PATHWAYS AS A TERTIARY TREATMENT OF EFFLUENT WASTEWATER

In a tidal wetland wastewater treatment plant, nitrate is removed by denitrification and by assimilation. These biogeochemical processes, however, do not result in complete removal of nitrates because of the short residence time of water in the treatment system. Thus, additional alternative denitrification pathways are required to remove excess nitrate from effluent water. The tidal wetland system used in this experiment currently converts 96% of ammonium to nitrate, and subsequently denitrifies 62% of the nitrate during treatment. The purpose of this research is to determine if additional heterotrophic and autotrophic denitrification pathways are effective tertiary treatment process for the removal of nitrogen from effluent in a tidal wetland system. The experiment was conducted using two identical mesocosm columns 140 cm tall x 46 cm wide. The heterotrophic column was filled with bleached wood chips and the autotrophic column was filled with a 3:1 by mass mixture of elemental sulfur and marble chips. For each experiment, 240 liters of treated effluent were cycled through the column hourly. Samples were collected hourly for the first 24 hours and then every 3 days for two weeks. The initial nitrogen concentration of the effluent was 9.15 mg N/L. Initial results show that the heterotrophic column reduced nitrogen concentrations by 95% after 1 day and 97% after 9 days, equilibrating at a concentrations of 0.26 mg N/L. The autotrophic column reduced nitrogen concentrations by 95% after 9 days, equilibrating at a concentration of 0.48 mg N/L. Similar experiments have obtained denitrification rates as high as 88%, thus these denitrification results are better than those observed in other studies. However, treatment by the heterotrophic column resulted in high dissolved organic carbon concentrations (>60 mg C/L) and a dark brown color. Treatment by the autotrophic column resulted in acidic pH (<3), high turbidity (>50 NTU), and high concentrations of calcium (>100 mg/L) and sulfate (>9000 mg/L). Initial data suggests both the autotrophic and heterotrophic denitrification pathways enhance removal of nitrogen. However, each treatment pathway causes significant additional water quality issues which would likely make both systems unsuitable for the purpose of greywater reclamation and reuse.