NITROGEN REMOVAL IN LABORATORY-SCALE CONSTRUCTED WETLANDS
1Achintya N. Bezbaruah, 2Tian C. Zhang, and 3John S. Stansbury
1,2,3Department of Civil Engineering, 205 PKI, University of Nebraska-Lincoln at Omaha Campus, Omaha, NE 68182-0178; 1Phone: (402) 554-3565, E-mail: ; 2Phone: (402) 554-3784, E-mail: ; 3Phone: (402) 554-3896, E-mail: .
As an alternative to conventional biological wastewater treatment systems, the laboratory-scale subsurface constructed wetland was used in this study for COD and nitrogen removal in a single reactor. Two reactors, 11.5' x 0 5' x 3' each, have been constructed side by side, and located in a controlled environment chamber. The reactors are fed with primary settled municipal wastewater from a local treatment plant. The first reactor is for COD removal, nitrification, and heterotrophic denitrification, while the second one is for COD removal, nitrification and sulfur/limestone (S/L) autotrophic denitrification. A polishing zone has been kept in both reactors and they are equipped with aeration pipes for optional compressed air aeration. Plants are not grown in the reactors so as to exclude their effect in the process. The parameters monitored are organic, ammonium, and hydraulic loading rates, and hydraulic retention times (HRTs), COD, NH4+-N, NO3-N, DO, pH, etc. The wetlands have been operated at several different temperatures and hydraulic retention times (HRTs).
Results so far obtained suggest ammonia-nitrogen removal and nitrification up to 65% in the first wetland. When compressed air aeration is not used, a stratified pattern of ammonia-nitrogen removal and nitrification has been observed, these being slightly faster at the surface than at the bottom. This suggests that DO may be one of the limiting factors for nitrogen removal in a constructed wetland. It seems that the limiting DO concentration is ca. 1 mg/l for nitrification to take place and hence, nitrogen removal. The temperature effect on COD and nitrogen removal has been investigated by evaluating the first-order reaction rate constant and temperature activity coeffficient (). The ammonia and nitrate removal rates do not increase much beyond 18°C. Sulfur-limestone autotrophic denitrification is very efficient (>95%), indicating that the rate-limiting step for nitrogen removal in the second wetland is nitrification. No SO42 is detected in the effluent of the second wetland, although it is detected immediately after the S/L section. In both reactors, NO3-N removal increases with an increase of HRT, while COD is removed very efficiently (up to 98%) at all HRTs. Currently, the effect of the ratios of COD/ammonium and COD/nitrate-N on nitrification and denitrification in these wetlands are being investigated. The results of this study seem to indicate that subsurface constructed wetlands can be used to treat wastewater generated by small communities.
Key words: constructed wetland, ammonia, nitrate, nitrification, denitrification, COD