AMMONIUM REMOVAL IN CONSTRUCTED WETLANDS WITH RECIRCULATING SUBSURFACE FLOW - REMOVAL RATES AND MECHANISMS

From June 1993 through February 1994, the removal of NH4-N was evaluat ed in constructed wetlands at the TVA constructed wetland research fac ility in Muscle Shoals, AL. The objectives were to determine rates for NH4-N removal and speculate on potential mechanisms for removal. Nine constructed wetland cells were used with approximate dimensions of 9. 1 x 6.1 x 0.6 m(3) and a recirculating subsurface flow system in a gra vel base. Treatments consisted of an unplanted (WO=control) and two po lycultural planting schemes (P1=Scirpus acutus, Phragmites communis an d Phalaris arundinacea; P2=Typha sp., Scirpus atrovirens georgianus an d Scirpus cyperinus) replicated 3 times. Salt solutions were added and recirculated in each cell resulting in initial concentrations of 50 a nd 300 mg l(-1) of NK4-N and COD, respectively, when fully diluted wit h wetland water. Salts were added to wetlands approximately every 6 we eks with the first addition on June 1, 1993 and the last addition on F ebruary 9, 1994 for a total of 6 time periods (times I, II, III, IV, V and VI). The COD of the waters was removed at rates ranging from 5.5 to 10 g/m(2)/d during times I through IV with no discernible differenc e amongst the planting treatments. Wetland cells with PI were more eff icient at removing NH4-N (1.1 g/m(2)/d) than P2 (0.6 g/m(2)/d) or WO ( 0.5 g/m(2)/d) at time I with differences decreasing by time IV (0.3 to 0.7 g/m(2)/d). During the winter (times V and VI), there were no diff erences in NH4-N removal amongst planting treatments with an average r emoval rate of 0.35 g/m(2)/d. There was a seasonal change in NH4-N rem oval in all the treatments, with the change most noticeable in the pla nted cells. The removal of NH4-N in WO was speculated to be due to a c ombination of sorption onto gravel, microbial assimilation, and nitrif ication at the air-water interface, The extra NH4-N removal in the pla nted cells diminished in the winter because the removal was most likel y due to a combination of enhanced nitrification from O-2 transport an d NH4-N uptake mediated by seasonal macrophyte growth.