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 evaluated in constructed wetlands at the TVA constructed wetland research facility 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 × 6.1 × 0.6 m3 and a recirculating subsurface flow system in a gravel base. Treatments consisted of an unplanted (WO=control) and two polycultural planting schemes (P1=Scirpus acutus, Phragmites communis and Phalaris arundinacea; P2=Typha sp., Scirpus atrovirens georgianus and Scirpus cyperinus) replicated 3 times. Salt solutions were added and recirculated in each cell resulting in initial concentrations of 50 and 300 mg l−1 of NH4-N and COD, respectively, when fully diluted with wetland water. Salts were added to wetlands approximately every 6 weeks with the first addition on June 1, 1993 and the last addition on February 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/m2/d during times I through IV with no discernible difference amongst the planting treatments. Wetland cells with P1 were more efficient at removing NH4-N (1.1 g/m2/d) than P2 (0.6 g/m2/d) or WO (0.5 g/m2/d) at time I with differences decreasing by time IV (0.3 to 0.7 g/m2/d). During the winter (times V and VI), there were no differences in NH4-N removal amongst planting treatments with an average removal rate of 0.35 g/m2/d. There was a seasonal change in NH4-N removal in all the treatments, with the change most noticeable in the planted cells. The removal of NH4-N in WO was speculated to be due to a combination of sorption onto gravel, microbial assimilation, and nitrification at the air-water interface. The extra NH4-N removal in the planted cells diminished in the winter because the removal was most likely due to a combination of enhanced nitrification from O2 transport and NH4-N uptake mediated by seasonal macrophyte growth.

A Continuous-Belt Herbicide Wiper

A continuous-belt herbicide wiper was developed to apply herbicides to weeds that are taller than cranberry (Vaccinium macrocarponAit.) plants. An endless belt with a sponge attached to the entire length of the belt moves through a reservoir containing a herbicide solution. A variable pressure wheel squeezes the excess liquid out of the sponge and returns it to the reservoir. The herbicide is deposited on the leaves of the weeds as the belt moves in a horizontal plane, perpendicular to the forward movement of the wiper. Control of Walter's sedge (Carex walterianaBailey), woolgrass [Scirpus cyperinus(L.) Kunth] and switchgrass (Panicum virgatumL.) was obtained when glyphosate [N-(phosphonomethyl)glycine], dalapon (2,2-dichloropropionic acid), and paraquat (1, 1′-dimethyl-4,4′-bipyridinium ion) were applied using the continuous-belt wiper. No injury to cranberry plants was observed.