Effect of water level fluctuation on nitrogen removal from constructed wetland mesocosms

Nitrogen removal processes were investigated at three frequencies of water level fluctuation, static, low and high (0, 2 and 6 d(-1)), in duplicate gr avel-bed constructed wetland mesocosms (0.145 m(3)) with and without plants (Schoenoplectus tabernaemontani). Fluctuation was achieved by temporarily pumping wastewater into a separate tank (total drain time similar to 35 min ). Intensive sampling of the mesocosms, batch-fed weekly with ammonium-rich (similar to 100 g m(-3) NH4-N) farm dairy wastewaters, showed rates of che mical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal increas ed markedly with fluctuation frequency and in the presence of plants. Nearl y complete removal of NH4-N was recorded over the 7 day batch period at the highest level of fluctuation, with minimal enhancement by plants. Redox po tentials (Eh) at 100 mm depth rose from initial levels of around - 100 to > 350 mV and oxidised forms of N (NO2 and NO3) increased to similar to 40 g m(-3), suggesting conditions were conducive to microbial nitrification at t his level of fluctuation. In the unplanted mesocosms with low or zero fluct uation, mean NH4-N removals were only 28 and 10%, respectively, and redox p otentials in the media remained low for a substantial part of the batch per iods (mid-batch Eh similar to + 100 and - 100 mV, respectively). In the pre sence of wetland plants, mean NH4-N removal in the mesocosms with low or ze ro fluctuation rose to 71 and 54%, respectively, and COD removal (>70%) and redox potential (mid-batch Eh> 200 mV) were markedly higher than in the un planted mesocosms. Negligible increases in oxidised N were recorded at thes e fluctuation frequencies, but total nitrogen levels declined at mean rates of 2.4 and 1.8 g m(-2) d(-1), respectively. NH4-N removal from the bulk wa ter in the mesocosms was well described (R-2 = 0.97-0.99) by a sorption-pla nt uptake-microbial model. First-order volumetric removal rate constants (k (v)) rose with increasing fluctuation frequency from 0.026 to 0.46 d(-1) wi thout plants and from 0.042 to 0.62 d(-1) with plants. As fluctuation frequ ency increased, reversible sorption of NH4-N to the media, and associated b iofilms and organic matter, became an increasingly important moderator of b ulk water concentrations during the batch periods. TN mass balances for the full batch periods suggested that measured plant uptake estimates of betwe en 0.52 and 1.07 g N m(-2) d(-1) (inversely related to fluctuation frequenc y) could fully account for the increased overall removal of TN recorded in the planted systems. By difference, microbial nitrification-denitrification losses were therefore estimated to be approximately doubled by low-level f luctuation from 0.7 to 1.4 g N m(-2) d(-1) (both with and without plants), rising to a maximum rate of 2.1 g N m(-2) d(-1) at high fluctuation, in the absence of competitive uptake by plants. (C) 1999 Elsevier Science B.V. Al l rights reserved.