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

Abstract

Nitrogen removal processes were investigated at three frequencies of water level fluctuation, static, low and high (0, 2 and 6 d −1), in duplicate gravel-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 ∼35 min). Intensive sampling of the mesocosms, batch-fed weekly with ammonium-rich (∼100 g m −3 NH 4-N) farm dairy wastewaters, showed rates of chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) removal increased markedly with fluctuation frequency and in the presence of plants. Nearly complete removal of NH 4-N was recorded over the 7 day batch period at the highest level of fluctuation, with minimal enhancement by plants. Redox potentials (Eh) at 100 mm depth rose from initial levels of around −100 to >350 mV and oxidised forms of N (NO 2 and NO 3) increased to ∼40 g m −3, suggesting conditions were conducive to microbial nitrification at this level of fluctuation. In the unplanted mesocosms with low or zero fluctuation, mean NH 4-N removals were only 28 and 10%, respectively, and redox potentials in the media remained low for a substantial part of the batch periods (mid-batch Eh ∼+100 and −100 mV, respectively). In the presence of wetland plants, mean NH 4-N removal in the mesocosms with low or zero 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 unplanted mesocosms. Negligible increases in oxidised N were recorded at these fluctuation frequencies, but total nitrogen levels declined at mean rates of 2.4 and 1.8 g m −2 d −1, respectively. NH 4-N removal from the bulk water in the mesocosms was well described ( R 2=0.97–0.99) by a sorption-plant uptake-microbial model. First-order volumetric removal rate constants ( k v) rose with increasing fluctuation frequency from 0.026 to 0.46 d −1 without plants and from 0.042 to 0.62 d −1 with plants. As fluctuation frequency increased, reversible sorption of NH 4-N to the media, and associated biofilms and organic matter, became an increasingly important moderator of bulk water concentrations during the batch periods. TN mass balances for the full batch periods suggested that measured plant uptake estimates of between 0.52 and 1.07 g N m −2 d −1 (inversely related to fluctuation frequency) 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 fluctuation 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.