Nitrate Removal Performance of Denitrifying Woodchip Bioreactors in Tropical Climates

Abstract

In Australia, declining water quality in the Great Barrier Reef (GBR) is a threat to its marine ecosystems and nitrate (NO3−) from sugar cane-dominated agricultural areas in the coastal catchments of North Queensland is a key pollutant of concern. Woodchip bioreactors have been identified as a potential low-cost remediation technology to reduce the NO3− runoff from sugar cane farms. This study aimed to trial different designs of bioreactors (denitrification walls and beds) to quantify their NO3− removal performance in the distinct tropical climates and hydrological regimes that characterize sugarcane farms in North Queensland. One denitrification wall and two denitrification beds were installed to treat groundwater and subsurface tile-drainage water in wet tropics catchments, where sugar cane farming relies only on rainfall for crop growth. Two denitrification beds were installed in the dry tropics to assess their performance in treating irrigation tailwater from sugarcane. All trialled bioreactors were effective at removing NO3−, with the beds exhibiting a higher NO3− removal rate (NRR, from 2.5 to 7.1 g N m−3 d−1) compared to the wall (0.15 g N m−3 d−1). The NRR depended on the influent NO3− concentration, as low influent concentrations triggered NO3− limitation. The highest NRR was observed in a bed installed in the dry tropics, with relatively high and consistent NO3− influent concentrations due to the use of groundwater, with elevated NO3−, for irrigation. This study demonstrates that bioreactors can be a useful edge-of-field technology for reducing NO3− in runoff to the GBR, when sited and designed to maximise NO3− removal performance.

Microbial consortia driving lignocellulose transformation in agricultural woodchip bioreactors

“…Genes for full-fledged denitrification were detected in seven MAGs where five of them also had genes for DNRA. 4, Figure S4), while none for DNRA, suggesting that denitrification is the dominant NO 3 reducing pathway in our data, as also indicated in previous WBR studies 9,11,[51][52][53] . Expressed enzymes included napAB (one enzyme), narGHI (14 enzymes), nirK (one enzyme), nirS (12 enzymes), norBC (six enzymes), nosZ (10 enzymes).…”

Quantifying Nitrogen Treatment by an In-stream Bioreactor in a Watershed Served by Septic Systems

“…It is also possible that the increase in ammonium concentrations could be due to inputs from groundwater seeps and/ or ammonification of DON. Past studies found elevated ammonium concentrations in outflow from denitrifying bioreactors, which was attributed to mineralization of DON from decay of carbonaceous material (Fenton et al 2016;Pfannerstill et al 2016;Manca et al 2021). Most studies on IBRs reported similar ammonium concentrations between inflow and outflow (Robertson and Merkley 2009;Elgood et al 2010;Maxwell et al 2022;Burb-ery and Abraham 2022) suggesting that DNRA and biodegradation was minimal.…”