water Article Cost-Effectiveness of Treatment Wetlands for Nitrogen Removal in Tropical and Subtropical Australia Emad Kavehei 1 , Syezlin Hasan 1 , Carla Wegscheidl 2 , Matthew Griffiths 3 , James C. R. Smart 1,4 , Carlos Bueno 5 , Liz Owen 6 , Kambez Akrami 7 , Mel Shepherd 3 , Scott Lowe 8 and Maria Fernanda Adame 1, *   Citation: Kavehei, E.; Hasan, S.; Wegscheidl, C.; Griffiths, M.; Smart, J.C.R.; Bueno, C.; Owen, L.; Akrami, K.; Shepherd, M.; Lowe, S.; et al. Cost-Effectiveness of Treatment Wetlands for Nitrogen Removal in Tropical and Subtropical Australia. Water 2021, 13, 3309. https://doi.org/ 10.3390/w13223309 Academic Editor: Jesus Gonzalez-Lopez Received: 28 October 2021 Accepted: 17 November 2021 Published: 22 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centre for Marine and Coastal Research Australian Rivers Institute, Griffith University, Nathan, QLD 4111, Australia; emad.kavehei@gmail.com (E.K.); s.hasan@griffith.edu.au (S.H.); j.smart@griffith.edu.au (J.C.R.S.) 2 Department of Agriculture and Fisheries, Queensland Government, Townsville, QLD 4814, Australia; carla.wegscheidl@daf.qld.gov.au 3 Department of Environment and Science, Queensland Government, Brisbane, QLD 4000, Australia; matthew.griffiths@des.qld.gov.au (M.G.); mel.shepherd@des.qld.gov.au (M.S.) 4 School of Environment and Science, Griffith University, Nathan, QLD 4111, Australia 5 Reef Catchments NRM (Mackay Whitsunday Isaac) Limited, Mackay, QLD 4740, Australia; carlos.bueno@reefcatchments.com 6 Jaragun EcoServices, Babinda, QLD 4861, Australia; liz@jaragun.com.au 7 Logan Water, Logan City Council, Logan, QLD 4114, Australia; kambezakrami@logan.qld.gov.au 8 Unitywater, 600 Market Drive, Caboolture, QLD 4506, Australia; scott.lowe@unitywater.com * Correspondence: f.adame@griffith.edu.au Abstract: Treatment wetlands can reduce nitrogen (N) pollution in waterways. However, the shortage of information on their cost-effectiveness has resulted in their relatively slow uptake in tropical and subtropical Australia, including the catchments of the Great Barrier Reef and Moreton Bay. We assessed the performance of constructed treatment wetlands (CW) and vegetated drains (VD) that treat agricultural runoff, and of sewage treatment plant wetlands (STPW), which polish treated effluent. Treatment performance was estimated as changes in concentration (dissolved inorganic nitrogen, DIN, and total nitrogen, TN; mg L −1 ) and annual load reductions (kg N ha −1 yr −1 ). We calculated their cost-effectiveness by comparing their N removal against the costs incurred in their design, construction, and maintenance. Overall, CWs and VDs reduced DIN concentrations by 44% (0.52 to 0.29 mg L −1 ), and STPW reduced them by 91% (2.3 to 0.2 mg L −1 ); STPWs also reduced TN concentrations by 72%. The efficiency varied among sites, with the best performing CWs and VDs being those with relatively high inflow concentrations (>0.2 mg L −1 of DIN, >0.7 mg L −1 of TN), low suspended solids, high vegetation cover and high length: width ratio. These high performing CWs and VDs removed N for less than USD 37 kg −1 DIN (AUD 50 kg −1 DIN), less than the end-of- catchment benchmark for the Great Barrier Reef of USD 110 kg −1 DIN (AUD 150 kg −1 DIN). When adequately located, designed, and managed, treatment wetlands can be cost-effective and should be adopted for reducing N in tropical and subtropical Australia. Keywords: cost-effectiveness metric; eutrophication; Great Barrier Reef; macrophytes; nitrogen; total suspended solids; treatment systems 1. Introduction Nitrogen (N) pollution in waterways is one of the most significant environmental challenges of our times, threatening the wellbeing of humanity [1]. More than a century ago, the discovery of artificial N fixation (the Haber–Bosch process) paved the way to industrial fertiliser manufacture, allowing for large-scale food production [2]. Simultaneously, fossil fuel combustion has contributed to elevated levels of reactive N in the atmosphere [3,4]. The increase in N in waterways due to agricultural and urban runoff, atmospheric deposi- tion, and sewage discharge has caused large scale degradation of aquatic ecosystems [4]. Water 2021, 13, 3309. https://doi.org/10.3390/w13223309 https://www.mdpi.com/journal/water