Greenhouses use large amounts of water in their operations (Robbins, 2010). Capturing and recycling irrigation runoff is one way that commercial greenhouses can conserve water while reducing fertilizer inputs and minimizing their environmental footprint. One of the difficulties in reusing nutrient solutions is the gradual accumulation of certain ions, especially Na+ and Cl–, which have a range of sources and at high concentrations can be damaging to greenhouse crops. These ions are often present in the water in low concentrations and they are also components of some fertilizer compounds added to the nutrient solution, for example KCl or NaNO3 (Robbins, 2010). Greenhouse crops do not commonly remove Na+ and Cl– so they leach from the substrate or remain in solution and their concentrations increase as the water is captured and recycled after irrigation. Both Na+ and Cl– can damage greenhouse crops when present at even relatively low concentrations (Stanghellini et al., 2005) so as ion concentrations accumulate above a species-specific threshold, recycled nutrient solutions become unusable. Therefore, for greenhouses to adopt sustainable water management practices, Na+ and Cl– need to be managed at concentrations below these threshold levels.
Current treatment options for removing Na+ and Cl– from recirculated greenhouse water (i.e., reverse osmosis and ultrafiltration) are often too expensive and impractical for the average grower (Gagnon et al., 2010). Greenhouse growers are therefore often forced to manage their water by discharging portions of it directly into the environment. One viable option for onsite wastewater treatment is the use of CWs, which are a technology already being used by greenhouse growers in various applications (Gagnon et al., 2010; Prystay and Lo, 2001; Seo et al., 2008; Vymazal, 2009). CWs are built to provide a favorable environment for the beneficial biological, chemical, and physical processes that occur in natural wetlands. They have a history of success in removing organics, forms of nitrogen, and suspended solids from a variety of different wastewaters (Tanner, 1996; Vymazal, 2010). Limited research has been published on the use of CWs for Na+ and Cl–; however, removal of these ions has been found when plants capable of hyperaccumulating Na+ and Cl– are included in these water treatment systems (Lymbery et al., 2006; Morteau et al., 2009; Nilratnisakorn et al., 2009; Shelef et al., 2012). This form of phytoremediation, known as phytodesalinization, could increase the Na+ and Cl–-removing capacities of CWs.
Certain plant species accumulate and store salt ions in their vacuoles to maintain a proper osmotic gradient and survive in saline environments (Manousaki and Kalogerakis, 2011; Munns, 2002). Plants capable of surviving in saline environments are known as halophytes. Adding halophytic plants that accumulate Na+ and Cl– into CWs could increase the removal of these ions from recycled greenhouse nutrient solutions.
The objectives of this study were to:
- Determine the plant tissue dry matter content of Na+ and Cl– from eight plant species grown in CW microcosms under controlled-environmental conditions (Expt. 1); and
- Quantify the mass of Na+ and Cl– removed by the top performing species (from the first experiment) in outdoor CW microcosms using a simulated greenhouse nutrient solution that contained typical concentrations of Na+ and Cl– (Expt. 2).
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