Comparison of Hydroponic Butterhead Lettuce Grown in Reject Water from a Reverse Osmosis System, Municipal Water, and Purified Water

A main constraint of hydroponics is the need for high-quality water. This study evaluated the viability of reverse osmosis concentrate backflush, or reject water, as a water source for hydroponic lettuce production. We compared the growth of ‘Casey’ butterhead lettuce produced using nutrient film technique hydroponics with municipal (tap) water (TW), reverse osmosis (RO) water, and reject water from an RO system [wastewater (WW)]. Characterization of water sources showed the following: RO had trace minerals; TW had the greatest potassium (K), calcium (Ca), and magnesium (Mg); and WW had greatest sulfur (S), sodium (Na), and chloride (Cl). No water source had detectable heavy metals. Mineral and heavy metal tissue contents were determined at mid-harvest and end-harvest, and nutrient budgets for nitrogen (N), S, Ca, Mg, Na, and Cl were calculated. Water sources did not impact lettuce growth during the 4-week hydroponic production cycle based on leaf greenness, diameter, fresh weight, or dry weight. In the nutrient reservoirs, Ca and Mg increased over time and were greatest in TW. The Cl and Na levels in WW were 106 and 203 mg·L −1 , respectively, and the electrical conductivity (EC) of this treatment increased to 3.5 dS·m −1 after 4 weeks; however, plant water uptake, water use efficiency, and percent moisture loss were unaffected. In shoot tissue, lettuce grown with WW had less Ca at mid-harvest and less Mg at mid-harvest and end-harvest compared with other treatments, suggesting antagonization of uptake by high salt. Tissue S was not different across treatments; however, there were differences in reservoirs of the water sources as S accumulated more from WW. Tissue Na and Cl levels were different among the treatments, and higher amounts in nutrient reservoirs translated to higher uptake by lettuce. Although the heavy metal content in all water sources was <0.01 ppm, arsenic, lead, and chromium were measured in lettuce tissue at levels that exceed recommended limits in most instances. Our results indicated that a water source of RO reject water from municipal feed water is a viable irrigation option for short-term hydroponic production of butterhead lettuce; however, EC reduction in the nutrient solution would be necessary when continuing sequential cropping.