Investigating the pilot-scale performance of a hydroponic nutrient solution as potential draw solution for fertilizer drawn forward osmosis and hydroponic agriculture of lettuce

Abstract

AbstractThe objective of this research is to test the feasibility of a large-scale application of fertilizer drawn forward osmosis using a concentrated hydroponic nutrient solution as the draw solution. This large-scale application will help to mitigate the ever-growing issue of freshwater resources, by providing an alternative source of freshwater for agriculture. This method can then be used to grow crops hydroponically in arid regions that have access to both brackish groundwater and freshwater. The nutrient solution was prepared by dissolving industrial-grade fertilizer salts in dechlorinated tap water and was tested as a draw solution (DS) on a pilot-scale forward osmosis membrane module. The feed solution (FS) for the process was NaCl dissolved in dechlorinated tap water, in a 10 g/L (10% w/w) NaCl concentration to represent the salinity level of brackish groundwater. The performance of the draw solution was measured based on the water flux, water recovery, specific solute flux, and salt rejection. Two hydroponic basins were utilized to hydroponically grow lettuce. The first basin was grown using a commercial hydroponic nutrient solution, while the other basin was grown using the diluted draw solution from the pilot-scale. The growth of the lettuce in both basins was compared based on wet and dry weights of the stem, percent reduction in weight by drying, total length, stem length, and the ratio between stem and total lengths. Results showed that average water flux on the pilot-scale was found to be 1.11 L/m2/hr (LMH), while it was found to be 7.7 LMH in previous research on the bench-scale; this is in line with previous experiments conducted on both bench-scale and pilot-scale forward osmosis. The highest value specific reverse solute flux on the pilot-scale was found to be 7.48 g/m2/h for the K+ ion, which is also comparable to the results obtained on the bench-scale. Finally, salt rejection was found to be high, the lowest value was 90.7%, indicating high membrane selectivity against feed ions. The lettuce grown hydroponically using the resultant diluted draw solution from the pilot-scale was comparable to the lettuce grown using a commercial nutrient solution, with the draw solution lettuce being 16.23% less in terms of weight than the commercial lettuce, and 27.18% in stem length. This decrease in size and weight of the lettuce grown with the diluted draw solution can be attributed to the ratio of macro- and micro-nutrients in the Resh Florida/California solution not being optimized for the species of lettuce grown (loose leaf). It was concluded that concentrated hydroponic nutrient solution is a valid draw solution for large-scale fertilizer drawn forward osmosis, and this application mitigates some of the drawbacks of fertilizer drawn forward osmosis and increases the feasibility of large-scale application of forward osmosis as a desalination technology. Graphical abstract