Optimization of an Indoor DWC Hydroponic Lettuce Production System to Generate a Low N and P Content Wastewater

Abstract

Hydroponic production raises economic and environmental issues related to the treatment, recovery or disposal of hydroponic wastewater, which can be rich in eutrophication-related nutrients, nitrogen (N) and phosphorus (P). Little focus has been put on the influence of the growth conditions on the N and P content in hydroponic wastewater, which is of uttermost importance when it is intended to reuse the wastewater for irrigation or other purposes with reduced impact on the environment. This study aimed to optimize an indoor non-recirculating deep-water culture (DWC) hydroponic system for lettuce (Lactuca sativa L. var. crispa) production, in terms of daily light integral (DLI) and volume of nutrient solution (NS) per plant, to maximize both the biomass production and the N and P removal, allowing for the wastewater to meet the criteria established for reusing in irrigation and minimizing the eutrophication impacts. A small-scale DWC hydroponic system with a fluorescent light fixture was built to study lettuce growth indoors for 35 days after transplanting (DAT). A first experiment was conducted under 14, 20 or 23 mol m−2 d−1 DLI and with 1.5 or 2 L of NS per plant. A pronounced inner leaf tip burn was observed, regardless of the volume of NS solution used, related to the unventilated conditions under high radiation. Total biomass was similar in all treatments and N and P removal was higher than 95% and 94%, respectively, at 35 DAT. Lettuces grown in 2 L of NS per plant exhibited higher average biomass. A second experiment was performed under 8, 10 or 12 mol m−2 d−1 DLI and with 2 or 3 L of NS per plant, making it possible to achieve healthy biomass at 35 DAT with higher water and light-use efficiency when compared to the first experiment. A DLI of 10 or 12 mol m−2 d−1 with 2 L of NS per plant and a DLI of 12 mol m−2 d−1 with 3 L of NS per plant made it possible to achieve both the best total biomass production and the highest N and P removal from water. Under those conditions, hydroponic wastewater complied with N and P criteria for reuse in irrigation, showing potential to be used as an alternative resource for agriculture and to minimize negative impacts on the environment.

Progress on the Use of Hydroponics to Remediate Hog Farm Wastewater after Vermifiltration Treatment

“…The hydroponic container arrangement was such that the containers were exposed to an approximate Daily Light Integral (DLI) of approximately 11 mol m −2 d −1 by hanging the light fixture at 40.5 cm above the containers with 2 middle lamps turned off. This DLI value fell within the optimal range for lettuce in this hydroponic system [35]. Photosynthetic Photon Flux Density (PPFD) measurements (LI-250Q PAR package; LI-COR, Lincoln, NE, USA) were conducted over each pot, 10 cm above the XPS boards.…”

“…Therefore, methodical studies needed to be carried out to better understand the viability of vermifiltered wastewater for hydroponic production, concurrently with the remediation of nutrients and other pollutants. A deep water culture hydroponic system was later optimized for efficient N and P removal [35]. The present study aimed to continue previous work, assessing the remediation of vermifiltered hog farm wastewater through hydroponic lettuce growth, as model culture, using the optimized treatment conditions.…”

“…Small 3 mm perforations were made around its lower half at a 7 cm distance to improve aeration. The cylinder was filled from bottom to top with layers of decreasing-size gravel and sand as described previously [35], topped by a layer of Siro TM (Leal & Soares, S.A., Mira, Portugal) vermicompost mixed with wood shavings from a local sawmill (apparent ratio 1:2 by volume). Internal void volume was previously estimated at 4.4 L [44].…”

“…Lettuce was hydroponically grown for 35 days after transplanting (DAT) on three liquid media: (i) vermifiltered wastewater (VW) (ii) supplemented vermifiltered wastewater (SVW) and (iii) a synthetic nutrient solution (NS) as control (Figure 2) with two replicates per treatment. A volume of 2 L per lettuce was used, previously found to be adequate to promote high N and P removal from NS under the DLI levels used in this experiment [35].…”

“…The use of vermifiltered wastewater as a liquid medium for hydroponic cultures may prove to be an interesting additional treatment stage. The ability of hydroponic culture to effectively remove nitrate and phosphate from synthetic media and wastewater has been reported in the literature [33][34][35]. Hydroponics comprises several techniques of soilless crop cultivation where the roots stay in direct contact with a nutrient-carrying liquid medium [36,37].…”