Abstract
Recirculating nutrient solutions were treated using dimensionally stable anode (DSA)-based regenerative in situ electrochemical hypochlorination (RisEHc) in a deep water culture hydroponic lettuce (Lactuca sativa) production system. Phytotoxic effects were noted and attributed to the formation of chloramines in treated nutrient solutions containing ammonium. The presence of chloramines resulted in a decrease in overhead biomass by 53% using 2.27 mA/cm2 and 83% with 4.55 mA/cm2. Adding ultraviolet light as a tertiary treatment stage allowed the photodecomposition of chloramines, which prevented phytotoxicity in crops and caused no significant differences in growth between treatments. Furthermore, using a nitrate-based fertilizer also served to prevent phytotoxic effects in crops and showed no significant differences in growth between the control and 4.55 mA/cm2. In addition, it was found that the electrochemical flow cell (EFC) treatment resulted in a 13% increase in shoot biomass using 2.27 mA/cm2. The study demonstrated that phytotoxic effects can be prevented with the use of regenerative in situ hypochlorination through proper management and monitoring techniques in recirculating hydroponic systems.
Closed loop hydroponics, where nutrient solution drainage is captured and reapplied to the crop, offers advantages over flow-through or single pass systems by way of water and nutrient resource use efficiency (Wilfried 2005). Increasing resource use efficiency reduces production costs and ultimately prevents environmental impacts associated with nutrient discharge to receiving water bodies (Ehret et al. 2001). Although there are many advantages, there are also production risks introduced when using a capture and reuse approach, namely, pathogen proliferation (Hong and Moorman, 2005). There are many types of systems currently used in industry for treating fertigation water (irrigation water with fertilizer). Examples include but are not limited to sand filtration, ultraviolet light, chlorination, ozonation [O3(aq)], advanced oxidation processes, peracetic acid (C2H4O3), and hydrogen peroxide (H2O2) (Stewart-Wade 2011).
Regenerative in situ electrochemical hypochlorination (RisEHC) using DSAs is a recently developed fertigation disinfection process (Lévesque et al. 2019). This process, where free chlorine (FC; hypochlorite and hypochlorous acid) is continuously regenerated in situ, holds promise in closed loop hydroponics, as it eliminates the need for hazardous chemical storage, accumulation of additives, and lowers energy costs compared with many other treatment systems (Lévesque et al. 2019). Previous research conducted by Lévesque et al. (2019) demonstrated that a chlorine-resistant phytopathogen, Rhizoctonia solani (Cayanan et al. 2009), could be completely inactivated using an EFC with DSAs. This inactivation was achieved by regeneratively releasing concentrations of FC below a reported phytotoxic threshold of 2.5 mg/L (Cayanan et al. 2008). There may, however, be limiting factors that need to be considered before widespread adoption of this technology for hydroponic systems is realized. Conventional chlorination has been shown to have negative impacts on plant growth when HOCl reacts with nitrogen compounds, from phytotoxic chloramines (NH2Cl, NHCl2, and NCl3), also called combined chlorine (CC) (Date et al. 2005; Kapalka et al. 2010; Lacasa et al. 2012). Earlier research showed the effluent concentration of chloramines released through electrochemical disinfection could have negative effects on plant growth (Date et al. 2005; Lévesque et al. 2019). It has also been demonstrated that a subsequent treatment with ultraviolet (254 nm) could reduce the concentration of chloramines through photodecomposition (De Laat et al. 2010). Preventing CC formation in these electrochemical systems can also be achieved by simply avoiding ammoniacal fertilizers, thereby eliminating the CC precursor (Meador and Fisher 2013).
While the phytotoxicity caused by conventional chlorination has been well studied, there is no information regarding the effect of the novel RisEHC, with DSA, system examined herein, where the disinfecting agent is continuously regenerated. The objective of the presented research was to evaluate plant responses and possible phytotoxic effects caused by treating different recirculating nutrient solutions via RisEHC. Further, the efficacy of using a post-electrochemical ultraviolet application to reduce the phytotoxicity effects, as well as changing the nitrogen source of the fertilizer, were examined.
Materials and Methods
Lettuce cultivation methods were adapted from previous research provided by Brechner et al. (1996). The lettuce cultivar used for all experiments in this study was Lactuca sativa cv. Patrona M.I. (P172P; Stokes Seeds Ltd., Thorold, Ontario, Canada).
Raising seedlings.
Rockwool starter plugs (Grodan ROXUL Inc., Kingsville, Ontario, Canada) were soaked and drained twice using deionized water to remove any residual materials within the growing medium. One seed was sown into each of the 98 cubes and hand-watered using a soluble fertilizer mix containing 0.25 g/L of 20 (NH4+-N)-8 (P2O5-P)-20 (K2O-K) (10561, 20–8–20; Master Plant-Prod Inc., Brampton, Ontario, Canada), 42 mg/L Ca2+ [Ca(NO3)2] (AA1066314; Thermo Fisher Scientific, Mississauga, Ontario, Canada) and 20 mg/L of Cl− (KCl) (LC187901, Thermo Fisher Scientific). A plastic humidity dome was placed on top of the rockwool slab, which was then placed under fluorescent bulbs at 100 μmol/m2/s for 2 d in an environmentally controlled chamber at 24 °C. The humidity dome was removed after 2 days and the fluorescent lighting was increased to 275 μmol/m2/s, followed by hand watering to achieve complete saturation using the same fertigation mixture. Plants were grown until roots protruded from the base of the mineral wool cubes (∼7 d after planting).
Experimental design and water treatment methodology.
Experiments were conducted inside the same controlled environment chamber used for seedling establishment. The conditions of the chamber were fixed across all experiments: 24 °C/22 °C day/night, 65% relative humidity, average photosynthetic photon flux density (PPFD) 278 ± 10 (SE) μmol/m2/s, and a 16-h photoperiod. Containers (38 L; Rubbermaid, Atlanta, GA, USA) were randomly assigned to three blocks and the PPFD was measured at each plant position using a LI-COR spectrometer (LI-180; LI-COR, Lincoln, NE, USA). Fertigation solution influent, effluent, and oxygenation ports were made in the reservoir lid. Solution was drawn from the reservoir and passed into the EFC (influent), and returned to the reservoir (effluent) after passage through the EFC. Each block contained three treatments with six subsamples per treatment. Treatments consisted of two different current densities (2.27 and 4.55 mA/cm2) and a control group (0 mA/cm2) without any current applied to the solution (Fig. 1A). Equipment and methods used for treating the nutrient solution with the EFC followed the same procedure previously described by Lévesque et al. (2019, 2020). In addition, all treatments ran at a flow rate of 720 mL/min, which provided a 30-s electrode contact time. Following the propagation stage, 54 uniform seedlings were selected, placed into net pots, and inserted into six holes in the reservoir lid (Fig. 1B). Each reservoir was filled with 30 L of nutrient solution that allowed the bottom 10 mm of the mineral wool cube to be submerged in nutrient solution.
Schematic of (A) randomized complete block design showing the location of all treatments and holes where tubing and plants are located. (B) Physical layout of reservoirs and plant allocation.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Schematic of (A) randomized complete block design showing the location of all treatments and holes where tubing and plants are located. (B) Physical layout of reservoirs and plant allocation.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Schematic of (A) randomized complete block design showing the location of all treatments and holes where tubing and plants are located. (B) Physical layout of reservoirs and plant allocation.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
At the prescribed contact time, the entire volume (30 L) of the reservoir was circulated through the EFC in 42 min, constituting a complete treatment cycle. After treatment, the EFC and tubing were drained and added back into each respective reservoir as not to cross-contaminate reservoirs with the solution from the previous treatment.
Three experiments were performed, each experiment was conducted over a total period of 2 weeks with consecutive treatments being applied five times per week (Monday–Friday). A summary of all three experiments and the solution contents are presented in Table 1.
Summary of the methodology used in each experiment and the components of the solution used for disinfection and supporting plant growth. The concentration of nitrogen was maintained across all experiments, although some minor differences were observed in Expt. 3. Potassium chloride (KCl) was added as the raw material for free chlorine production (20 mg/L of Cl−).
In Expt. 2, the volume of the ultraviolet chamber was 1.5 L, which provided the solution with a 2-minute photolysis exposure period using the EFC flow rate of 720 mL/min. The transmittance of the ultraviolet system was 76% and intensity was measured at 135 mJ/cm2. The control group was not subjected to electrochemical treatment but was treated using ultraviolet disinfection. In Expt. 3, the pH of the solution required an adjustment to 5.8 using KOH, which resulted in K+ being 184.1 ± 1.23 mg/L higher than in Expts. 1 and 2. This was due to the addition of KH2PO4, which resulted in lowering the pH. The concentration of total nitrogen (130 mg/L N) and phosphorus (55 mg/L H2PO4) in solution was consistent throughout all three experiments. Micronutrients were added for the purpose of maintaining the same concentration of chelated iron across each of these experiments (10046, Chelated micronutrient mix, Master Plant-Prod Inc.) although other micronutrients fluctuated in Expt. 3.
Fertigation solution and plant biomass sample analysis.
Following treatment of each reservoir, three solution subsamples were collected for FC and total chlorine analysis. The concentrations of FC and total chlorine were measured using N,N-diethyl-p-phenylenediamine (DPD) Test ’N Tube cuvette and colorimeter (Hach Company, Loveland, CO, USA). The difference between total chlorine and FC was calculated to determine the concentration of CC (NH2Cl, NHCl2, and NCl3) in bulk solution. After treatment, the pH, electrical conductivity (EC), oxidation-reduction potential (ORP), and dissolved oxygen content (DO) from each reservoir were measured in the bulk solution. The solution pH was adjusted to between 5.8 and 6.0 using 1M HNO3 and 1M KOH, while the EC was maintained between 1000 and 1100 μS/cm2 using a 25x stock solution. However, the EC for Expt. 3 was initially determined to be ∼1200 μS/cm2 due to the pH adjustment using KOH. The EC was maintained in this case between 1100 and 1200 μS/cm2 throughout the experiment. A sample was collected once a week from each reservoir to measure macronutrient ions using a Shimadzu (Kyoto, Japan) high-performance liquid chromatography system consisting of a DGU-20A3 degasser, an SIL-10AP autosampler, two LC-20AT pumps, two CDD-10A VP conductivity detectors, a CTO-20AC column oven, and a CBM-20A system controller. The full suite on nutrient ions analyzed were NH4+, NO3−, H2PO4, K+, NO2, SO42−, Cl2−, Mg2+, and Ca2+. Only those that showed a treatment effect are presented for clarity. An additional three samples per week were collected from each reservoir to determine the microbial characteristics of the solution. Serial dilutions and culturing methodology were performed using the same methods as described by Lévesque et al. (2019). Half of the plates used antibiotics (0.1 g/L streptomycin and 0.05 g/L ampicillin) in potato dextrose agar to measure the number of fungal colonies in solution, and other plates were antibiotic free for bacterial enumeration. The total microbial density was determined by adding the number of fungal and bacterial colonies together.
After 2 weeks of growth, plants were harvested on day 15 and measured individually for biomass parameters such as leaf area (LA; cm2), leaf fresh weight (g), leaf dry mass (mg), number of leaves, root length (m), and root dry mass (mg). Final dry weight measurements were performed by placing fresh plants in paper bags, dried at 65 °C over a 4-day period in a drying oven (OV-510A-2; Blue M-Electric Company, New Columbia, PA, USA), and then weighed (MS32001L; Mettler-Toledo, Greifensee, Switzerland). The total LA of each plant was measured by removing leaves and passing through a leaf area meter (LI-3100C, LI-COR Inc.).
Statistical analysis.
All statistical analyses were performed using JMP version 14.0 (SAS Institute Inc., Cary, NC, USA). Residuals were analyzed to verify model assumptions (random distribution, independent of treatment and design effects, common covariance, and normal distribution by testing for heterogeneity and normality for all data). Statistical analysis on plant growth parameters [number of leaves, LA, leaf fresh weights, leaf dry weights, root dry weights, and shoot/root ratio (mg/mg)] was conducted by performing a randomized complete block design (RCBD) one-way analysis of variance between water treatment current densities. Individual macronutrients and micronutrients effects were determined with a repeated measures RCBD analysis. The same analysis was performed for microbial inactivation across time between all treatments.
Results
Microbial inactivation.
In Expt. 1, microbial counts in the control group increased over time (P = 0.0073) between weeks 1 and 3 (Fig. 2). Both EFC treatments resulted in reductions in total colony-forming unit (CFU) counts; 16 ± 2 CFU/mL with 2.27 mA/cm2 and complete inactivation using 4.55 mA/cm2 in the effluent solution before entering the reservoir. In the bulk solution, the reduction in total CFU was not as large compared with the control benchmark values. Treatments reduced counts to an average of 80 ± 11 (CFU/mL) at 2.27 mA/cm2 and 29 ± 6 (CFU/mL) at 4.55 mA/cm2. In Expt. 2, differences in microbial counts for bulk and effluent solutions between all treatments (P < 0.0001) were observed. The control group had the highest microbial density at 1518 ± 83 (CFU/mL) followed by 2.27 mA/cm2 at 1181 ± 100 (CFU/mL) and 614 ± 90 with 4.55 mA/cm2 in bulk solution. All treatments reached complete inactivation after passing through the EFC and ultraviolet treatments. The control group passed through the EFC without power and reached complete inactivation due to the incorporation of ultraviolet disinfection. In Expt. 3, total CFU counts increased over time (P = 0.0019). Compared with the control, treatment in the EFC reduced total CFU counts (P < 0.0001), to 14 ± 3 (CFU/mL) at 2.27 mA/cm2 and 1 ± 0.37 (CFU/mL) at 4.55 mA/cm2 in the effluent solution. The microbial density in the bulk solution was 93 ± 70 (CFU/mL) at 2.27 mA/cm2 and 37 ± 6 (CFU/mL) using 4.55 mA/cm2.
Microbe inactivation rates through different experiments in the effluent (blue) and bulk (red) of the fertigation solution, while using various current densities. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Microbe inactivation rates through different experiments in the effluent (blue) and bulk (red) of the fertigation solution, while using various current densities. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Microbe inactivation rates through different experiments in the effluent (blue) and bulk (red) of the fertigation solution, while using various current densities. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Characterization and composition of fertigation solutions following EFC treatment.
Concentrations of FC and CC were monitored following treatment of each reservoir (Fig. 3). In all experiments, the control was free of any FC or target CC compounds. In Expt. 1 (DSA + NH4+), there was an increase in both FC and CC with increased current density (P < 0.0001) and treatment time (P = 0.0371) (Fig. 3A). In Expt. 2 (DSA + NH4+ + ultraviolet), it was also shown to have significant differences for both FC and CC with current density over time (P = 0.0081:FC, P = 0.0305:CC) (Fig. 3B). However, the concentrations released by both electrochemical treatments were minor and only varied between 0 and 0.05 mg/L. In Expt. 3, FC increased with increasing current density (P < 0.0001) but did not increase or accumulate over time (P = 0.6058) (Fig. 3c). Similarly, chloramines increased with current density (P < 0.0001) but not with time (P = 0.8157; Fig. 3C).
Concentration of free chlorine (blue) and combined chlorine (red) across (A) Expt. 1 [dimensionally stable anode (DSA) + NH4+], (B) Expt. 2 (DSA + NH4+ + ultraviolet), and (C) Expt. 3 (DSA + NO3−) with varying current densities applied to bulk solution over time. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Concentration of free chlorine (blue) and combined chlorine (red) across (A) Expt. 1 [dimensionally stable anode (DSA) + NH4+], (B) Expt. 2 (DSA + NH4+ + ultraviolet), and (C) Expt. 3 (DSA + NO3−) with varying current densities applied to bulk solution over time. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Concentration of free chlorine (blue) and combined chlorine (red) across (A) Expt. 1 [dimensionally stable anode (DSA) + NH4+], (B) Expt. 2 (DSA + NH4+ + ultraviolet), and (C) Expt. 3 (DSA + NO3−) with varying current densities applied to bulk solution over time. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
For Expt. 1, the pH, EC, and ORP were shown to have differences over time (P < 0.0001), whereas only ORP was determined to exhibit an interaction between treatment and time (P = 0.0187) (Fig. 4A–C). Furthermore, DO measurements had carried with current density (P < 0.0001) (Fig. 4D). Expt. 2 also showed differences in pH, EC, and ORP with time (P < 0.0001), and ORP also had an interaction between treatment and time (P = 0.0379). The DO content also varied between treatments (P < 0.0001). In Expt. 3, differences were observed in pH, ORP, and DO (P < 0.0001). For all experiments, pH was shown to decrease over time, EC was maintained between 1000 and 1200 across experiments, and ORP varied between 200 and 550 mV and DO between 7 and 9 mg/L.
Water quality parameters measured in the reservoir (A) pH, (B) electrical conductivity (EC), (C) oxidation-reduction potential (ORP), and (D) dissolved oxygen after each treatment across all experiments performed in the study. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Water quality parameters measured in the reservoir (A) pH, (B) electrical conductivity (EC), (C) oxidation-reduction potential (ORP), and (D) dissolved oxygen after each treatment across all experiments performed in the study. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Water quality parameters measured in the reservoir (A) pH, (B) electrical conductivity (EC), (C) oxidation-reduction potential (ORP), and (D) dissolved oxygen after each treatment across all experiments performed in the study. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Ionic species (from the fertilizer) were monitored posttreatment. Differences, or lack thereof, are detailed in Fig. 5. For clarity of presentation, only those species that were shown to vary (NO2− and NH4+) are presented. In Expt. 1, NO2− increased in the control group (m = 0.068, a = 0.128 mg/L; P = 0.0002). Although unexpected, this small increase could be due to microbial activity (Li et al. 2019). In Expt. 2, NO2− increased in all treatment groups, with the largest changes occurring in the EFC treatments (Fig. 5). Again, in Expt. 3, NO2− increased for 2.27 mA/cm2 (1.75 mg/L; P = 0.0006) and 4.55 mA/cm2 (1.36 mg/L; P = 0.0054). Nitrite was consistent between weeks 2 and 3. Also, there were significant increases for NH4+ with 2.27 mA/cm2 (4.13 mg/L; P = 0.0300) and 4.55 mA/cm2 (4.45 mg/L; P = 0.0001).
Ion concentrations over time with varying current densities applied to the solution under different experiments. Because of the absence of both NH4+ and NO2− in Expt. 3, the lines are overlapping and blocking the blue line. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Ion concentrations over time with varying current densities applied to the solution under different experiments. Because of the absence of both NH4+ and NO2− in Expt. 3, the lines are overlapping and blocking the blue line. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Ion concentrations over time with varying current densities applied to the solution under different experiments. Because of the absence of both NH4+ and NO2− in Expt. 3, the lines are overlapping and blocking the blue line. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Plant growth response.
Electrochemical treatments, in the presence of ammoniacal nitrogen and in the absence of ultraviolet post-EFC treatment (Expt. 1), resulted in reduced plant vigor (Fig. 6A). A decrease in root biomass and root browning was apparent in the 2.27 mA/cm2 treatment, and severe stunting was observed in the 4.55 mA/cm2 treatment (Fig. 7D). Shoot biomass was also reduced with increasing current density treatments. With the addition of ultraviolet radiation (Expt. 2), the suppression of root and shoot biomass production was considerably less that in Expt. 1, but still reduced in the highest current density treatment (P = 0.0535) (Figs. 6B and 7D). Root browning was the leading symptom for suppressing the development of root and shoot biomass for these experiments. The electrochemical treatments seemed to affect root morphology through a visual assessment, with higher amounts of lateral branching of seminal roots in comparison with the control group. In the absence of ammoniacal components in the treated solution (Expt. 3), there was a slight stimulatory effect at 2.27 mA/cm2 relative to the control (Figs. 6C and 7).
Photograph of plants following 14 d of growth after transplant and treated with the electrochemical flow cell (EFC) system under (A) fertilizer containing NH4+, (B) fertilizer with ammonium but with added ultraviolet light, and (C) fertilizer that contained only NO3− and was treated with the EFC system.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Photograph of plants following 14 d of growth after transplant and treated with the electrochemical flow cell (EFC) system under (A) fertilizer containing NH4+, (B) fertilizer with ammonium but with added ultraviolet light, and (C) fertilizer that contained only NO3− and was treated with the EFC system.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Photograph of plants following 14 d of growth after transplant and treated with the electrochemical flow cell (EFC) system under (A) fertilizer containing NH4+, (B) fertilizer with ammonium but with added ultraviolet light, and (C) fertilizer that contained only NO3− and was treated with the EFC system.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Plant performance following treatment of the solution under three different experiments, while using three current densities. Measured plant parameters were (A) number of leaves, (B) leaf area, (C) leaf fresh weight, (D) leaf and root dry weights, as well as the shoot/root ratio. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Plant performance following treatment of the solution under three different experiments, while using three current densities. Measured plant parameters were (A) number of leaves, (B) leaf area, (C) leaf fresh weight, (D) leaf and root dry weights, as well as the shoot/root ratio. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Plant performance following treatment of the solution under three different experiments, while using three current densities. Measured plant parameters were (A) number of leaves, (B) leaf area, (C) leaf fresh weight, (D) leaf and root dry weights, as well as the shoot/root ratio. Error bars are ± SEM, n = 3.
Citation: HortScience 58, 1; 10.21273/HORTSCI16734-22
Discussion
The application of the RisEHC system for the treatment of fertigation solutions was effective for controlling pathogens (Lévesque et al. 2019); however, the process can lead to the production of phytotoxic chloramines if not properly managed. This study sought to characterize these effects and develop remediation protocols. In the first experiment, the treatment system realized a bulk solution microbial reduction of 98.9% at 2.27 mA/cm2 and 99.6% at 4.55 mA/cm2 (Fig. 3). The concentration of FC in the bulk solution following treatment was stable at 0.17 ± 0.03 mg/L for the 2.27 mA/cm2 treatment and 0.41 ± 0.08 mg/L at 4.55 mA/cm2 (Fig. 5). These residuals are below the reported phytotoxic threshold of 2.5 mg/L for most major crops (Cayanan et al. 2008). However, the concentration of CC (0.15 ± 0.03 mg/L at 2.27 mA/cm2; 0.39 ± 0.08 mg/L at 4.55 mA/cm2) clearly had phytotoxic effects (Figs. 6 and 7). In the absence of chloramine mitigation strategies, there was a decrease of 53% in leaf biomass at 2.27 mA/cm2 and 82% at 4.55 mA/cm2. The same trend was also demonstrated for root growth and development, with a decrease of 68% and 83%, respectively, for the 2.27 mA/cm2 and 4.55 mA/cm2 treatments. These results confirm the chloramine phytotoxic threshold of 0.18 mg/L reported (for lettuce) by Date et al. (2005).
A consideration of any fertigation solution treatment technology is its influence on nutrient composition. The RisEHC process examined here did not alter the concentration of any nutrient ion species, although the control group had an increase of NO2−, which did not have the RisEHC process (Fig. 7C). The increase in NO2– within the control group could be associated with biofilm development, which can moderately increase NO2– in solution over time (Li et al. 2019). Previous research by Hoque et al. (2007) found that NO2− could have phytotoxic effects on plants and they estimated a 1% loss in biomass per 1 mg/L NO2—N. The study also found that other ions, such as NO3−, could decrease within plant tissues with increasing concentrations of NO2− in solution (Bingham et al. 1954; Oke 1966; Phipps and Cornforth 1970). Although increases in NO2− were found in Expt. 1, the concentrations were maintained at low levels (0.13 mg/L), and any influence on plant health was apparently negligible. Electrochemical treatments did not cause significant increases in NO2− due to oxidation processes at the anode (Lacasa et al. 2012; Watanabe et al. 2002). In addition, electrochemical treatments produced both free and CC, which have been shown to suppress the development of biofilms (Williams et al. 2005).
When ultraviolet was added as a post-EFC treatment step, all microbes were inactivated in the effluent solution and in turn, no significant differences were found among all three treatment groups (Fig. 2). There were significant differences in the microbial density in the bulk solution between treatments. However, the percent difference in the number of microbes inactivated in the control and the EFC treatments was between 3% and 10%, which indicates that ultraviolet possessed strong capabilities for disinfection. The concentration of FC in the effluent solution following treatment with 2.27 mA/cm2 was 0.04 ± 0.01 mg/L and 0.05 ± 0.02 mg/L with 4.55 mA/cm2. Likewise, the concentration of CC was 0.02 ± 0.01 mg/L using 2.27 mA/cm2 and 0.03 ± 0.01 mg/L with 4.55 mA/cm2. The concentration of chloramine was reduced using photodecomposition processes (De Laat et al. 2010). However, ultraviolet treatment also reduced the concentration of FC, thereby limiting residual disinfection capacity and the efficacy to control microbes in the bulk solution. Nonetheless, the ultraviolet-based reduction of chloramine greatly improved the health of plants in Expt. 2 compared with the first experiment (Fig. 6A and B). Although improvements were observed, there was still a reduction in LA and root dry weight in the highest current density treatment relative to the control (LA: 15% and 13% at 2.27 mA/cm2 and 4.55 mA/cm2, respectively; root dry mass 14% at 4.55 mA/cm2) (Fig. 7). Although chloramines were reduced, the concentration of NO2− increased to 7.69 mg/L under the 2.27 mA/cm2 treatment (Fig. 7C). Increased NO2− was also detected in the control group, although the concentration was considerably lower. Nitrite has been observed by others as a product of photodecomposition of chloramines as well as by the formation and decay of aminyl radicals (NH2•) (De Laat et al. 2010; Laszlo et al. 1998). The presence of NO2− likely contributed to decreases in LA and root mass; however, further investigation with longer growth cycles would be needed to confirm these effects (Hoque et al. 2007). Alternative methods for removing chloramines without the release of NO2− is through the use of ascorbic acid or activated carbon filters (Basu and De Souza 2011; Kochany and Lipczynska-Kochany 2008). Alternatives techniques such as mentioned should be investigated for future trials to determine whether plant health becomes less variable between treatments.
When the fertigation solution contained only NO3− as a nitrogen source (Expt. 3), the effluent solution’s microbial density was reduced by 99.8% using 2.27 mA/cm2 and 99.99% using 4.55 mA/cm2. In the bulk solution, microbial densities were reduced by 98.9% with 2.27 mA/cm2 and 99.6% using 4.55 mA/cm2. The concentration of FC in solution was 0.17 ± 0.04 mg/L while using 2.27 mA/cm2 and 0.30 ± 0.05 mg/L with 4.55 mA/cm2, which was below the reported phytotoxic threshold of 2.5 mg/L (Cayanan et al. 2009). The concentration of CC was 0.03 ± 0.01 mg/L for both 2.27 and 4.55 mA/cm2, which is six times lower than the reported phytotoxic threshold of 0.18 mg/L (Meador and Fisher 2013). This CC did not result in any differences in plant performance between the control group and the EFC treatment using a current density of 4.55 mA/cm2 (Fig. 7). However, there were significant increases in biomass production at a current density of 2.27 mA/cm2 with a 13% increase in leaf biomass (P = 0.0048) and a 16% increase in root biomass (P = 0.0074) and LA (P = 0.0008) being observed. As in the previous experiment, NO2− increased, but to a lesser degree (∼2 mg/L). Further study is required to determine if NO2− accumulates over longer term grow cycles and treatment periods.
Increases in plant yield could be explained by some contributing factors, such as higher concentrations of DO following treatment (Fig. 6D). The increase in DO following electrochemical treatment is a known effect, as oxygen evolution is a side reaction during many water-based electrochemical processes (Comninellis and Vercesi 1991; Hu et al. 2004; Särkkä et al. 2009). It has been well documented that a solution’s DO content directly affects the final yield of plants (Gislerød and Kempton 1983; Urrestarazu and Mazuela 2005; Zeroni et al. 1983; Zheng et al. 2007); however, final yields were not found to be significantly different while using the current density of 4.55 mA/cm2, which had a higher DO concentration as well as higher inactivation rates. Other factors, such as the concentration of FC, may have contributed to the differences in plant yield between electrochemical treatments. Tomato plants were also shown to have a 10% increased yield after treatment using a nutrient solution with 1 mg/L of FC (but not with 2 mg/L) through conventional chlorination methods (Dannehl et al. 2016). Likewise, lettuce grown through drip irrigation had a 17% increase in fresh weight following nutrient solution treatment with 0.55 and 5.5 mg/L of sodium hypochlorite (Premuzic et al. 2007). Further investigation is needed with longer growth cycles as well as different hydroponic systems (e.g., nutrient film technique, aeroponics) to confirm the increase in plant yield following treatment with RisEHC.
Conclusion
Effective fertigation solution remediation is key in realizing the full potential of long-term recirculating hydroponic systems. The RisEHC system evaluated here was demonstrated to be effective at reducing microbial populations in laboratory-scale hydroponic growth trials; however, the production of chloramines in the presence of ammoniacal compounds/fertilizers led to phytotoxicity in some scenarios. In the current study, chloramine phytotoxicity was addressed by either excluding ammoniacal fertilizers, or through decomposition using ultraviolet radiation after electrochemical treatment, a practice that would further enhance microbial inactivation. RisEHC is an effective fertigation solution remediation tool when chloramine production is avoided or mitigated.
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