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Article

Hydroponic Wastewater Treatment with Microalgae: A Sustainable Alternative for Irrigating Pelargonium × hortorum

by
Alejandro Rápalo-Cruz
1,2,
Cintia Gómez-Serrano
3,
Cynthia Victoria González-López
3,
Miguel Urrestarazu-Gavilán
1 and
Silvia Jiménez-Becker
1,*
1
Department of Agronomy, CIAIMBITAL, Higher Engineering School, Agrifood Campus of International Excellence (CeiA3), Ctra. Sacramento s/n., 04120 Almería, Spain
2
Faculty of Agricultural Sciences, Universidad Nacional de Agricultura, Road to Dulce Nombre de Culmí, Km 215, Barrio El Espino, Catacamas 16201, Honduras
3
Department of Chemical Engineering, Higher Engineering School, Agrifood Campus of International Excellence (CeiA3), Ctra. Sacramento s/n., 04120 Almería, Spain
*
Author to whom correspondence should be addressed.
Horticulturae 2025, 11(5), 547; https://doi.org/10.3390/horticulturae11050547
Submission received: 21 April 2025 / Revised: 12 May 2025 / Accepted: 16 May 2025 / Published: 19 May 2025
(This article belongs to the Special Issue Effects of Biostimulants on the Growth and Development of Horticultural Crops)
Browse Figures
Versions Notes

Abstract

:
Microalgae are an effective solution for the treatment and valorization of wastewater generated in hydroponic systems. In the current context of sustainability and resource management, the search for ecological alternatives in agriculture is essential. This study investigated the use of wastewater from hydroponic systems, purified by microalgae, for the irrigation of Pelargonium × hortorum. An experiment was designed under controlled conditions in which different irrigation treatments were applied. Hydroponic leachates treated by microalgae were used at 100%, 75%, and 50% (diluted using tap water), in addition to tap water as a negative control and nutrient solution as a positive control. The treatment system was established in a raceway photobioreactor, which allowed the proliferation of microalgae that act as bioremediators for the elimination of pollutants and the removal of nitrogen and phosphorus. The growth parameters, biomass, and general health of the Pelargonium × hortorum plants were evaluated, complemented with physicochemical analyses of the water carried out during the experimental period. These analyses showed that the water obtained after the purification process retained nutrients that can be reused for irrigation. The results indicated that plants irrigated with treated water showed significant improvements in height, diameter, number of leaves, leaf area, leaf dry weight, and flower dry weight compared to those irrigated with tap water. In conclusion, the study shows that the treatment of hydroponic wastewater by means of microalgal cultivation represents a viable and ecological alternative for the irrigation of ornamental plants such as Pelargonium × hortorum. The implementation of this system contributes both to the reduction of pollutants and to the optimal use of water resources, establishing a solid basis for future research in which additional nutrients could be incorporated to balance the nutrient solution studied.

Graphical Abstract

1. Introduction

The global population is expected to reach 9.5 billion, which means that crop productivity needs to double to meet the growing population’s food demand [1]. As both the global population and economic activity expand, the pressures on water, land, and food resources intensify [2]. On the other hand, soil-less farming techniques such as hydroponics have gained significant attention worldwide for their resource efficiency and ability to produce high-quality fresh products [1]. By providing optimal conditions for plant growth, hydroponics can enhance agricultural production [3].
However, agricultural wastewater presents serious environmental risks and requires proper treatment before discharge [4]. In hydroponic systems, nutrient dosing often exceeds plant requirements, leading to nutrient-rich effluents. Effective wastewater treatment and nutrient recovery from these systems remain a major challenge [5]. Improving wastewater recycling processes is essential for ensuring sustainable agriculture [6].
A promising solution to this challenge lies in integrating microalgae into hydroponic systems. Microalgae-based treatment technologies have emerged as an effective solution for advanced wastewater treatment and nutrient recovery, making significant contributions to several Sustainable Development Goal [7]. Hydroponic effluent, enriched with inorganic nutrients, serves as an affordable and efficient medium for microalgal biomass production and subsequent resource recovery [2]. Through phytoremediation, microalgae can efficiently remove contaminants from wastewater while simultaneously generating valuable biomass for agricultural applications [4]. Reusing drainage from soil-less crops to grow microalgae presents a dual opportunity; it mitigates environmental challenges while harnessing valuable resources [6].
Once treated, the phytodepurated water can be reintroduced into agricultural systems, providing a sustainable water source while retaining residual nutrients that are beneficial for plant growth. Recent studies have shown that wastewater from microalgal production can be repurposed for irrigation, providing essential nutrients to crops [8]. However, caution is advised due to high concentrations of Na and Cl, which could negatively impact plant health if not properly managed [9].
The current study focuses on Pelargonium × hortorum, one of the best-selling ornamental plants in Europe [10], highlighting its significant economic importance. This species is widely used for seasonal decoration of balconies, terraces, and green spaces, valued not only for its aesthetic appeal but also for its adaptability to diverse environmental conditions, including moderate salt tolerance [11].
The purpose of this study is to evaluate the efficacy of hydroponic effluent phytoremediation by microalgae, enabling its use in water reuse for agricultural irrigation, with special attention to its application in Pelargonium × hortorum crops.

2. Materials and Methods

2.1. Microalgal Production

In this study, the strain Chlorella vulgaris was provided by the microalgae bank of the Chemical Engineering Department of the University of Almeria, Spain. The initial inoculum was grown in a 500 mL flask and progressively scaled up to a volume of 3000 L in a tubular photobioreactor, until a concentration of 1 g·L−1 was achieved. Once this concentration was reached, the culture was transferred to a raceway photobioreactor of 80 m2, with a culture depth of 15 cm, representing a total volume of 12,000 L (Figure 1). The operating conditions of the system were maintained in a continuous regime, with a dilution rate of 0.2 day−1, pH control at 8 by CO2 injection, and dissolved oxygen regulation through air injection, with both variables managed according to system demand. At this stage, the culture medium used for microalgal growth consisted of horticultural leachates from the hydroponic cultivation of tomato (Solanum lycopersicum), obtained from the greenhouse of the IFAPA center in La Cañada.
Subsequently, to separate the microalgal biomass from the liquid phase, the differential centrifugation technique, commonly used in this type of process, was used. A GEA Westfalia Separator OTC-3-02-137 batch centrifuge (Oelde, Germany) was used. A total culture volume of 100 L was processed, separating two phases: the sediment, consisting of the algal cells, and the supernatant, corresponding to the purified liquid medium intended for irrigation, which was continuously collected in an outlet tank.

2.2. Pelargonium Experiment Facilities

The study was conducted in a 170 m2 greenhouse located at the University of Almeria (coordinates: 36°49′38″ N, 2°24′20″ W) in Spain. The installation had a centralized ventilation system, which activated forced ventilation, implemented by means of a helical exhaust fan, located at the west end of the greenhouse and equipped with a 500 r.p.m. motor, which was switched on for days when the indoor temperature exceeded 25 °C. In addition, the glasshouse had a natural ventilation system consisting of a side window with mosquito netting and a sliding door made of a metal frame with perforated mesh.
A PCE-HT 114 data logger was used to record environmental conditions, capturing temperature and relative humidity data every 10 min. The logger was placed on a shelf at a height of 1.30 m, corresponding to the level of the plants. During the experiment, the thermal conditions varied considerably, with a maximum temperature of 27.56 °C, a minimum of 12.35 °C, and a mean of 18.35 °C. Relative humidity fluctuated between 91.92% and 39.40%, with a mean of 72.40%. As for light, the average daily integral was 17.50 mol m−2 day−1.

2.3. Plant Material

Pelargonium × hortorum var. Silvia plants were supplied by the nursery Plantas de Andalucía in Almería, Spain. Transplanting was carried out in 1.5 L pots filled with coconut fiber substrate with the following characteristics: pH: 5.8–6.8; EC: 0.25–0.50 dS/m; cation exchange capacity: 60–130 meq/100 g; total organic matter: 94/98 (w/w, dry basis, %); C:N ratio: 80:1; air content: 17.5%; available water: 26%; reserve water: 2.3%; residual water: 32%; total porosity: 78%; and bulk density: 0.08 (g/cm3).

2.4. Treatments

Six different irrigation water treatments were evaluated, each with 10 replicates to ensure the reliability of the results. Irrigation was applied, on average, every two days, adjusted according to weather conditions. On cloudy days, the irrigation volume was reduced, as substrate evaporation and leaf transpiration decreased and crop water demand was lower, while on very hot days, it was proportionally increased. At the beginning of each irrigation session, a sample of the drainage water was collected to verify that approximately 20% of the volume applied had percolated, which made it possible to accurately calculate the daily dose required. Throughout the experiment, the plants received on average 100 mL of solution per irrigation.
The treatments studied were as follows: The NS treatment corresponded to a complete nutrient solution, serving as the positive control due to its optimal nutrient content. INTEL W referred to water collected directly from the leachate of a hydroponic system, without undergoing any treatment, which provided a baseline for understanding the initial condition of the effluents. The 100%OW treatment consisted entirely of hydroponic leachate water that had been treated using microalgae. The 75%OW treatment was a mixture composed of 75% microalgae-treated hydroponic leachate and 25% tap water, while the 50%OW treatment combined 50% treated leachate with 50% tap water. TAP W represented the tap water used as a negative control.
Nutrient solution: This solution was prepared by mixing the following chemical compounds in 20 L of tap water: 2.55 mL nitric acid (HNO3), 10.04 g calcium nitrate (Ca(NO3)2), 8.64 g potassium nitrate (KNO3), 2.28 g ammonium nitrate (NH4NO3), 1.76 g potassium sulphate (K2SO4), and 5.44 g monopotassium phosphate (KH2PO4).

2.5. Sampling and Analyses

2.5.1. Water Analysis

Anions and cations were quantified using high-performance liquid chromatography (HPLC) with a Metrohm 883 Basic IC Plus system. For anion analysis, a Metrosep A Supp 7—250/4.0 column was utilized, maintained at 45 °C. The analysis lasted 28 min per sample with a flow rate of 0.8 mL/min, using 3.6 mM sodium carbonate (Na2CO3) solution as the eluent. Conversely, cation determination was carried out using a Metrosep C6-150/4.0 column at a constant temperature of 45 °C. The cation analysis was completed in 20 min per sample at a flow rate of 1.1 mL/min, employing an eluent composed of 3 mM nitric acid (HNO3) and 1 mM oxalic acid.

2.5.2. Biometric Parameters

At the end of the trial, plant growth parameters were measured, with the exception of flowers, which were assessed weekly during the experiment by counting the number of flowers on each plant. Other measurements were plant height (from base to apex), number of leaves, leaf length and width, and total plant diameter. Leaf area was estimated non-destructively using the formula S = a + bLW, described by Giuffrida et al. (2011) [12], where S represents leaf area, L is leaf length (in cm), W is leaf width, and the coefficients a (0.07) and b (0.68) are species-specific. A standard measuring ruler was used for these measurements (Figure 2).
At the end of the growing season, fresh and dry matters were assessed. Plants were separated into roots, stems, leaves, and flowers, and the fresh weight of each part was recorded using a COBOS G M5-1000 balance (accuracy 0.005 g). After washing, the samples were dried on filter paper in an EFN500 oven at 65 °C for 72 h to remove all moisture. The dry weights were then measured after the samples had dried completely.

2.6. Experimental Design and Statistical Analysis

A completely randomized design was employed, comprising 6 treatments with 10 replications each, resulting in 60 experimental units overall. Treatments were randomly assigned to ensure that each unit had an equal chance of receiving any given treatment under uniform conditions. An analysis of variance (ANOVA) was performed to identify significant differences among treatments, and a least significant difference (LSD) test was subsequently applied to compare the treatment means. All statistical analyses were conducted using Statgraphics Centurion 19 software.

3. Results

3.1. Irrigation Water Quality

3.1.1. Irrigation Water Analysis

Table 1 shows the chemical analysis of the irrigation water used. The pH ranged from 5.3 to 9.4, with the lowest value recorded in the nutrient solution (NS, pH 5.3) and the highest in tap water (Tap W, pH 9.4). In the leached water treatments, INTEL W showed a pH of 6.4, which increased to a range between 8.5 and 9.0 in 100%OW, 75%OW, and 50%OW after purification by microalgae. The electrical conductivity (EC) ranged from 0.4 to 3.75 dS·m−1, with Tap W being the lowest (0.4 dS·m−1) and 100%OW being the highest (3.75 dS·m−1). Additionally, INTEL W (3.4 dS·m−1) and NS (2.3 dS·m−1) had intermediate levels, along with 75%OW (2.86 dS·m−1) and 50%OW (2.2 dS·m−1).
Nitrogen supports protein synthesis and cell division, boosting vegetative growth [13]. Additionally, nitrogen is a key element in chlorophyll, the molecule responsible for photosynthesis, which converts light energy into chemical energy [14,15]. Plants mainly absorb nitrogen in the form of nitrate (NO3−) and ammonium (NH4+), with nitrate being the predominant form [16,17]. Regarding nutrient concentrations, the nitrate content (NO3−) was highest in NS (634.86 ppm), followed by INTEL W (418.24 ppm), but it decreased drastically after purification to 151.27, 114.42, and 77.67 ppm for 100%OW, 75%OW, and 50%OW, respectively. By contrast, tap water had a lower concentration, (2.24 ppm). A similar pattern was observed for ammonium (NH4+), with the highest value in NS (34.05 ppm), followed by INTEL W (16.1 ppm), and further reductions after purification to 0.83, 0.62, and 0.35 ppm in 100%OW, 75%OW, and 50%OW, respectively, Tap W remained nearly negligible at 0.02 ppm. Phosphorus plays an important role in various cellular processes, such as maintenance of membrane structures, synthesis of biomolecules, and formation of high-energy molecules [18]. It participates in signal transduction pathways that regulate various plant responses and adaptations [19]. For phosphorus (PO43−), NS had the highest concentration (188.7 ppm), followed by INTEL W (154.75 ppm), but levels dropped significantly after purification to 22.29, 17.28, and 11.46 ppm in 100%OW, 75%OW, and 50%OW, respectively, with Tap W showing the lowest value (0.02 ppm). In the case of potassium, it helps maintain ionic homeostasis by balancing the uptake and distribution of other ions, such as sodium (Na+). This balance is particularly important under stress conditions such as salinity and drought [20,21]. Potassium (K+) showed a maximum value in NS (304.61 ppm) and decreased successively in INTEL W (146.92 ppm), 100%OW (139.34 ppm), 75%OW (105.52 ppm), and 50%OW (71.58 ppm), with Tap W (2.48 ppm) having the lowest concentration.
For calcium (Ca2+), INTEL W (106.28 ppm) outperformed even NS (97.02), while the purified treatments decreased progressively to 73.18, 63.04, and 52.54 ppm in 100%OW, 75%OW, and 50%OW, respectively. Tap water (Tap W) had the lowest value of 29.11 ppm. By contrast, the nutrient solution had the lowest magnesium concentration (10.49 ppm), compared to the highest values in 100%OW (79.35 ppm) and INTEL W (75.34 ppm), indicating significant amounts of these elements in the leached waters. Regarding sulphates (SO42−), 100%OW marked the maximum peak (304.82 ppm), followed by INTEL W (258.61 ppm) and 75%OW (230.67 ppm). NS (89.6 ppm) showed an intermediate concentration, while Tap W (12.66 ppm) had the lowest value. Likewise, chloride (Cl-) values increased markedly after purification, reaching 510.29 ppm in 100%OW, higher than in INTEL W (326.57 ppm), 75%OW (423.25 ppm), and 50%OW (228.09 ppm), NS (nutrient solution) (146.99 ppm) had the lowest concentration, while Tap W(158.9) exhibited a moderate value. Finally, sodium (Na+) reached its maximum in 100%OW (296.31 ppm), while NS (84.91 ppm) and Tap W (85 ppm) recorded the lowest concentrations. INTEL W (189.14 ppm), 75%OW (250 ppm), and 50%OW (191 ppm) showed a considerable contribution, although lower than 100%OW. These trends confirmed the effectiveness of purification in reducing nitrogen and phosphorus but also highlighted increases in salinity and alkalinity, which should be considered when reusing treated water for irrigation.

3.1.2. Plant Biometric Parameters

Figure 3 shows the average values of height and diameter of Pelargonium × hortorum evaluated under different irrigation sources. The treatment with nutrient solution (NS) registered the highest average height (22 cm), showing significant statistical differences (p < 0.05) compared to the other treatments. Regarding height, the INTEL W (17.6 cm), 100%OW (15.2 cm), and 75%OW (18.2 cm) treatments presented intermediate values without significant differences among them, while the 50%OW (11.8 cm) and Tap W (9.6 cm) treatments showed considerably lower heights. For diameter, NS obtained the best result (27.3 cm), while the INTEL W treatment (23.1 cm) was significantly superior to the treatments purified by microalgae: 100%OW (20.8 cm), 75%OW (18.6 cm), and 50%OW (17.2 cm).
Figure 4 shows the average values for the numbers of flowers and leaves. Regarding flowers, no statistically significant differences were observed, indicating that NS, INTEL W, different percentages of purified water (100%OW, 75%OW, and 50%OW), and tap water (Tap W) resulted in similar numbers of flowers. By contrast, the number of leaves showed significant differences (p < 0.05). The NS (20 leaves) and INTEL W (20.6 leaves) treatments achieved the highest production, with no significant differences between them, and outperformed the other treatments. The purified water treatments showed intermediate values (100%OW: 14.8 leaves; 75%OW: 15 leaves; 50%OW: 15 leaves), while tap water had the lowest value (11.2 leaves).
Figure 5 compares leaf area, a key indicator of photosynthetic performance and overall plant growth. The positive control (NS) treatment achieved the highest value (636.44 cm2), while the negative control (Tap W) had the smallest leaf area (113.76 cm2), highlighting the limitations associated with the absence of nutrients. INTEL W had an area of 431 cm2, showing a significant difference in comparison. Treatments with leached greenhouse water treated by microalgae showed leaf areas of 288.70 cm2 (100%OW), 272.06 cm2 (75%OW), and 206.33 cm2 (50%OW). These results indicated that, although microalgal purification partially reduced nutrient levels, all of these treatments significantly improved leaf development compared to tap water.
Figure 6 shows the results of the dry weights of the stems, leaves, roots, flowers, and total dry weight. The NS treatment showed the highest stem dry weight (1.23 g), significantly outperforming the others. By contrast, the 100%OW treatment showed the lowest value (0.586 g). A higher stem dry weight suggested a more robust structure and more efficient nutrient conduction, positively impacting plant health and productivity. The other treatments showed the following values: INTEL W (0.785 g), Tap W (0.709 g), 75% OW (0.692 g), and 50%OW (0.644 g), with no significant differences among them.
Similarly, the NS treatment achieved the highest leaf dry weight (2.34 g), with significant differences with respect to the other treatments, reflecting an optimal nutritional supply that favored leaf development. The leaf, an essential organ for photosynthesis and gas exchange, is fundamental for the production of photoassimilates and general growth. In second place, INTEL W obtained 1.611 g, showing significant improvements over the other treatments. The treatments with purified water (100%OW, 75%OW, and 50%OW) recorded values between 1.077 and 1.239 g, with no significant differences between them, but all were higher than the Tap W treatment (0.627 g), confirming that nutrient deficiency limited leaf formation. In terms of root dry weight, the 50%OW (0.643 g) and Tap W (0.593 g) treatments reached the highest values, with no significant differences between them, and both surpassed INTEL W, 100%OW, and 75%OW. The NS treatment (0.497 g) fell at an intermediate level, showing no significant differences from the other treatments. It is important to note that the root system plays a crucial role in water and nutrient uptake, as well as in anchoring the plant to the substrate.
No statistically significant differences in flower dry weight were detected between treatments, with values ranging from 0.48 g (INTEL W) to 0.269 g (75%OW). Although flowers are the reproductive organ and their development can be influenced by nutritional, climatic, and genetic factors, in this experiment, the quality and type of irrigation water did not significantly affect flower biomass production. Total dry weight, which integrates the biomass accumulated in stem, leaves, roots, and flowers, is a key indicator of overall plant growth. In this study, NS had the highest value (4.435 g), underlining the positive effect of a complete nutrient supply. The INTEL W (3.057 g) treatment showed a significant higher value than 100%OW (2.285 g) and Tap W (2.239 g), although it did not differ significantly from the 75%OW (2.606 g) and 50%OW (2.78 g) treatments (Figure 6).

4. Discussion

The use of microalgae in the purification of wastewater from agriculture and hydroponic systems offers multiple advantages. These include the purification of root exudates secreted by plants containing allelochemicals such as sugars, organic acids, and amino acids, which can affect productivity [22]. In addition, the nutrient solution derived from hydroponic waste has high concentrations of nitrogen (N) (150–600 mg·L−1) and phosphorus (P) (30–100 mg·L−1) [23], which, if released into the environment, could cause significant environmental problems. In the present study, microalgal purification was found to significantly reduce nitrate concentrations, achieving a 63% reduction compared to the input water in the 100%OW treatment, along with an 86% reduction in phosphates and 95% reduction in ammonium, confirming their ability to assimilate these nutrients. In addition, the microalgae Chlorella vulgaris has been shown to contribute significantly to the removal of Escherichia coli in wastewater; this interaction, which can evolve from mutualistic to competitive depending on the environmental conditions, has been shown to be a significant contributor to the removal of Escherichia coli in wastewater [24]. This process is essential to prevent eutrophication and enable water reuse in agricultural and hydroponic systems [25]. However, after purification, an increase in certain ions, such as sodium and chlorides, has been detected, which requires proper management to prevent adverse effects on plants; in this context, the increase in electrical conductivity (EC) serves as an indicator of the concentration of dissolved salts [26]. In the evaluated treatments, the EC was 0.4 dS·m−1 in Tap W and 3.75 dS·m−1 in 100%OW. The high EC of the 100%OW treatment was due to the fact that, after purification, nutrients still remained in the treated water, and the accumulation of salts. This situation of high EC levels in irrigation water can lead to an increase in soil salinity, negatively affecting both plant growth and yield [27], and requires the use of this water with crops that are tolerant to salinity.
The pH directly influences the availability of nutrients and the efficiency of their uptake by roots [28]. Each plant species has specific pH preferences but, generally speaking, the optimal range for most crops is between 5.4 and 6.8 [29]. Nutrient solution with a low, adjusted pH (5.3) facilitated the dissolution and utilization of nutrients. By contrast, water treated by microalgae showed an increase in pH from 6.5 in INTEL W to values between 8.5 and 9, which was attributed to the photosynthetic process whereby microalgae absorb carbon dioxide [30]. The test results showed significant variations in the water’s physicochemical properties, which depended both on its origin and on the remediation process applied. Hydroponic effluent wastewater after phytodepuration by microalgae can serve as a source of both water and nutrients, providing significant amounts of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) [31]. However, its nutritional profile does not fully meet the requirements of a standard nutrient solution (NS). In previous work, the presence of elevated levels of chloride (Cl−) and sodium (Na+) posed a risk to salt-sensitive plant species, possibly leading to ion imbalances, osmotic stress, or reduced growth [32].
The analysis of the biometric parameters of Pelargonium × hortorum provides insight into the direct influence of water quality on plant development. The NS treatment was notable for producing plants with greater height, diameter, leaf area and total biomass. An imbalance in nutrition can lead to significant yield reductions and physiological problems [33]. The treatments with microalgae-purified water, although showing decreases in parameters such as height and diameter compared to nutrient solution, offered significant advantages over the use of tap water (Tap W). As the percentage of microalgae-treated water (OW) in the mix decreased, a progressive reduction in nutrients was observed, which had an impact on growth. A balanced supply of nutrients is essential for proper plant growth and development. Ensuring that plants receive adequate amounts of essential nutrients in the correct proportions helps to optimize plant health and productivity [34].
Leaf area, which directly affects the plant’s ability to capture light for photosynthesis and to exchange gases with the atmosphere [33], showed higher values in this trial in plants irrigated with NS. This increase had a direct correlation with the number of leaves, since greater leaf development translates into a significant expansion of leaf area, as demonstrated in this research. This condition favored not only a higher efficiency in the conversion of light energy into biomass but also more vigorous vegetative growth [35]. Although the use of purified water represented a qualitative improvement over tap water, the absence or reduction of certain essential nutrients in its composition limited its optimal productive potential.
The standard nutrient solution (NS) treatment proved to be the most effective in maximizing aerial and total biomass, with the highest values for stem dry weight (1.23 g), leaf dry weight (2.34 g), and total dry weight (4.435 g). Stem dry weight is a key indicator of mechanical strength and xylem conduction capacity, elements that determine the efficiency of water and nutrient transport to the photosynthetic organs [36]. Higher leaf dry weight is directly associated with greater photosynthetic area and photoassimilate production, which results in a higher growth rate and improved resilience to environmental stress [37]. Finally, by measuring total dry weight, researchers can optimize the use of resources such as water and nutrients. For example, in a study on winter wheat, the relationship between dry matter accumulation and water use efficiency was analyzed to improve yield and resource management [38].
In the intermediate treatments (INTEL W, 75%OW, and 50%OW), although aerial biomass did not reach NS values, the dry weight obtained suggested that partial nutrient inputs can maintain a moderate growth rate without drastically compromising leaf structure. Furthermore, the increase in root dry weight in 50%OW and Tap W showed an allometric adjustment in favor of root development, known as root foraging, which allows the plant to explore a larger volume of substrate in the face of nutrient deficiencies [39]. The constancy in flower dry weight, with no significant differences, indicated that the investment in reproduction can remain stable in moderate ranges of water and nutritional availability.

5. Conclusions

In conclusion, hydroponic effluent wastewater, after phytodepuration by microalgae, has considerable concentrations of essential nutrients, especially nitrogen and potassium, which reduces dependence on chemical fertilizers. However, this treatment increases the alkalinity and salinity of the water, raising parameters such as pH, electrical conductivity, and concentrations of sodium and chloride.
Despite these chemical variations, Pelargonium × hortorum shows a remarkable tolerance to high salinity levels, which confirms its suitability for trials with treated water.
Compared to tap water, the phytodepurated effluent significantly improved plant growth indicators (number of leaves, leaf area, and height), although it did not reach the performance of a complete nutrient solution.
The results showed that phytodepuration by microalgae is an effective strategy that contributes to the decontamination of water resources and, simultaneously, the recovery of nutrients for crops, thus reducing the use of synthetic fertilizers.
Some recommendations would be to adjust the nutrient profile of the treated water to resemble that of conventional nutrient solutions, balancing macro and micronutrients; implement pH and conductivity control systems after the phytodepuration process in order to optimize conditions for plant growth; and carry out additional studies with other horticultural species to validate the tolerance and effect of phytodepurated water on different crops.

Author Contributions

Conceptualization, C.G.-S. and S.J.-B.; methodology, C.G.-S. and S.J.-B.; software, S.J.-B.; validation, C.G.-S.; formal analysis, A.R.-C.; investigation, A.R.-C.; resources, C.V.G.-L.; data curation, A.R.-C.; writing—original draft preparation, A.R.-C.; writing—review and editing, S.J.-B. and M.U.-G.; visualization, C.G.-S. and M.U.-G.; supervision, S.J.-B.; project administration, C.V.G.-L. All authors have read and agreed to the published version of the manuscript.

Funding

Project PID2021-126564OB-C31 was funded by MICIU/AEI/10.13039/501100011033 and by FED-ER, EU. Thanks to the National University of Agriculture for the Doctoral scholarship awarded to Alejandro José Rápalo Cruz.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Horticulturae 11 00547 g001
Figure 1. The 3000 L tubular photobioreactor (a). The 1200 L raceway photobioreactor (b).
Figure 1. The 3000 L tubular photobioreactor (a). The 1200 L raceway photobioreactor (b).
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Figure 2. Pelargonium × hortorum plants ready to be measured for their biometric parameters.
Figure 2. Pelargonium × hortorum plants ready to be measured for their biometric parameters.
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Figure 3. Plant height and plant diameter (cm) for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW: 100% treated water, 75%OW: 75% treated water + 25% tap water, 50%OW: 50% treated water + 50% tap water, TAP W: 100% tap water.
Figure 3. Plant height and plant diameter (cm) for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW: 100% treated water, 75%OW: 75% treated water + 25% tap water, 50%OW: 50% treated water + 50% tap water, TAP W: 100% tap water.
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Figure 4. Number of flower and number of leaves for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW: 100% treated water, 75%OW: 75% treated water + 25% tap water, 50%OW: 50% treated water + 50% tap water, TAP W: 100% tap water.
Figure 4. Number of flower and number of leaves for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW: 100% treated water, 75%OW: 75% treated water + 25% tap water, 50%OW: 50% treated water + 50% tap water, TAP W: 100% tap water.
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Figure 5. Leaf area (cm2) for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW: 100% treated water, 75%OW: 75% treated water + 25% tap water, 50%OW: 50% treated water + 50% tap water, TAP W: 100% tap water.
Figure 5. Leaf area (cm2) for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW: 100% treated water, 75%OW: 75% treated water + 25% tap water, 50%OW: 50% treated water + 50% tap water, TAP W: 100% tap water.
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Figure 6. Dry weights of stems, leaves, roots, flowers, and total dry weight (g) for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW 100% purified water, 75%OW 75% purified water + 25% tap water, 50%OW 50% purified water + 50% tap water, TAP W 100% tap water.
Figure 6. Dry weights of stems, leaves, roots, flowers, and total dry weight (g) for the different irrigation water treatments. Data are expressed as mean values ± standard error. Different letters indicate statistically significant differences (p < 0.05) according to the LSD test. NS: nutrient solution, INTEL W: leachate input water, 100%OW 100% purified water, 75%OW 75% purified water + 25% tap water, 50%OW 50% purified water + 50% tap water, TAP W 100% tap water.
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Table 1. Chemical analysis of irrigation water sources.
Table 1. Chemical analysis of irrigation water sources.
NSINTEL W100%OW75%OW50%OWTAP W
pH5.306.408.508.709.009.40
EC dSm−12.303.403.752.862.200.40
NO3− ppm634.86418.24151.27114.4277.672.24
NH4+ ppm34.0516.100.830.620.350.02
PO43− ppm188.70154.7522.2917.2811.460.02
K+ ppm304.61146.92139.34105.5271.582.48
Ca2+ ppm97.02106.2873.1863.0452.5429.11
Mg2+ ppm10.4975.3479.3562.9246.1412.04
SO42− ppm89.60258.61304.82230.67158.2412.66
Cl− ppm146.99326.57510.29423.25228.09158.90
Na+ ppm84.91189.14296.31250.00191.0085.00
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MDPI and ACS Style

Rápalo-Cruz, A.; Gómez-Serrano, C.; González-López, C.V.; Urrestarazu-Gavilán, M.; Jiménez-Becker, S. Hydroponic Wastewater Treatment with Microalgae: A Sustainable Alternative for Irrigating Pelargonium × hortorum. Horticulturae 2025, 11, 547. https://doi.org/10.3390/horticulturae11050547

AMA Style

Rápalo-Cruz A, Gómez-Serrano C, González-López CV, Urrestarazu-Gavilán M, Jiménez-Becker S. Hydroponic Wastewater Treatment with Microalgae: A Sustainable Alternative for Irrigating Pelargonium × hortorum. Horticulturae. 2025; 11(5):547. https://doi.org/10.3390/horticulturae11050547

Chicago/Turabian Style

Rápalo-Cruz, Alejandro, Cintia Gómez-Serrano, Cynthia Victoria González-López, Miguel Urrestarazu-Gavilán, and Silvia Jiménez-Becker. 2025. "Hydroponic Wastewater Treatment with Microalgae: A Sustainable Alternative for Irrigating Pelargonium × hortorum" Horticulturae 11, no. 5: 547. https://doi.org/10.3390/horticulturae11050547

APA Style

Rápalo-Cruz, A., Gómez-Serrano, C., González-López, C. V., Urrestarazu-Gavilán, M., & Jiménez-Becker, S. (2025). Hydroponic Wastewater Treatment with Microalgae: A Sustainable Alternative for Irrigating Pelargonium × hortorum. Horticulturae, 11(5), 547. https://doi.org/10.3390/horticulturae11050547

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