Research Article | Open Access
Liangchang Zhang, Yurong Xue, You Wang, Chengbo Zhan, Weidang Ai, Xingyan Wang, "The Feasibility Research on Reusing High Salinity Wastewater as a Plant Nutrient Medium for Plant Hydroponics in CELSS", Space: Science & Technology, vol. 2022, Article ID 9853421, 11 pages, 2022. https://doi.org/10.34133/2022/9853421
The Feasibility Research on Reusing High Salinity Wastewater as a Plant Nutrient Medium for Plant Hydroponics in CELSS
The reuse of wastewater is a key problem in controlled ecological life support system (CELSS). In this study, the feasibility to reuse the treated wastewater (including sanitary wastewater and urine wastewater) as a plant hydroponic medium was verified. Two salt-tolerant plants, apium graveolens Linn (celery) and mesembryanthemum cordifolium L.F. (cordifolium), were cultivated with the reused medium or Hoagland nutrient solution, and the characteristics of crop growth, hydroponic medium composition, and elements absorption by plants were investigated. The results showed that the reused medium replenished with micronutrient elements could maintain the normal growth of cordifolium and celery crops. The biomass productivity of cordifolium cultivated by the recycled medium (121.53 g FW/strain) was higher than that cultivated by Hoagland nutrient solution (98.33 g FW/strain). The nutrient elements accumulation in plant dry mass indicated that plants could effectively utilize the recycled mineral elements in wastewater, and cordifolium exhibited better stability and adaptability to salt-induced environment. The absorption capacity of Na and Cl elements in cordifolium cultivated within the reused medium was the highest, which was 4.54% DW and 2.63% DW, respectively. This study demonstrated the feasibility to directly reuse domestic wastewater as the main ingredient for plant hydroponics, which could provide insights into design and operation of plant hydroponics system and water circulation system in CELSS.
Controlled ecological life support system (CELSS) is an effective way to guarantee the survival of astronauts for the long-term manned deep space exploration by continuously regenerating almost all life-sustaining materials such as food, oxygen, and water . Water cycle is an important part of CELSS system, which involves the supply of drinking water and plant irrigation water, and the recycle of urine, sanitary waste water, and air condensate water. The recovery of urine and sanitary wastewater is very difficult due to the seriously pollution, meanwhile, it is very important for establishing a high mass closure life support system, since in those wastewater, there are many mineral elements such as N, K, Mg, and Ca, that are necessities for plant growth and food production. Using urine and sanitary wastewater for plant culture is an effective way of wastewater reuse, but there are three main challenges. Firstly, some salt content in wastewater especially sodium chloride is high, but it is not necessary for plant growth and may inhibit plant growth . Secondly, the wastewater contains organic components such as surfactants, which also has a negative impact on plant growth . Thirdly, if wastewater is used as the main nutrient source, the proportion of mineral elements in wastewater might be inconsistent with the proportion of plant growth needs, which will also affect the normal growth of plants.
Over the past 20 years, researchers have been exploring appropriate wastewater treatment and reuse methods.
As early as 1997, Lisovsky et al. studied the effect of human urine as supplement on the growth of wheat. Through the comparative study of wheat cultivation in Knoppe nutrient solution and Knoppe nutrient solution added with human urine, it was proved that the addition of urine could meet 70% of the nitrogen needs in the process of wheat growth [ 4], and the nutritional components of the harvested wheat were not affected. Then, in the 2-person-103-day integrated experiment in Bios-3, all the urine produced by the two people were directly added to the wheat nutrient solution system. The results showed that the addition of urine did not reduce the yield of wheat, but the content of sodium chloride in the nutrient solution and the inedible part of wheat (straw and root) increased significantly . This reuse method reduced the supply of exogenous nutrient elements for wheat growth to a certain extent, but the longer-term accumulation effect of sodium chloride in the nutrient solution system needed to be further discussed. More importantly, in the process of directly reusing urine the nutrient solution without treatment, the decomposition and emission of urea and volatiles in the urine would cause atmospheric pollution, which also needs special attention. In order to investigate the effect of sodium chloride in urine on crop growth and explore the recovery mode of NaCl, Polonskiy and Gribovskaya  carried out the growth of a variety of salt tolerant vegetables (lettuce, celery, Chinese cabbage, etc.) under different sodium chloride concentrations, and found that there was no salt in the edible part of vegetables. The content of sodium chloride increased, with the increase of sodium chloride concentration and time, which preliminarily indicated the feasibility of extracting sodium salt from vegetables. Balnokin et al.  studied the use of simulated mineralized urine to cultivate Salicornia, and found that the content of sodium chloride in every 360 g of Salicornia could meet the daily salt intake needs of one person. However, the results of Tikhomirova et al.  showed that the addition of preliminary mineraliz human inhibited the growth of salt absorbing crops, and its biomass yield was lower than that cultivated with the nutrient solution prepared from simulated mineralized urine.
As for the reuse of sanitary wastewater rich in surfactants, Levine et al.  carried out the biodegradation of single component surfactant IGEPON TC-42 directly using wheat and soybean hydroponic culture system, and established the degradation pathway of the surfactant in the culture system, but the affection of the surfactant on the growth of wheat and soybean had not been reported. Subsequently, Nelson and Wolverton’s demonstrated that urea and surfactants had toxic effects on plant growth in soil or wetland ecosystems . In order to alleviate the negative impact of urine or sanitary wastewater on plant growth, the wastewater can be pretreated and then added into the plant culture system. Vairavan et al.  added the simulated early planet base wastewater into the nutrient solution of chives after treated by a bioreactor. It was found that the addition of treated wastewater could also improve the yield of chives, however, because the biological pretreatment was not complete, the nitrification and organic degradation processes continued to proceed in the plant culture system. Zabel et al.  used bioreactor to treat synthetic urine and then used the treated urine for tomato cultivating. They found that the yield of tomato decreased compared with Hoagland nutrient solution extraction system, and the yield decreasing might be due to the imbalance of mineral elements in synthetic urine.
The above research aims to explore a technical approach to wastewater reuse for the manned CELSS system, and the wastewater reuse methods selected by different manned integration experiments were also different. In 2014, in the 105 day integration experiment carried out in “Lunar Palace 1”, the reduced pressure distillation technology was used to treat urine; distillation condensate was used for plant irrigation; however, most of the salts and organic matter in urine were discharged out of the system in the form of distillation residue to avoid its impact on plant growth [13, 14]. And in the 4-person-180-day integrated experiment (carried out in Shenzhen, China, 2016), the urine and sanitary wastewater of the four crew members were mixed and treated in a two-stage bioreactor. The treated wastewater was recycled to the wheat hydroponic nutrient solution to supplement the water and mineral nutrients required for wheat growth. The 180 day operation results showed that this wastewater reuse method had no negative effect on wheat yield, but it should be noted that the concentration of sodium chloride in its nutrient solution continued to rise as the experiment proceeded .
According to the above research, it can be concluded that the reuse of urine and sanitary wastewater for plant culture after stabilization treatment and removal of harmful organic substances is a relatively safe and reliable way of water circulation in CELSS. However, there are still some problems to be solved, such as whether it is feasible to use the treated wastewater as the main nutrient source, and the sodium chloride accumulation in nutrient solution system. In this paper, the fresh human urine and commercial hygiene products were used to prepare mixed wastewater, and the wastewater was treated by two-stage BF-MBR process  to complete organic mineralization and nitrification transformation. The feasibility of cultivating plants with treated wastewater as the main nutrient source was studied, and a comparative study was conducted to investigate plant growth and nutrient absorption.
2. Materials and Methods
2.1. Hydroponic Media Preparation
Two hydroponic media were used for hydroponic cultivation. One was the reused medium prepared by recovery domestic wastewater from CELSS, which was the effluent from the two-stage MBfR system with anaerobic and aerobic process . The characteristics of the effluent were shown in Table 1. The domestic wastewater influent was prepared from raw urine and hygiene products, which had the characteristics of high salinity, high concentration of detergents, and complex organic matter composition and difficult biodegradation. The domestic wastewater was prepared according to the amount of domestic wastewater generated in the previous CELSS integration experiment and the level of pollutants in wastewater , as shown in Table S1.
The other medium was Hoagland nutrient solution, which was a standard plant nutrient solution. The composition of the two nutrient solutions was shown in Table 2. In comparison, the reused medium had higher concentration of nitrogen (N) and potassium (K) and roughly equal magnesium (Mg) and sulfur (S) concentration. However, the reused medium lacked of phosphorus (P) and the micronutrients (B, Mo, Cu, Zn, Fe, and Mn). Therefore, NaH2PO4·2H2O was added to the reused medium to compensate phosphate content to 47.122 mg/L. In addition, a concentrated solution of micronutrients was replenished when the plants represent maldevelopment due to lack of them.
a Supplemented by Na2HPO4·2H2O; b supplemented by micronutrients concentrate.
2.2. Crop Selection
In order to adapt the high salinity content in recovery wastewater, in this study, two kinds of crops, apium graveolens Linn (celery) and mesembryanthemum cordifolium L.F. (cordifolium), which showed good salt tolerance in previous research, were selected. Celery needs a lot of nitrogen fertilizer in the growth process, and the recovery wastewater could meet its growth requirement better. Moreover, celery is a kind of health vegetable. Because of its unique flavor, it can also be used as seasoning to bring taste enjoyment to crewmembers. Cordifolium is also a green leafy vegetable with high yield and high nutritional value. The evergreen plant can be harvested for many times to meet the long-term operation of CELSS system.
2.3. Plant Cultivation
The plant cultivation device was composed of plant cultivation boxes, floating boards, diversion plates, and the LED lighting system. There were 4 cultivation boxes (A, B, C, and D) with the same size of mm. The floating boards were made of high-quality PVC plates with a thickness of 5 mm. Each board has cultivation holes with a diameter of 3.5 cm. The overflow port, connecting port, and draining port of the cultivation box were all assembled with standard parts, which were convenient for the interface connection during the test. The floating board was installed inside the hydroponic cultivation box, on which the plants were planted. The diversion plates were welded into an inverted T shape from two 5 mm thick pieces with dimensions of mm and mm, respectively, and placed in the cultivation box to guide the water flow, so that all plants can absorb enough nutrients. The illuminance of the LED lighting system was 29.5 W/m2 in average, with light intensity ratio 1 : 1 : 1 of red : blue : white.
50 seedlings of each crop with uniform growth were selected for the hydroponic cultivation experiment. The root of each plant was cleaned and wrapped by absorbent sponge before the plant was put into the assigned cultivation hole. Celery was cultivated in box A and C, while cordifolium was planted in box B and D. For box A and B, the plants were cultivated with reused medium, and for box C and D, Hoagland nutrient solution was used. All nutrient solutions were continuously circulated at the rate of 600 ml/min between the cultivation box and nutrient solution storage tank, which led to a cycle period of h. For the first two weeks (0 ~ 13 d), the hydroponic media were at half strength, and then the storage tanks were supplied with full strength hydroponic media to gradually increase the nutrient concentration (Table 3). Each box was equipped with two aerators of the same size to maintain the dissolved oxygen concentration (). Light exposure time was set as 10 h (8:00-18:00) per day during the first 13 d, and then turn to 12 h (8:00-20:00) per day growing period. The pH, electrical conductivity and elements concentration of nutrient solution circulation tanks were measured regularly.
2.4. Analysis Method
Electrical conductivity (EC) and pH of nutrient solution were measured every day by portable EC (WTW, Multi 3620 IDS) and pH meter (Thermo, ORION STAR A214), respectively. The total nitrogen in plant dry mass was determined by the Kjeldahl Nitrogen Determination method. The ion concentrations of Sodium (Na+), ammonia-nitrogen (NH4+-N), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) were determined by cationic chromatography (Thermo Fisher-Aquion ion chromatography, Dionox ionPac-CS12A cationic chromatography, and ion chromatography), and the chloride (Cl-), nitrate-nitrogen (NO3--N), phosphorus (PO43--P), and sulfate (SO42--S) concentrations were determined by anionic chromatography (DIONEX ICS-90 ion chromatography and IonPac-AS14 ion chromatography). The concentration of micronutrient elements, including boron (B), copper (Cu), manganese (Mn), zinc (Zn), iron (Fe), and molybdenum (Mo), were determined by inductively coupled plasma spectrometer (Thermo Fisher-Icap 7400, inductively coupled plasma method).
After harvest, the crop raw mass were measured; subsequently, the samples were sterilized for 2 hours at 105°C and then kept at 75°C in an oven until a constant dry mass was taken. The dried biomass was digested in high performance microwave digestion system with mixture of nitric acid and hydrogen peroxide, and then the element concentration in the digested solution was determined by the above-mentioned methods. Besides, the total nitrogen in plant was determined by the Kjeldahl Nitrogen Determination method.
3. Results and Discussion
3.1. Crop Appearance
The growth state can be directly evaluated through crop appearance, for example, abnormal leaf color indicates the possible lack of nutrition . As shown in Figure 1, the celery cultivated in reused medium exhibited severe yellowing and wilting in the first 23 days; a concentrated liquor of micronutrient elements was added into the circulation tank of the corresponding cultivation box on the 24th day to promote microelement concentration to the same level of Hoagland medium. Then, the celery sprouted new green shoots on the 32nd day, and the appearance restored completely on the 43rd day. Therefore, it could be considered that the microelement in the recovery wastewater could not meet the growth needs of celery. The celery cultivated with Hoagland nutrient solution was verdant and robust during the experiment without any supplementary nutrients.
As same as celery, the cordifolium grown in reused medium showed a microelement shortage symptom that many leaves turned to gloomy yellow green, and a concentrated micronutrient liquor was replenished on the 30th day; then, the cordifolium appearance return normal on the 40th day. However, the cordifolium plants cultivated in Hoagland nutrient solution showed the same symptoms as cordifolium grown with reused medium on the 30th day, and there was no sign of improvement in several days after a concentrated micronutrient liquor supplement until the nutrient solution pH was regulated to acidic condition (Figure 1). The nutrient solution supplement of the four test groups during the testing time were shown in Table 3.
Over the whole growth periods, the celery and the cordifolium could be cultivated using the recovery wastewater with micronutrient elements replenishment, furthermore, the cordifolium cultivated in reused medium showed better performance than that in standard Hoagland nutrient solution.
3.2. The Characteristics of the Hydroponic Media
3.2.1. pH and Electrical Conductivity
The pH and electrical conductivity (EC) of all hydroponic media were monitored every day during the experiment. As shown in Figure 2, the pH of the reused medium, both for celery and cordifolium, was lower than Hoagland nutrient solution, that is, due to the relatively lower initial pH in the recovery wastewater; on the other hand, there are relatively higher content of mineral elements (K, Na) in the recovery wastewater, and the crops absorbed a large amount of cations and released H+, thereby reducing the nutrient solution pH. Specifically, the pH of the reused medium for celery was neutral, within a range of , over the whole period, while the pH of Hoagland nutrient solution for celery was slightly alkaline within a range of . However, the pH of the reused medium for cordifolium showed an obvious trend of acidification, and the minimum pH was 3.08 at harvest time (day 54). As for the Hoagland nutrient solution for cordifolium, the pH was stable around during the first 35 days, then, it was manually adjusted to 6.0 on day 36 because mineral elements would precipitate in an alkaline environment, which would have impacts on absorption of nutrients and the plant growth, since then, the pH of D box represented a decreasing trend due to cationic mineral elements absorption. Generally, the pH change in hydroponic media was affected by the ion absorption and metabolism of plant roots. For the consideration of acid-base balance in the solution, the pH drop dramatically in reused media for cordifolium indicates that the cations Na+ and K+ were largely absorbed by plants which was similar to the results of the ion absorption mechanism of the mesembryanthemum crystallinum Linn studied by Feng et al. .
Generally, in the absence of nutrient supplement, the EC tendency of hydroponic solution will be influenced by both plant transpiration and nutrient absorption, EC increase with transpiration and decrease with nutrient absorption. As shown in Figure 2, the EC of the reused medium was higher than that of Hoagland nutrient solution during the whole planting period. The EC of reused medium for celery exhibited a sharp rise from the day 41, when the crop growth was restored after microelement replenish. The EC of Hoagland nutrient solution for celery gradually rose from the beginning of experiment, reflecting the continuous growth of the plants. The EC of reused medium for cordifolium also had an increasing trend from the beginning, although the plant growth appeared not good before the addition of micronutrients, indicating the lack of micronutrients probably did not influence its transpiration. Obviously, the EC of Hoagland medium for cordifolium showed a trend of going down around day 15, which might be attributed to the precipitation of mineral elements in an alkaline environment, resulting a decrease of ion concentration . On day 42, the EC of Hoagland nutrient solution for cordifolium had a significant jump due to the addition of macronutrient elements. For both crop, the reused medium had faster increase in EC than Hoagland nutrient solution, which indicated that the plants transpiration cultured with recovery wastewater was stronger than that of plants cultured with Hoagland nutrient solution.
3.2.2. Concentration of Macronutrient Elements
The concentration of macronutrient elements in all hydroponic media was investigated over the experimental period as shown in Figure 3. The sharp rise of macronutrient concentration of the Hoagland nutrient solution for cordifolium cultivation was due to addition of macronutrient elements liquor on day 42. The N and K concentrations of the reused medium for celery and cordifolium had significant increases, from mg/L and mg/L to about mg/L and mg/L, respectively, which were much higher than Hoagland medium. Although the NaCl concentration of the reused medium was high and kept a rising trend, the two crop’s growth maintained normal.
During the experiment, the total input of macronutrient elements in hydroponic media and the residual at harvest were shown in Table 4. The difference of K content in the media for cordifolium was great, indicating a significant absorption of K+ and excretion of H+, which explained the drop of pH in these media.
Note: (1) Adding tap water; (2) Normal growth time.
The NaCl difference of the media for cordifolium was much higher than that of the media for celery, which indicated that the crop cordifolium could absorb more NaCl in the reused medium, thereby providing a method to relieve the NaCl accumulation in the water cycle subsystem of CELSS.
3.2.3. Micronutrient Elements
Micronutrient elements are essential for plant growth regardless of their low quantity. A remarkable enhancement of plant growth could be achieved from the joint addition of B, Cu, Mn, and Zn Mo . On the 16th, 29th, 43rd, and 54th day, the micronutrient elements concentration in each medium was tested. The nominal concentrations were shown in Table 1. As shown in Figure 4, the B and Zn concentrations could meet the requirements of the normal crop growth in the whole experimental stage. In the early stage, there was a shortage of Mo in the reused medium for both crops until the microelement solution was replenished. The Fe and Mn concentrations of each nutrient solution were fluctuating and sometimes much lower than the nominal concentration, which could be a result of these two elements precipitating under alkaline conditions. The results suggested that the symptoms of yellow leaves and stunted growth were mainly due to the low content of Fe and Mn elements in the hydroponic media. After pH adjustment, the micronutrient elements concentrations in Hoagland nutrient solution were significantly increased, and the growth of cordifolium plants was return to normal (Figure 1).
So, to maintain crop growth in Hoagland medium, it is necessary to prevent sedimentation by pH adjustment. From this point of view, the reused medium would be more conducive to nutrients absorption by plants, since the reused medium was acidic and had a better pH buffer capacity. However, it was necessary to supplement the micronutrient elements to the reused medium to keep the crop growth.
3.3. Plant Production
The height of the plants (shoot part) was recorded after the harvest, as shown in Figure 5, and Table 5 listed the biomass weight of all crops harvested in the experiment. The average height of celery cultivated by the reused medium was 20.6 cm and was significantly lower than the average height of celery cultivated by Hoagland nutrient solution, which was 35.0 cm. And the biomass yield of celery cultivated in reused medium (20.81 g FW/d) was also significantly lower than that cultivated in Hoagland nutrient solution (31.98 g FW/d). The main reason was the deficiency of micronutrient elements in reused medium for celery in the early stage, and high salinity would also limit the plant growth; the same inhibition was demonstrated by Feng et al., who studied the influence of NaCl stress on the growth of celery .
However, the average height of the cordifolium cultured by the reused medium was 21.6 cm, which was higher than that cultured by Hoagland nutrient solution (17.2 cm). And the shoot mass of cordifolium cultivated in the reused medium (52.26 g FW/d) was higher than that cultivated in Hoagland nutrient solution (43.096 g FW/d), suggesting that the reused medium was better for the plant growth. Overall, the cordifolium plants produced more biomass than celery in one growth cycle. Therefore, it could be concluded that cordifolium was more adaptable to the reused medium. In addition, we conducted statistical difference analysis on the edible parts dry weight per plant of the two crops cultivated in the two nutrient solution systems. And the value of the dry weight per plant of celery in the two nutrient solution systems was 28.21; the value was , and the value of cordifolium was 9.12; the value was 0.004, suggesting an extremely significant difference in the dry weight of the edible part of celery and cordifolium in the two nutrient solution systems.
3.4. Nutrient Assimilation in Plants
Mineral elements content in dry mass was measured to evaluate nutrient assimilation in biomass. Figure 6 showed the weight of macronutrient elements and NaCl in shoot dry mass (SDM) and root dry mass (RDM). Almost all kinds of nutrient element assimilated by the celery cultivated in the Hoagland medium were higher than that cultivated in reused medium except , that might be attributed to the absorption disorder of in alkaline condition (Figure 2). As for cordifolium, the cordifolium cultivated with the reused medium had higher N, P, K, and S but lower Ca and Mg content than those cultivated with Hoagland nutrient solution. This was consistent with the difference of nutrient content in the two hydroponic medium.
As Figure 6 showed, the two crops showed different NaCl absorption capacity, and the crop cordifolium had the higher performance than celery. The cordifolium cultivated in reused medium obtained the highest assimilation of NaCl with weight ratio 6.7%, that is much higher than Rawahy’s study in another article, in which the total content of Na+ and Cl- in tomato shoots was 1.2% on the basis of dry biomass . Furthermore, more than 95% of the NaCl absorption was concentrated in edible part. This provided a possible new way for the recycling of NaCl in CELSS. It can be calculated that about 3.6 m2 of cordifolium planting area can meet the demand of NaCl regeneration cycle for one person. And it can be inferred that the NaCl absorption capacity of this crop could be further improved when the planting process is optimized, or the NaCl concentration in the culture solution is increased. These results indicated cordifolium had good stability and adaptability to salt-induced hydroponic environment. Chen studied the physiological response of heart leaf diurnal flower to salt stress, and found that salt stress increased the enzyme activity in the plant, and then improved the physiological resistance , which is consistent with the conclusion of this experiment. In comparison, NaCl accumulation had no obvious difference in the two celery cultivated in the two type of nutrient solution, therefore, it can be concluded that the celery could adapt to reused medium with high salinity but could not absorb extra NaCl.
This study demonstrated the feasibility to reuse domestic wastewater as a plant nutrient medium for plant hydroponics. The cordifolium cultivated with the reused medium added with micronutrients exhibited the best growth and productivity among all the tests. After supplementation of the micronutrient elements and phosphorus, the reused medium could meet the growth demand for cordifolium and celery crops. The salt absorption by cordifolium had been proved, which provided a new way for NaCl extraction and recovery from wastewater in the future. The results in this study provided a practical basis for the design of domestic wastewater recycling and reusing in CELSS, and further study will focus on the integration of the wastewater treatment system and plant hydroponics.
The data used to support the findings of this study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
Liangchang Zhang is the corresponding author and responsible for data analysis. Yurong Xue is responsible for paper revision. You Wang contributed to draft manuscript writing and statistical analysis. Chengbo Zhan is in charge of grammar modification. Weidang Ai and Xingyan Wang offered many valuable help and suggestions.
This work was supported by the Foundation of National Key Laboratory of Human Factors Engineering (Grant No. SYFD061908 and 6142222200710).
Table S1 The component of 20 L domestic wastewater. (Supplementary Materials)
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