Material

The Sesuvium portulacastrum plants were collected from high-salinity areas near salt fields in Ly Nhon commune, Can Gio district, Ho Chi Minh City, where soil salinity is 65‰. Each plant cutting, averaging 10 cm in length, was planted in foam planting beds (25 x 40 cm) with 3 cm diameter holes, stabilized using water-absorbing foam sheets. The plants were cultivated in seawater supplemented with Masterblend hydroponic solution (Masterblend 5-12-25 and Calcium nitrate 15.5- 0-0). At the start of the experiment, the plants were 20 days old, averaging 9.20 ± 1.45 cm in height and 2g in weight, with 84 plants per tray.

Whiteleg shrimp and tilapia used in the study were raised at the Institute for Environmental and Resources experimental site. The tilapia were included to consume excess shrimp feed, thereby improving food utilization efficiency in the aquaculture system.

The water for shrimp farming was sourced from salt fields in Ly Nhon commune, with a salinity level of 10%, diluted to achieve a 10‰ salinity level and a pH of 7.6 for the experiment.

Experimental Procedure

Batch System: To evaluate the growth of Sesuvium portulacastrum and the removal of contaminants from shrimp farming wastewater over 7 days in a batch reactor without circulation.

1. Preparation of planting environment

Germinate Sesuvium portulacastrum plants in planting beds within a seawater environment. To enhance growth, add 5 grams of mineral salt to 5 liters of seawater. The mineral salt composition is as follows:

5. Calculation of Processing Capacity: The processing capacity of the hydroponic system could be calculated using the following formula:

Where:

Aquaponic system: To evaluate the treatment capacity of the plant in a continuous system over 70 days.

Aquaponic System Design

The experimental setup for Sesuvium portulacastrum cultivation included three identical models as seen in Fig. 2, each consisting of four tanks:

The system also included a 35W air pump for Tank 1, with a capacity of 2.4 m³/hour, and a 28W water pump for Tank 2, with a capacity of 1350 liters/hour.

Control System Design

The control setup also comprised three identical models described in Fig. 3, which are similar to the experimental system but without S. portulacastrum plants, as follows:

Fig. 1.

Schematic of the batch system model during 7 days.

Fig. 2.

Schematic of the aquaponic system.

The control system included a similar air blower and water pump addition with A 34 mm circulating water pipe.

Initial Conditions

The initial water supply for the experiment had a salinity of 10‰ and a pH of 7.6. The concentrations of N-NH₄⁺, N-NO₂^-, N-NO₃^-, and total phosphorus were 3.91, 0.35, 53.6, and 33.2 mg/L, respectively.

Feeding Regime

Whiteleg shrimp were fed a diet containing 39% crude protein four times daily (7:30 am, 12 pm, 5 pm, and 9 pm). The experiment lasted 70 days and was divided into 10 weeks, with feeding amounts increased weekly as follows: Week 1: 10 g/feeding; Week 2: 12 g/ feeding; Week 3: 16 g/feeding; Week 4: 20 g/feeding; Week 5: 20 g/feeding; Week 6: 25 g/feeding; Week 7: 25 g/feeding; Week 8: 30 g/feeding; Week 9: 30 g/feeding; Week 10: 35 g/feeding. The same feeding regime was applied to both experimental tanks.

Procedure

A 28W submersible pump was installed in the fish tank (Tank 2) to circulate water from the bottom of the fish tank to the planted tank (Tank 3). Tanks 1 and 2 were connected via PVC pipes (diameter: 42 mm), allowing water to flow from the shrimp tank to the fish tank, where tilapia consumed the leftover shrimp feed. The water then flowed to Tank 3, where S. portulacastrum plants absorbed inorganic substances (N-NH₄⁺, N-NO₂^- , N-NO₃^-) and other nutrients (replaced by circulation in the control experiment). Due to the tank design, the water flowed from Tank 3 to the sedimentation tank (Tank 4) and then back to the shrimp tank (Tank 1), completing the water cycle. The total area of floating vegetation on the two tanks was about 1.5 m². The experimental tanks were placed in a greenhouse with a glass roof, utilizing natural light ranging from 10.09 to 39.0 klx (with direct sunlight from 9 am to 2 pm). The average temperature was 28–29°C, with a maximum of 33°C and a minimum of 27°C.

Fig. 3.

Layout of the control culture setup.

Monitoring and Data Collection

Daily monitoring included dissolved oxygen (DO), pH, and salinity. Parameters such as N-NH₄⁺, N-NO₂^-, N-NO₃^-, and total phosphate levels were analyzed at the beginning (day 0) and every 7 days. The shrimp growth was also monitored based on the weight and body length at the beginning and end of the experiment (70 days) with some monitoring indices:

The growth of S. portulacastrum was also determined at the end of the experiment, including indicators such as average stem height and average fresh weight.

Data Analysis

Water samples were collected and processed systematically throughout the day to ensure thorough data acquisition. Specifically, two liters of water were extracted from the shrimp tank and transported to the laboratory. In the laboratory, the samples were filtered using a 0.45 µm pore size filter paper to remove sediment, algae, and other particulate matter. Following filtration, the samples underwent decontamination according to standardized procedures.

The analysis involved measuring various parameters: Ammonium (N-NH₄⁺) levels were determined using SMEWW 4500-NH3 B&F:2017, phosphate (P-PO₄^3-) levels were analyzed as per SMEWW 4500-P.D:2017, nitrate (N-NO₃^-) was assessed with SMEWW 4500-NO₃^- .E:2017, and nitrites (N-NO₂^-) were evaluated according to SMEWW 4500-NO₂ B:2017. Daily measurement indicators included salinity, measured with an EC170 Extech (USA), pH, determined with an AZ 86021 (Taiwan), and dissolved oxygen (DO), assessed using a Milwaukee Mi605 (USA). Tree length was measured with a ruler, and fresh weight was recorded using a Nhon Hoa Scale with an accuracy of ±1 g.

Phytoplankton samples were collected using a Juday phytoplankton net with a mesh size of 20 µm, following SMEWW 10200B:2012 guidelines. These samples were fixed in situ with a saturated formalin solution, achieving a final concentration of approximately 4%. Marine phytoplankton identification was performed as previous study [16]. Quantitative analysis was carried out with a Sedgewick Rafter counting chamber, as outlined in SMEWW 10200:2017.

Data processing was executed using Microsoft Excel. The analysis involved calculating mean and standard deviation, with statistical significance tested using a p-value threshold of < 0.05. Results were visualized with descriptive statistics and charts to clearly communicate the findings.

Batch System

The efficacy of Sesuvium portulacastrum (S. portulacastrum) in treating nutrient-rich water from shrimp farming was assessed. The results, as depicted in Fig. 4a, indicate that S. portulacastrum significantly reduced nutrient concentrations compared to the control tank. Specifically, the concentration of ammonia (N-NH₄⁺) decreased from 3.91 ± 0.01 mg/L to 0.05 ± 0.002 mg/L, achieving a reduction of 98.7%. Nitrate (N-NO₃^- ) levels fell from 53.48 ± 0.006 mg/L to 2.24 ± 0.046 mg/L, representing a 95.8% reduction. Similarly, nitrite (N-NO₂^-) levels dropped from 0.31 ± 0.015 mg/L to 0.02 ± 0.002 mg/L, a decrease of 92.7%. Total phosphorus concentrations decreased from 33.60 ± 0.015 mg/L to 9.55 ± 0.026 mg/L, which is a reduction of 71.6%.

While in the control tank (which did not utilize S. portulacastrum) demonstrated lower nutrient removal efficiencies: 30.69% for N-NH₄⁺, 34.04% for N-NO₂^- , 14.82% for N-NO₃^- and 23.22% for total phosphorus. These results suggest that S. portulacastrum is substantially more effective at nutrient removal compared to traditional methods. The higher efficiencies observed in the S. portulacastrum treatment tank— 68.02% for N-NH₄⁺, 58.72% for N-NO₂^-, 80.99% for N-NO₃^-and 48.36% for total phosphorus—underline its superior performance in nutrient reduction.

Fig. 4.

Variation a) and accumulation b) of nutrients in the batch reactor.

Fig. 4b illustrates the nutrient removal capacity per gram of S. portulacastrum. Each gram of the plant can process 0.397 mg of N-NH₄⁺, 0.030 mg of N-NO₂^-, 5.276 mg of N-NO₃^-, and 2.477 mg of total phosphorus. These findings align with Buhmann et al. (2015) [17] and Hu et al. (2015) [18], who reported the effectiveness of plants in reducing excess nutrients in aquaponic systems.

Aquaponic System

Fig. 5 shows the variations in nutrient concentrations in both the aquaponic system with S. portulacastrum and the control system over 70 days. High ammonia (NH₃) levels can adversely affect shrimp health, impairing organs such as gills and hepatopancreas, and increasing susceptibility to disease. The nitrification process, facilitated by S. portulacastrum, efficiently converts NH₃ to less harmful nitrate (NO₃⁻) and nitrite (NO₂⁻), enhancing water quality and shrimp health [19]. The increased dissolved oxygen levels in the S. portulacastrum tank further support the nitrification process.

In the control tank, nutrient concentrations increased over time due to waste accumulation, leading to elevated levels of NH₄⁺, NO₃^-, NO₂^-, and PO₄^3-, particularly after day 14. This finding is consistent with previous studies indicating reduced efficiency in nutrient removal without plant intervention [20].

During the treatment, temperature and salinity were maintained at 28–29°C and 10 ppt, respectively. Fig. 6 illustrates that pH levels in the S. portulacastrum cultivation tank fluctuated between 7.49 and 7.96 but exhibited a decreasing trend over 70 days. Dissolved oxygen (DO) levels remained stable within the range of 6.10 to 6.86 mg/L. In the control tank, pH decreased from 7.95 to 6.97, and DO decreased from 6.70 mg/L to 5.07 mg/L. Both tanks maintained pH and DO levels within the permissible limits set by the Vietnamese national technical regulation on aquaculture water quality (DO ≥ 3.5 mg/L; pH: 7.0 – 9.0).

These results suggest that S. portulacastrum not only improves nutrient removal but also helps in maintaining better water quality parameters, consistent with He et al. (2022) [21].

Fig. 5.

Nutrient content variables in aquaponic and control system.

The good quality of water was evidenced by the analysis of phytoplankton composition and density in both tanks. Table 1 summarizes the findings. A total of 21 phytoplankton species were identified, with Bacillariophyceae (38.1%), Cyanobacteria (33%), and Chlorophyceae (28%). Although the species composition was similar between the tanks, the density of phytoplankton was notably different. The lower density of parasitic plants in the S. portulacastrum tank (2,842,400 individuals/L) compared to the control tank (9,442,310 individuals/L) indicates effective management of algal populations [22].

Shrimp growth and survival were significantly better in the S. portulacastrum tank. Table 2 summarizes shrimp growth and feed conversion metrics. The shrimp in the S. portulacastrum tank showed an average biomass increase from 4.20 ± 1.46 g to 19.01 ± 1.81 g, with a survival rate of 92.67% and a feed conversion ratio (FCR) of 1.62. In the control system, the average biomass increased to 16.30 ± 1.16 g, with a survival rate of 68% and an FCR of 3.13. These results reflect the positive impact of S. portulacastrum on shrimp health and productivity [23].

The growth of S. portulacastrum over the 70-day trial was substantial. The average plant height increased from 9.20 ± 1.45 cm to 39.28 ± 8.306 cm. The total fresh biomass of the S. portulacastrum reached 9,627.33 grams, with a fresh yield of 6.42 kg/m². These findings confirm that S. portulacastrum is well-suited for use in aquaponic systems, supporting water recirculation and reducing pollution discharge [17].

Comparison with Previous Studies

Numerous studies worldwide have explored the use of plants for treating water, soil, and air pollution. These studies have utilized a range of plant species, including woody, herbaceous, terrestrial, and aquatic plants. However, research specifically targeting the use of salt-tolerant plants for treating high-salinity wastewater (salt ≥ 10‰) has been relatively limited. The following Table 3 summarizes recent studies on salt-tolerant plant species used for brackish wastewater treatment:

The effectiveness of different plant species in wastewater treatment is complex. The treatment efficacy is influenced by several factors, including the plant species, the type of cultivation system (e.g., floating beds, sand, gravel), and the design of the treatment system. Additionally, the involvement of microorganisms, which are crucial for treatment, depends on environmental factors such as nutrient availability, pH, and salinity.

Fig. 6.

Variation of pH and DO parameters in aquaponic and control system.

For instance, the study by Zhang et al. [11] using S. portulacastrum involved treating 3 liters of aquaculture wastewater with four plants at an experimental temperature of 24°C. However, the pH and salinity parameters were not provided. These factors are significant as they can affect both plant and microbial activity. While Quoc et al. [13] confirmed the qualitative effectiveness of Sonneratia Caseolaris in treating wastewater with nitrogen and phosphorus components, a detailed quantitative analysis was not included.

Fernando García-Ávila [15] reported a nitrogen removal rate of 20 mg N/m²/day for Phragmites australis. This is compared to the primary production requirements for nutrient uptake by Rhizophora apiculata, which are approximately 109.43 to 173.549 kg N/ha/year and 5.467 to 8.12 kg P/ha/year, respectively [24]. The findings showed that the treatment efficiency reached 383 mg N/m²/day, demonstrating the superior performance of S. portulacastrum. This high nitrogen removal efficiency is likely due to the species' rapid growth rate and its ability to accumulate nutrients as plant proteins.