1.Introduction
Well-being trend pursuing for agricultural safety and health has spread around the world, which leads to gradual increase in production and demand for organic food (Lee, Park, & Lim., 2011). The food is any substance consumed to provide nutritional supportfor the body (Park, & Jeong, 2012). Recently, the consumption of crustaceans among seafood has increased worldwide, and among them, the consumption of shrimp has increased remarkably. Shrimp is a diet that loved for seasoning, tempura, and grilling, and an increase in shrimp farms to meet the consumption of shrimp is inevitable. In response to the domestic demand for shrimp, when farms increase, countermeasures against water pollution and odors caused by farm wastewater are also necessary. Long-term contact with wastewater discharged from shrimp farms can cause diseases such as cholera. In addition, chemicals used to prevent shrimp disease (FAO, 2018) mixed with feed containing toxic gases and ammonia nitrogen, which causes odor, and shrimp feces. It can have serious adverse effects on the ecosystem.
In fact, in the Philippines of Southeast Asia, where shrimp farms are densely distributed, there is a case where groundwater contaminated by the farm discharge water, making it impossible to use it as drinking water. Recently, the biofloc shrimp farming method as an eco-friendly shrimp farming method has been in the spotlight of academia and shrimp farmers. However, currently, the shrimp farming industry in Korea is 90% of the festival-style farms raised in natural fields (Baek, & Jeong, 2019). In the festival-style farming method, the mortality rate of shrimp due to chemicals, shrimp feces, and feed debris is close to 50%, causing environmental pollution and odor problems in nearby water systems. Therefore, in order to activate the biofloc shrimp distribution method, this study installed and tested an experimental equipment to compensate for the lack of microbial growth demand, which is a disadvantage of the biofloc shrimp farming method, at a biofloc shrimp farm located in Seocheon-gun, Chungcheongnam-do. An ultrasonic generator and an air ejector installed as a device to facilitate the metabolism of microorganisms to organic substances to help the growth of microorganisms. The ultrasonic generator and the air ejector facilitate the metabolism of microorganisms to organic substances by decomposing various organic substances and suspended substances in water into organic carbon form through cavitation and water hammer action, respectively. Therefore, this study attempts to study a method to reduce odor and improve water quality environmentally and economically by changing the distribution method of shrimp to consumers.
2.Theoretical Background
2.1. Shrimp Farming
Aquaculture is an aquatic environment mainly controlled by human activity (Zhou, Zeng, Hou, Liu, Weng, & Huang, 2020). The uncontrolled growth of shrimp farming has led to negative environmental impacts in many countries, which has increased concerns regarding the expansion of this activity (Poersch, Bauer, Kersanach, & Wasielesky, 2020). Globally, shrimp aquaculture has undergone a rapid development in the last decades, as it can help to satisfy the increasing food demand of a growing population (Dorber, Verones, Nakaoka, & Sudo, 2020).
Shrimp farming refers to cultivating freshwater or sea shrimp. In Korea, from the 1990s, farming began to flourish around the west coast, where large amounts of prawns produced. However, due to the mass mortality of the white spot virus, which is a viral disease, giant shrimp replaced by white-legged shrimp farming, which has a low incidence of disease. Shrimp culture industry has threatened by a tension of diseases that considered as a primary concern and a limiting factor (Flegel, 2006).
This trend is the same in the world. According to the FAO World Aquaculture Trends published by the Food Agriculture Organization (FAO), shrimp farming is the largest among crustaceans in recent years, and the production of white leg shrimp is the dominant. Shrimp farming and distribution methods generally referred to as shrimp farms in Korea, meaning a festival farm run in the open field. However, with the recent emphasis on eco-friendly industry, biofloc shrimp farming in plants is increasingly highlighting as a new shrimp farming distribution method.
2.2. Shrimp Farming Distribution
2.2.1. Shrimp Culture Ponds
The consumption of seafood is important to gain animal protein from the diet, and the global demand for seafood has increased in recent years because of its health advantages over meat proteins from terrestrial livestock (Stentiford, Neil, Peeler, Shields, Small, Flegeld, Vlak, Jones, Morado, Moss, Lotz, Bartholomayi, Behringer, Hautonm, & Lightnerm, 2012). In general, it refers toa culture that is cultivated in the open field, that is, outdoors, on the coast. It is a method of farming and circulating in an open-air nursery with a depth of 1.2 to 2 m on an average 3.3 ha coastal site. In fact, the distribution system is really in transition (NGO, 2021). Shrimp Culture Ponds are much lower in initial cost than plant farms, or water tank farms, because they do not require additional facilities other than facilities to supplement the site and cultured water.
Due to the characteristic of flow-through type, the standard of pollution level of discharged water is set low, so no special purification facilities are required. However, there are many restrictions due to natural conditions such as season, sunshine, air volume, and weather. In addition, since festival farms exposed to the outdoors, they are vulnerable to diseases such as infectious diseases, and chemicals are inevitable.
2.2.2. Biofloc Culture
During the last decade, biofloc technology (BFT) emerged as a potential solution for improving the sustainability of aquaculture (Bossier, & Ekasari, 2017). Biofloc can use as food or as a nutrient source for farmed shrimp, either through direct intake from culture water or as an ingredient (Castro, Pinto, & Nunes, 2021). The application of biofloc technology has been tested in an extensive range of culture species, including Pacific white shrimp, Litopenaeus vannamei (Tinh, Hai, & Verreth, 2021). Biofloc aquaculture is a distribution name for a farm that named biofloc aquaculture by the World Aquacultural Society (WAS) in Las Vegas, Nevada, USA in February 2006.
In biofloc farming, various suspended substances such as feces and feed debris of aquaculture organisms decomposed by microorganisms that artificially or naturally generated in the water of the farm, improving the water quality environment, and furthermore, the microorganisms themselves become nutrients for consumption of the aquaculture organisms. As an eco-friendly method, it prevents contamination of the surrounding ecosystem due to discharged water by eliminating the return water from the farm. A purification facility is required because it cleans and discharges sediment at the bottom of the tank. Therefore, the initial cost ofbiofloc farms is higher than that of festival farms.
2.3. Water Quality Environment for Distribution
The optimum aquatic environment for shrimp farms should maintain at least 3.5 to 6.0 ppm of dissolved oxygen. In addition, the concentration of NH3 also changes depending on the carbon dioxide concentration and pH. In the afternoon when the concentration of carbon dioxide is low and the pH is high, the concentration of NH3-N reaches its peak. NH3-N is toxic to shrimp at concentrations of 0.1 ppm or higher, so the farms try to keep the concentration below 0.1 ppm. In addition, H2S influenced by pH and water temperature, and it causes severe toxicity to shrimp even at 0.03 ppm, which smells of rotten eggs, and causes death at 0.1 ppm, so shrimp farms try to keep the concentration of H2S as low as possible. In order to meet these demanding conditions for distribution of aquaculture, frequent exchanges are essential in festival-style farms, and the discharge of toxic substances deposited during recovery adversely affects the water quality environment and seawater in the surrounding area. Likewise, in biofloc aquaculture, toxic substances are contained in water, but they mitigated by microbial degradation.
2.3.1. Optimal Frequency for Microbial Activation
The optimal frequency radiation in water serves to decompose suspended and pollutants in the water. Microorganisms help the metabolism of microorganisms through decomposition substances due to frequency and consequently contribute to the activation of microorganisms. (Lee, & Kim, 2004). However, when the frequency of the ultrasonic wave is less than 60 kHz, the cavitation phenomenon is insufficient, and when the frequency of the ultrasonic wave is more than 120 kHz, even aquatic microorganisms are killed due to the influence of cavitation. It found that the power of 200 W and the frequency of 60 kHz to 80 kHz to affect microbial activation using ultrasonic waves are optimal.
2.3.2. Definition of Air Ejector and its Impact
Aerators are essential for maintaining the dissolved oxygen level in shrimp culture operations (Hwang, Seo, & Lee, 2019; Jayanthi, Balasubramaniam, Suryaprakash, Ravisankar, & Vijayan, 2021; Kim, Lee, Lee, & Kim, 2009). An ejector is a type of pump that moves a low-pressure fluid by using the velocity of a fluid injected at high pressure (Im, Jung, & Kim, 2014). In other words, when the fluid flows through a narrow passage, the speed increases and the pressure decreases, but when flowing through a wide passage, the speed decreases and the pressure increases. Fine bubbles generated in the air ejector using this principle. The generated microbubbles rise to the surface and burst, helping to raise the amount of dissolved oxygen, helping shrimp farming.
2.3.3. Overview of Microbial Carriers
A microbial carrier is a habitat for useful microorganisms, and useful microorganisms in the carrier repeat death and proliferation. Materials used as carriers include soil, compost, ceramics, sawdust, bark, wood chips, activated carbon, cork, polyvinyl, and polyurethane. As an economic problem due to frequent replacement cycles caused by brittleness due to its large hardness or the inability to withstand the pressure in the wastewater treatment equipment, polyvinyl alcohol (PVA) and polyurethane (PU) mainly used as carriers for the wastewater treatment equipment. In this experiment, a carrier made of PVA material, which is easy to obtain on the market and can promote durability and an economical efficiency, used.
3.Research Method
3.1. Research Flow Chart
The research process showed in Figure 1. The research device installed in a biofloc shrimp farm located in Seocheon-gun, Chungcheongnam-do, and samples collected before and after treatment by the experimental device. The concentration of complex odor and H2S measured to determine the degree of reduction of odor-causing substances after installation of the research device. DO, pH, Turbidity, NO2-N, and NH3-N were measured to determine the effect of improving water quality by decomposition of organic substances, and the number of common bacteria, E. coli, and Bacillus bacteria were measured to determine the degree of microbial activation.
Figure 1: Research Flow Chart
3.2. Experimental Materials and Number of Experiments
For the research experiment, an experimental device installed directly at a biofloc shrimp farm located in Seocheon-gun. Samples used in the experiment collected in the field for collection of complex odors and water quality analysis, and collected samples were stored refrigerated below 4℃. The experiment run over 10 rounds. Complex odor analysis and microbial population analysis carried out once per round, and water component analysis of H2S, Turbidity, DO, pH, NH3-N, and NO2-N performed four times per round.
3.3. Experimental Device
The experimental setup shown in Figure 2. It consists of a culture tank (raw water), an air ejector, an ultrasonic device, and a microbial carrier. The culture tank was made of a PVC material, 6,000 mm long × 2,400 mm long × 1,200 mm high, a rectangular parallelepiped, and the air ejector was made with an inlet diameter of 25 mm, an outlet diameter of 30 mm, and an injection part diameter of 10 mm. The ultrasonic generator manufactured using a 1,500 W oscillator for AC and a chemical reaction device through a vibrator.
Figure 2: Schematic Diagram of Ultrasonic Culture Device
3.4. Analysis Equipment
3.4.1. Analysis Equipment
The analytical equipment used in the experiment to measure pH, H2S, Turbidity, NO2-N and NH3-N is a pH meter, gas chromatography (GC), Turbidity meter, UV/Vis Spectrometer, and UV/V is Spectrometer.SS, pH, NO2-N, NH3-N were analyzed in accordance with the water pollution process test standards, and complex odor and H2S were analyzed according to the odor pollution test standards was conducted.
3.4.2. Method for Measuring and Analyzing Odor
The odor analysis agent checks the presence or absence of the odor recognition test by having all the preliminary judges smell the test solution of 1 degree of the odor recognition test. If the preliminary judge does not recognize the smell, it excluded from the subject of the judge selection test. For the panel that has passed the test, the test solution for recognizing odor strength is unsealed in a well-ventilated place to smell the odor in the order of 1 to 5 degrees to recognize the degree of odor strength. On the other hand, as a method for measuring the concentration of hydrogen sulfide, the suction box method used as the sampling method and the low-temperature concentration-capillary separation tube gas chromatography analysis method and the low-temperature concentration-filling type separation tube GC analysis method are used.
3.4.3. Microbial Medium Culture Method
A sample taken and the sample itself cultured in a medium for initial microbial identification. Then, 1 ml of the initial sample put into a 9 ml test tube, diluted 10 times, and the sample cultured again in medium. The same method can use to produce a sample that diluted 10 times continuously, and this method can dilute up to 10 times to obtain an approximate number of microorganisms.
4. Research Results and Discussions
4.1. Effect of Reducing Odor-causing Substances
Table 1 shows the results of 10 experiments. In a result of verification using the T-test technique, the T value was -24.80, and the p-value was 1.35 × 10-9, which is less than the significance probability of 0.05, confirming that this experimental device is effective in reducing the odor of the shrimp farm. As can be seen in 3, the complex odor decreased from a maximum of 1,159 times to a minimum of 300 times.
Table 1:The Change of Complex Odor (Dilution Factor)
Note: *n = 10, **t = -24.80, ***p = 1.35 × 10-9
In the case of hydrogen sulfide, the result values are as shown in Table 2, and the results analyzed four times for each time. The significance level of the p-value for each round was less than 0.05, indicating that the experimental apparatus had an effect on the reduction of hydrogen sulfide. Hydrogen sulfide reduced from 1.07 ppm to 0.25 ppm on average.
Table 2:The Change of H2S (ppm)
4.2. Water Quality Improvement Effect
SS, or Turbidity, measuredto determine the degree of treatment of suspended solids in the process of processing the cultured water of the shrimp farm by the experimental device. The degree of changes in DO, pH, NO2-N, and NH3-N due to the decomposition of suspended substances and organic substances due to microbial activation in the raw water of shrimp farming measured. Fig. 3 and 4 show the average of the results of the 1st to 10th experiments of each item in a graph. The p-value of the experiment per round of each item did not exceed the significance level of 0.05. Therefore, it confirmed that this experimental apparatus had an effect on reducing Turbidity, DO, pH, NO2-N, and NH3-N. As for the pH, it found that the pH concentration increased due to the treatment of organic substances, that is, ammonia, of microorganisms, and it confirmed that suspended substances, NH3-N and NO2-N also decreased due to decomposition. In addition, dissolved oxygen appears to have increased due to the microbubble and cavitation action of the air ejector and ultrasonic waves.
Figure 3:The Graph of Turbidity Changes
Figure 4:The Graph of DO, pH, NO2-N and NH3-N Changes
4.3. Microbial Culture Results
The culture of the medium of general bacteria and E. coli made with Petri film medium of company, and the experiment performed with yellow medium for general bacteria and brown medium for E. coli. In the case of Bacillus bacteria, Agar medium containing nutrients used, and the color of the medium was red. As a result, of conducting T-test with the result of the 10th experiment, the proliferation result of general bacteria, E. coli, and Bacillus bacteria showed that the p-value was less than the significance level of 0.05. It confirmed that this experimental device has an effect on the growth of microorganisms.
Table 4:The Changes of Escherichia coli(CFU/mL)
Note: *n=10, **t = 4.34, ***p = 1.85 × 10-3
Table 5:The Changes of Bacillus sp. (CFU/mL)
Note: *n=10, **t = 2.69, ***p = 0.02
4.4. Effect of Experimental Device on Shrimp Growth and Survival
In terms of the survival rate of shrimp, the survival rate of shrimp for shipment by the conventional shrimp biofloc farming method is about 80% on average. However, the survival rate of the shrimp, which confirmed at the time of shipment after installing this experimental device, was more than 90%. Looking at the growth side of shrimp, the average size of white leg shrimp on the market is 15 to 16 cm, and the average weight is 25 to 30 g. The average size of the shrimps confirmed at shipping in the farm where this experimental device installed was 16.5 cm and the weight was 31 g, and they grew to the size and weight of oversized or king shrimps sold on the market.
5. Conclusions
Brand experience results from stimulation and leads to pleasant results, and consumers want to repeat this experience (Budi, Hidaya, & Mani, 2021). In this study, by installing ultrasonic waves, air ejectors, and microbial carrier tanks in the shrimp farm to treat cultured water with an experimental device, the following conclusions can draw by enhancing microbial activation. In this study, the conclusions are divided into theoretical and practical implications.
In conclusion, the theoretical aspect of the study is set, and practical conclusions for the theory are described based on the experimental results.
5.1. Sonochemistry reaction by ultrasonic generator
When ultrasonic waves are irradiated in water, bubbles are generated due to the effect of generating sound pressure by the ultrasonic waves, and these bubbles are repeatedly contracted and exploded by the continuously irradiated ultrasonic waves. This is called a cavitation phenomenon, and in this study, it is expected that the use of the cavitation phenomenon can create an environment in which microorganisms can more smoothly metabolize suspended substances due to the resulting decomposition of suspended substances.
In practical implications, in this and previous studies, when the water quality was improved using an ultrasonic culture device, as a result of culturing the culture medium with a microbial activity of 108, it was confirmed that microorganisms such as general bacteria, Bacillus bacteria, and E. coli were significantly increased. Therefore, it could be concluded that the negative chemical reaction by the ultrasonic generator has a positive effect on the activation of microorganisms.
5.2. Water hammer action by air ejector
In an air ejector using Bernoulli's law that the total energy is constant in a continuously flowing fluid system, pressure energy is converted into velocity energy at the front end of the ejector where the nozzle diameter decreases as high-pressure fluid passes through the air ejector. The pressure drops at the nozzle neck, and a low-pressure fluid is supplied due to this reduced pressure. Microbubbles are generated by this layer push phenomenon. As a result of this phenomenon, microbubbles are generated at the tip of the nozzle, which is expected to increase the amount of dissolved oxygen as these microbubbles rise to the surface and burst.
In practical implications, as a result of treating the shrimp farm nursing water using an ultrasonic culture device, there was an improvement effect of about 15% from 6.79 ppm to 7.98 ppm. Unlike wastewater research, this study was conducted with aquaculture water that can live in real shrimp, not wastewater. Therefore, the DO value before treatment was not so low that the improvement effect was not remarkable. However, the original purpose of increasing the microbial proliferation activation using ultrasonic waves and air ejectors, the purpose of this study, was confirmed by the noticeable increase in microbial activity.
5.3. Increased microbial activation rate due to carrier installation
Different results were expected from previous studies in which experiments were conducted using only ultrasonic waves and air ejectors. It was expected to have a positive effect on the growth of microorganisms by securing the metabolic time of the underwater microorganisms by installing a culture tank containing a carrier of 3 cm in width and 2 cm in height.
In practical implications, when the study was first planned, the experiment was conducted with only ultrasonic waves and air ejectors, as in previous studies. However, unlike previous studies in which the experiment was conducted in a closed environment by collecting wastewater, the rate of microbial activation is serious due to the semi-open experimental environment in which the water collected from the device must be supplied back to the farm for a certain period of time in the actual farm. It was low enough. However, as shown in the text result, by installing a carrier, it is possible to confirm the degree of remarkable activation of unlife.
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