In 2019, the degree of food self-support in Taiwan was 32.08%, which was lower than that in the previous year by 2.4%. Taiwan does not have the ability to control the availability of food in the face of a food crisis. This study used an aquaponics system to implement the mutualism of fish, flowers, vegetables, and water, as well as to implement the cyclic utilization of water so that vegetables and fruit could be produced to relieve food shortages in the world.
Water Resource Per Capita
Taiwan receives abundant rainfall; however, it covers a small territory and has a dense population, as well as steep hillsides and a non-uniform time and space rainfall distribution. The utilizable water resource per capita is 1/7th of the global average, and the stream water is polluted by the factory and human discharge. The degree of food self-support in Taiwan was 32.08% in 2019, which was 2.4% lower than that in the previous year [1].
Aquaponics: Floating Islands
In this study, the concept of aquaponics, combined with floating islands, could solve four problems at once, namely, it could be used to implement the mutualism of fish, flowers, vegetables, and water. Moreover, the cyclic utilization of water can also be implemented. This equilibrium system could avoid water quality deterioration, form a symbiotic relationship between fish and plants, and it could produce vegetables and fruit on its own, thereby relieving the world food shortage. The term “aquaponics” is a combination of “aquaculture” (fish) and “hydroponics” (the soilless culture of plants) [2].
The plants in an aquaponics system can absorb nutrients from the excrement of fish in the system and purify the water, as well as reduce the water waste and produce additional crops, but the addition of an element is needed because fish mainly supply nitrogen [3]. In addition, the start-up period highly influences the N budget [4].
Aquaponics is a system that integrates recirculating aquaculture with fish-vegetable production and represents an innovative and sustainable practice [5]. A well-managed aquaponics system can increase the efficiency of nutrient retention and reduce the waste of water resources and nutritive salt discharge, which pollute the environment while earning profits simultaneously from the two economic products of fish and vegetables [5].
It is believed that the sustainable food production of aquaponics and the cyclic utilization of the nutritive elements in water will be accepted extensively by common people in the near future [6]. The waste-derived from aquaponics and aquaculture can be converted by microbes into soluble nutrients that are absorbed by plants. After these processes, the water turns into clean and safe water and flows back into the aquaculture system. The number of feeding services will influence the excretion of fish, and uneaten feed and excrement will influence the water quality [7].
Recirculating Aquaponic System
A Recirculating Aquaponic System (RAS) is a water circulation system that can co-produce fish and plants and may be used to establish a superior fish culture technology that can protect good fishery conditions, provide better water quality control, reduce the wastage of water resources, and enhance sewage management and the alimentary cycle [8,9,10,11,12] indicated that the plants should adapt well and have a strong tolerance and absorptive capacity for pollutants. However, the RAS equilibrium and maintenance management must be designed [13].
The dissolved oxygen in water may be derived from atmospheric dissolution, natural or artificial aeration, and the photosynthesis of aquatic plants. If the water is polluted by organic matter, aquatic microorganisms will consume the dissolved oxygen in the water, while decomposing the organic material, thereby inducing lower dissolved oxygen levels in the water and even causing an oxygen deficiency [14].
Global aquaculture has grown steadily and rapidly in recent decades [15]; if it is combined with an aquaponics system, it could increase the production of multiple farming resources, and the sustainable utilization of natural resources could be achieved. RAS can increase fish growth by 2.4 g per day, the amount of harvested vegetables is 22 kg, the nutritive salt in water is removed, the ammonia-N removal is 83%, the nitrite-N removal is 87%, the nitrate-N removal is 70%, the total phosphorus removal is 60%, the total suspended solids removal is 88%, and the 5-day biochemical oxygen demand removal is 63% [16].
The combination of an aquaponics system and a recirculating system can purify the nutritive salt in the water and maintain the cleanliness of the water to help fish and vegetables grow normally, thus implementing the sustainable utilization of ecological resources.
Nitrifier
Aquaponics can generate a nitrifier, in which NH4+ is converted into NO3− by nitrification. NO2− conversion also occurs during the course of nitrification. Ref. [17] observed that the reduction of the NH4+, NO2− and NO3− concentrations in an aquaponics system could reduce the dissipation of NO2−.
The vegetables that are applicable to an aquaponics system include leafy vegetables, such as Lactuca sativa, Spinacia oleracea, and Brassica chinensis Linn, as well as fruit plants, such as Solanum lycopersicum and Cucunis anguria L. [18,19]. In terms of the effect of the transmission of nitrogen in water on Lycopersicon esculentum and Brassica campestris L. subsp. chinensis planted in an aquaponics system, the Lycopersicon esculentum has a larger root surface area than Brassica campestris L. subsp. chinensis, and therefore it has abundant symbiotic nitrobacteria and can treat the nitrogen matter in water effectively [20].
Another advantage of aquaponics is that nitrogen is an element that is necessary for the growth of all organisms. Fish feed, which contains a high protein level, is the main source of nitrogen in fish culture and accounts for 50–70% of the fish production cost [21]. In comparison to general aquaculture systems, where only 25% of the nitrogen is absorbed by fish and about 70% of the ammonia is discharged into the environment, the nutrition transmission benefit of aquaponics has been observed [22].
Ecological Conservation
In terms of ecological conservation, the [23] indicated that when a floating island is isolated from the land, flying insects, spiders, fish, and amphibians preying on insects, as well as birds preying on amphibians and fish, come, in turn, to form a small floating island of a food chain ecosystem. Artificial floating islands have the following advantages: they are exempt from land costs and can save land area; the plant root systems suspended in water physically remove the suspended solids in the water; contaminants are absorbed and used by biosynthesis, and a bio-membrane adhesion medium is provided [24,25,26].
Besides the abovementioned functions, an artificial floating island provides organisms with habitat and conservation, increases the biological habitats in the water, and provides a safe place for diversified organisms, such as birds, fish, insects, and amphibians. Artificial floating islands have been used successfully to restore birds in Canada [27,28].
The purpose of this research is to build an artificial floating island that can grow vegetables and flowers on the water. In addition, the floating island will preserve its main function of purifying the water and increasing biodiversity. Based on the concept of sustainable agriculture, our research and design of floating islands will provide a potential solution for circular agriculture in Taiwan.
Comparison Between the Experimental and Control Groups
According to the comparison between the experimental and control groups, the water quality of the groups with the equipment that was installed was relatively stable and remained within the effluent regulations. The difference in the pH values of the vegetable and flower groups was slight, whereas the difference of the control group was 1.2 times lower. The average difference in the dissolved oxygen content of the vegetable and flower groups was about 1 mg/L.
The mean difference of the control group was 2 mg/L, and the maximum difference was 6.5 mg/L. The NH3-N content of the control group was 68 times that of the experimental groups due to the NH3-N derived from dead fish and leftover feed in April. In terms of the NO2 content, due to nitrification in July, the NH3-N decomposed into NO2, and the NO2 level suddenly increased to 13 mg/L, which was unsuitable for the aquatic organisms, and it was five to six times higher than of the vegetable and flower groups.
Fish Growth Rate
In terms of the fish growth rate, the water body of the control group changed too much in the early stage; the Oreochromis mossambicus could not bear the water pollution in the mid-and late-stages, and the fish all died. Thus, it could be seen that the use of floating islands with plants contributed to the improvement of the water quality and provided a favorable living environment for the fish.
Therefore, it is suggested that vegetables and flowers be planted, using the floating island design of this study and that either pet fish or edible fish be cultured, by using aquaponics. This system can purify water and provide a growing environment for fish schools, so as to achieve sustainable resources, purify ambient water, provide food for humans, and to mitigate the food crisis.
Our study points out that the Aquaponics System does help to improve the water quality, that vegetables or flowers can obtain a good growth rate by using this system, and that green energy can provide power to irrigate automatically and to convert the nutrients in the water into fertilizer.
Although the circular farming facility is simple, it provides useful data and can be a reference for local farmers. It not only saves energy and prevents water pollution, but it also provides a new option of arable land for the people.
Source: Huang, C. C., Lu, H. L., Chang, Y. H., & Hsu, T. H. (2021). Evaluation of the Water Quality and Farming Growth Benefits of an Intelligence Aquaponics System. Sustainability, 13(8), 4210.
Useful Article: Engineering Considerations And Perspectives For Nitrogen Recovery Via Aquaponics
