Dwindling natural resources are forcing us to look into more sustainable ways of living. Aquaponics is a developing sustainable food production method coupling aquaculture and horticulture together in one circular system mimicking nutrient and water cycles in nature. The methodology of aquaponics has gained increased interest in recent years and the number of aquaponic practitioners has increased greatly since 2007 [1
Aquaponics is a combination of recirculating aquaculture systems (RAS) and hydroponics, which is growing plants without soil in circulating water containing nutrients [2
]. A typical aquaponic system (Figure 1
) consists of a fish rearing tank, a solids removal tank, a biofilter, a hydroponic unit, and optionally a sump tank [3
]. In RAS, it is important to maintain good water quality for the fish health by removing solid waste and dissolved nutrients that would become toxic to the fish at high concentrations. The release of large amounts of nutrients into the environment can cause eutrophication, leading to imbalances in the ecosystem such as algal bloom and changes in the local fauna [4
]. Conversely, in hydroponics, it is beneficial to keep high nutrient concentrations for good growth and health of the plants. Combining these two production systems thus reduces nutrient waste through reuse, making the food production system more sustainable.
The water flowing from the fish rearing tank is first mechanically filtered to remove suspended solids that will adhere to plant roots and may affect rhizosphere health. Accumulated solids can lead to depleted dissolved oxygen in the water, due to a high biological- and chemical oxygen demand (BOD and COD) and elevated ammonia levels [5
]. Organic particles can also lead to reduced effectiveness of the biofilter [6
]. After removal, the solids can be mineralized to release nutrients and then re-introduced into the aquaponic system, where the released nutrients can be of further use for the plant growth [7
After solids removal, the water is passed through biomedia (biofilter), whereby the biological conversion takes place via a two-step bacterial nitrification of the ammonia in the fish waste. In the first step, ammonia is converted to nitrite, and in the second step, nitrite is converted to plant-usable nitrate. The nitrate-enriched water then flows to the hydroponic unit, where the plants remove nutrients from the water by utilizing them for growth. The main types of hydroponic setups are deep water culture (DWC), nutrient film technique (NFT), and flood-and-drain (F&D) systems, and these differ mostly in the method of irrigation [7
The success of recirculating aquaponic systems relies on a healthy environment for both fish and plants, by securing good aeration, a suitable pH in the system, efficient biological conversion of harmful ammonia to beneficial nitrate and the plant uptake of dissolved nutrients. Plants utilize nitrogen mostly as nitrate (NO3−
) or ammonium (NH4+
), but the total ammonium nitrogen (TAN) in the fish waste consists of two forms, i.e., ammonia (NH3
) and ammonium (NH4+
). Generally, biological membranes are highly permeable to NH3
but impermeable to NH4+
, consequently causing NH3
toxicity to fish at high concentrations [8
]. The NH3
balance, and thereby the level of toxicity of TAN, is related to the pH within the system, with NH3
predominating at higher pH [8
]. Nitrite, the intermediate product of bacterial conversion of ammonia to nitrate, is also toxic. Its toxicity is due to its affinity to the Cl−
binding mechanism in fish gills [9
]. It can cause methemoglobinemia and it also has an effect on the osmoregulatory and endocrine systems in fish [10
]. However, nitrite can be detoxified in aquatic animals at high environmental oxygen concentrations [11
]. The potential toxicity within the aquaponic system can be prevented by good design, planning, and management. Water temperature, pH, and good aeration to maintain sufficient dissolved oxygen are critical parameters that need to be regularly monitored and controlled.
Most aquaponic systems are small-scale hobbies or research units built by enthusiasts mostly interested in making their own food in environmentally positive cultivation [1
]. However, a few startup companies in Europe have taken the step towards commercial production [7
]. Goddek et al. [12
] suggested aquaponics as an important driver for the development of integrated food production systems and identified opportunities to fill the gap between research and implementation of commercial aquaponics. The interest in aquaponics has been awakened by both conventional aquaculture and horticulture companies since the aquaculture needs to minimize organic waste from fish farming and the horticulture is looking for environmentally friendly fertilizer, and in some cases, possibilities to diversify the business. For large-scale production systems, several challenges need to be addressed. One of these is to optimize the design and the production ratio between fish and plants and setting up pilot units that can be scaled up to commercially viable units, as conducted in the present study.
The living components of the aquaponics system drive the system design. Tilapia (Oreochromis niloticus
) is one of the most popular fish species in aquaculture and in aquaponics. A market test was carried out in Iceland in 2011 and fresh tilapia was very well received and could be sold at the same price as cod fillets in the Icelandic market. The popularity of tilapia in aquaculture is due to its omnivorous nature, rapid breeding, and fast growth. Higher productivity has been reached by all-male cultures [13
]. The Food and Agricultural Organization of the United Nations (FAO) has suggested that aquaculture of tilapia should replace agriculture of livestock in poorer regions because it has lower feed conversion ratio (FCR), based on similar feed as used for livestock, i.e., 1.6 for tilapia versus 8.8 for cereal-fed beef [14
]. Tilapia is also very hardy and tolerant of a wide range of water quality, such as a large temperature variation (15–30 °C), although thrives at the optimum of 26–28 °C. Other species of fish have also been tested in aquaponic systems. Rainbow trout (Oncorhyncus mykiss
) and brown trout (Salmo trutta
), for example, are already popular, dominant aquacultural species in Europe. They need better water quality than tilapia and are reared at a lower temperature (<17 °C), therefore requiring a larger biofilter and more aeration. Trout has been cultured with good results in a test aquaponic system at Nibio in Norway [7
Various species of plants have successfully been cultivated in aquaponic systems (Table 1
]. To date, they are all edible species. Many studies show that leafy green plants, such as various cultivars of lettuce (Lactuca sativa
) and basil (Ocimum basilicum
), give good yield in aquaponic systems [16
Aquaponics startup companies have met several challenges not least due to the small pilot units and simple setups not using best practices within both the aquaculture and horticulture parts. The main goal of this project was to design and implement a pilot commercial aquaponic system suitable for implementation and incorporation into already operating greenhouse farming in Iceland. Different aquaponic crops were tested and the production ratio between plants and fish was determined for each system. The effect of flow rate on the plant yield and nutrient removal was also assessed. Smaller tests were carried out adding Australian red claw crayfish (Cherax quadricarinatus) and red worms (Eisenia fetida) to the system. Moreover, the main aquaponics products from the tests (tilapia, pak-choi, and herbs) were offered to a consumer test group to receive their feedback on quality and acceptance for aquaponics products.
The idea of adding an aquaculture unit into smaller greenhouse farms is believed to be successful in diversifying the business. Not only do the fish and fertilizing water provide an added value, but the circular production system offers new innovative ideas for novel high value products such as crayfish and new services such as educational and experience tourism. However, further research is needed to optimize the combined production unit.
The principal conclusions highlight the importance of an effective filtering system to maintain a healthy system and a production ratio of more than 4 kg of leafy greens per kg of farmed fish. The following recommendations on system design for commercial aquaponic production systems are based on the findings of this research.
(1) Removal of solids is very important to maintain good water quality and prevent the system from collapsing. This can be done by a settling tank or a microscreen drumfilter. For commercial production systems, the drumfilter would be preferred as it can remove finer particulates.
(2) Commercial NFT systems are recommended for the hydroponics. They use less water than DWC, are easy to maintain and clean, and for well controlled large commercial systems they would not be vulnerable to mishaps. For the longevity of the F&D system, the bed has to be cleaned regularly as solids will build up in the media, regardless of filtration methods. For large systems, this would become time consuming and expensive.
(3) An improved fish feed with lower sodium and fat content would be preferable for future development to keep the system healthy and improve the sustainability and efficiency of fish farming.
(4) Sludge should be re-used to obtain a zero-waste system. As examples, crayfish grow well on solid waste from the system and vermiculture or anaerobic digestion can provide fertilizer.
(5) Leafy green vegetables with similar requirements as lettuce and pak-choi should be a good choice for production in a larger scale system. Further research into plant choices with regards to their physiological responses to environmental factors in the aquaponic systems is necessary.
(6) For less compromise between optimal conditions for fish rearing and hydroponic plant production, a decoupled design would be preferred as the plants prefer low pH and high nutrient concentrations for optimal growth, all of which have a negative effect on the fish.