Aquaponics /ˈækwəˈpɒnɨks/ is a sustainable food production system that combines a traditional aquaculture (raising aquatic animals such as snails, fish, crayfish or prawns in tanks) with hydroponics (cultivating plants in water) in a symbiotic environment. In the aquaculture, effluents accumulate in the water, increasing toxicity for the fish. This water is led to a hydroponic system where the by-products from the aquaculture are filtered out by the plants as vital nutrients, after which the cleansed water is recirculated back to the animals. The term aquaponics is a portmanteau of the terms aquaculture and hydroponic.
Aquaponic systems vary in size from small indoor or outdoor units to large commercial units, using the same technology. The systems usually contain fresh water, but salt water systems are plausible depending on the type of aquatic animal and which plants.[citation needed] Aquaponic science may still be considered to be at an early stage.
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[edit]Function
Aquaponics consists of two main parts, with the aquaculture part for raising aquatic animals and the hydroponics part for growing plants.[1][2] Aquatic effluents resulting from uneaten feed or raising animals like fish, accumulates in water due to the closed system recirculation of most aquaculture systems. The effluent-rich water becomes toxic to the aquatic animal in high concentrations but these effluents are nutrients essential for plant growth.[1] Although consisting primarily of these two parts, aquaponics systems are usually grouped into several components or subsystems responsible for the effective removal of solid wastes, for adding bases to neutralize acids, or for maintaining water oxygenation.[1]Typical components include:
- Rearing tank: the tanks for raising and feeding the fish;
- Solids removal: a unit for catching uneaten food and detached biofilms, and for settling out fine particulates;
- Biofilter: a place where the nitrification bacteria can grow and convert ammonia into nitrates, which are usable by the plants;[1]
- Hydroponics subsystem: the portion of the system where plants are grown by absorbing excess nutrients from the water;
- Sump: the lowest point in the system where the water flows to and from which it is pumped back to the rearing tanks.
Depending on the sophistication and cost of the aquaponics system, the units for solids removal, biofiltration, and/or the hydroponics subsystem may be combined into one unit or subsystem,[1] which prevents the water from flowing directly from the aquaculture part of the system to the hydroponics part.
[edit]Nitrification
Nitrification, the aerobic conversion of ammonia into nitrates, is one of the most important functions in an aquaponics system as it reduces the toxicity of the water for fish, and allows the resulting nitrate compounds to be removed by the plants for nourishment.[1] Ammonia is steadily released into the water through the excreta and gills of fish as a product of their metabolism, but must be filtered out of the water since higher concentrations of ammonia (commonly between 0.5 and 1 ppm)[citation needed] can kill fish. Although plants can absorb ammonia from the water to some degree, nitrates are assimilated more easily,[2] thereby efficiently reducing the toxicity of the water for fish.[1] Ammonia can be converted into other nitrogenous compounds through healthy populations of:
- Nitrosomonas: bacteria that convert ammonia into nitrites, and
- Nitrobacter: bacteria that convert nitrites into nitrates.
In an aquaponics system, the bacteria responsible for this process form a biofilm on all solid surfaces throughout the system that are in constant contact with the water. The submerged roots of the vegetables combined have a large surface area, so that many bacteria can accumulate there. Together with the saliency of ammonia and nitrites in the water, the surface area determines the speed with which nitrification takes place. Care for these bacterial colonies is important as to regulate the full assimilation of ammonia and nitrite. This is why most aquaponics systems include a biofiltering unit, which helps facilitate growth of these microorganisms. Typically, after a system has stabilized ammonia levels range from 0.25 to 2.0 ppm; nitrite levels range from 0.25 to 1 ppm, and nitrate levels range from 2 to 150 ppm.[citation needed] During system startup, spikes may occur in the levels of ammonia (up to 6.0 ppm) and nitrite (up to 15 ppm), with nitrate levels peaking later in the startup phase.[citation needed] Since the nitrification process acidifies the water, non-sodium bases such as potassium hydroxide or calcium hydroxide can be added for neutralizing the water's pH[1] if insufficient quantities are naturally present in the water to provide a buffer against acidification. In addition, selected minerals or nutrients such as iron can be added in addition to the fish waste that serves as the main source of nutrients to plants.[1]
A good way to deal with solids buildup in aquaponics is the use of worms, which liquefy the solid organic matter so that it can be utilized by the plants and/or animals.
[edit]Hydroponics subsystem
Plants are grown as in hydroponics systems, with their roots immersed in the nutrient-rich effluent water. This enables them to filter out the ammonia that is toxic to the aquatic animals, or its metabolites. After the water has passed through the hydroponic subsystem, it is cleaned and oxygenated, and can return to the aquaculture vessels. This cycle is continuous. Common aquaponic applications of hydroponic systems include:
- Deep-water raft aquaponics: styrofoam rafts floating in a relatively deep aquaculture basin in troughs.
- Recirculating aquaponics: solid media such as gravel or clay beads, held in a container that is flooded with water from the aquaculture. This type of aquaponics is also known asclosed-loop aquaponics.
- Reciprocating aquaponics: solid media in a container that is alternately flooded and drained utilizing different types of siphon drains. This type of aquaponics is also known asflood-and-drain aquaponics or ebb-and-flow aquaponics.
- Other systems use towers that are trickle-fed from the top, nutrient film technique channels, horizontal PVC pipes with holes for the pots, plastic barrels cut in half with gravel or rafts in them. Each approach has its own benefits.[3]
Most green leaf vegetables grow well in the hydroponic subsystem, although most profitable are varieties of chinese cabbage, lettuce, basil, roses, tomatoes, okra, cantaloupe andbell peppers.[2] Other species of vegetables that grow well in an aquaponic system include beans, peas, kohlrabi, watercress, taro, radishes, strawberries, melons, onions, turnips,parsnips, sweet potato and herbs.[citation needed] Since plants at different growth stages require different amounts of minerals and nutrients, plant harvesting is staggered with seedings growing at the same time as mature plants. This ensures stable nutrient content in the water because of continuous symbiotic cleansing of toxins from the water.[4]
[edit]Aquaculture subsystem
Freshwater fish are the most common aquatic animal raised using aquaponics, although freshwater crayfish and prawns may also be used.[5] In practice, tilapia are the most popular fish for home and commercial projects that are intended to raise edible fish, although barramundi, Silver Perch, Eel-tailed catfish or tandanus catfish, Jade perch and Murray cod are also used.[2] For temperate climates when there isn't ability or desire to maintain water temperature, bluegill[6] and catfish[7] are suitable fish species for home systems.Koi[8] and goldfish[9] may also be used, if the fish in the system need not be edible.
[edit]Normal operations
Aquaponic systems do not typically discharge or exchange water under normal operation, but instead recirculate and reuse water very effectively. The system relies on the relationship between the animals and the plants to maintain a stable aquatic environment that experience a minimum of fluctuation in ambient nutrient and oxygen levels. Water is only added to replace water loss from absorption and transpiration by plants, evaporation into the air from surface water, overflow from the system from rainfall, and removal of biomass such as settled solid wastes from the system. As a result, aquaponics uses approximately 2% of the water that a conventionally irrigated farm requires for the same vegetable production.[citation needed] This allows for aquaponic production of both crops and fish in areas where water or fertile land is scarce. Aquaponic systems can also be used to replicate controlled wetland conditions that are useful for water treatment by reclaiming potable water from typical household sewage.[citation needed] The nutrient-filled overflow water can be accumulated in catchment tanks, and reused to accelerate growth of crops planted in soil, or it may be pumped back into the aquaponic system to top up the water level..
The three main inputs to the system are water, feed given to the aquatic animals, and electricity to pump water between the aquaculture subsystem and the hydroponics subsystem. Spawn or fry may be added to replace grown fish that are taken out from the system to retain a stable system. In terms of outputs, an aquaponics system may continually yield plants such as vegetables grown in hydroponics, and edible aquatic species raised in an aquaculture.
http://en.wikipedia.org/wiki/Aquaponics
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