The Case for Aquaponics

Aquaponics is an ancient form of agriculture which has undergone a modern revival since the 1990s.  It is born out of two industries, which give it its name: AQUAculture and hydroPONICS = AQUAPONICS.
 
Solid and dissolved wastes accumulate in the water within a recirculating aquaculture system (RAS) necessitating filtration for the regular removal of the solid wastes and continuous conversion of the dissolved wastes into a less toxic form.  This results in water being discharged from the RAS daily as the filters are cleaned.  However, both waste types contain nutrients that can be utilised by plants, primarily nitrate and phosphate, but also a host of micronutrients.  Redirecting this wastewater stream to an area within the RAS that is planted, results in lush growth as these crops utilise the nutrients for their metabolic requirements.  The plants in turn reduce the nutrients to the point that, in a correctly balanced aquaponics system, all the wastes produced by the fish are utilised by the plants, allowing the cleaned water to again return to the fish in a continuous cycle.
 
Aquaponics offers several advantages over a fish-only system including the production of a second crop for minimal extra cost.  The plant crop also commonly generates more sales than the fish do as the fish wastes supports large quantities of plants.  Plant crops also attain market size sooner than the fish, thereby improving cash flow during the start up phase of the business.  As aquaponics systems have no discharge, they are sustainable and environmentally responsible; well suited to areas where biodiversity concerns preclude the use of systems that discharge wastes into the environment.
 
Types of Aquaponics
There are different types of aquaponics systems of which the ebb and flow system is the mostly widely used.  In this instance a 300mm deep bed of 20mm diameter stones is placed inside a strong container and water flows into this growbed continuously.  An automatic siphon device activates once the bed fills, causing it to empty, whereupon the siphon breaks allowing the bed to refill.  This continuous cycle of full and empty provides water and nutrients to the roots of the plants and desirable microbes living in the bed, alternated with abundant oxygen as the bed drains.  The result is a super healthy environment for the microbes and the plant roots, and the solid and dissolved fish wastes are broken down by these microbes into elemental forms that are readily available for uptake by the plants.  Consequently, these systems produce no waste.  Ebb and flow aquaponics is suitable for producing virtually all crop types including the fruiting and leafy plants.
 
An alternative form of aquaponics is the deep water culture (DWC) method whereby polystyrene rafts are floated on a trough of 30cm deep water.  Holes are cut into the polystyrene and plants are inserted into the holes so that the leaves are above the polystyrene but the roots are in the water.  Heavy aeration below the sheets ensures that the dissolved oxygen concentration remains high for the plant roots.  Although slightly cheaper to install than ebb and flow systems, DWC aquaponics is only well suited to growing leafy greens.
 
The final form of aquaponics involves running a thin film of water inside a pipe.  Plants are planted through a row of holes in the top of the pipe such that their roots dangle into the film of water, hence the name nutrient film technique (NFT).  Due to the dual disadvantages of the roots drying out rapidly during power failures, along with the heating or cooling of the thin film of water due to fluctuating air temperatures, this technique is not very popular.
 
The small stream of waste water generated by cleaning the mechanical filters in a DWC or NFT system can be treated and returned to the system, or utilised to fertilise soil grown crops.
 
A further development that has enhanced the efficiency of aquaponics is what is known as a decoupled aquaponics system.  In traditional aquaponics the plants and fish are situated in a single system where the water flows directly from the fish to the plants and back.  Given that the water cycles through the growbeds roughly 4x per hour, the temperature of the water in the plant and fish systems is the same.  Large scale installations, however, have the option of separating the two components (plants and fish) so that the wastewater from cleaning the fish filters is discharged into a highly aerated area known as a mineralisation tank, whereafter it flows to the plants at a slow rate of about 30% of the plant system volume per day.  If the replacement is too slow the plants will reduce the nutrient levels to suboptimal concentrations for their optimal growth, but this 30% replacement avoids such nutrient depletion within the plant growing area.  The cleaned plant water is directed to header tanks to replace the water lost when cleaning the filters.  In this way about 10% of the volume of the fish system is replaced daily allowing for the fish and plant systems to be operated at completely different temperatures.  It also allows for the establishment of multiple plant systems containing different plant species each held under their own optimal temperature regimes.
 
Less than 10% of the total water volume in an aquaponics system is lost weekly due to evaporation and the harvesting of crops, and this needs to be replaced with clean water.
 
Indoor vs Outdoor Installations
These systems can be installed outdoors in places where the climate suits the temperature requirements of the fish and plants throughout the year, but more commonly, the infrastructure is erected inside a greenhouse for temperature control.  Heat is trapped by day and heat loss at night is reduced.  Crop quality is further enhanced by protecting the plants from rain and pests.
 
Additives
Aquaponics is an amazing technology that allows for two crops to be produced on one primary input, fish feed.  Depending on which plants are farmed, chelated iron will need to be added, and potassium (K) and calcium (Ca) are often also needed.  The pH of the water needs to be carefully managed to maintain the optimal 6.5 required by the plants, requiring buffer to be added.  Normally the buffer is a K or Ca compound, leaving only the chelated iron to be added as a supplement.
 
Electricity
Aquaponics utilises pumps to circulate the water continuously and the fish tank is normally aerated to ensure adequate oxygen is available for the fish.  Therefore, aquaponics is dependent on stable electrical supply.  Solar systems, with battery backup for nighttime operation, are popular, but grid power can be used if it is reliable.  Whichever power supply is selected an auto-start generator is an obligatory emergency accessory as with any form of RAS, including aquaponics.
 
In Summary
Where aquaponics becomes the agricultural method of choice is where the price of vegetables or plants is high, which typically corresponds to arid climates or areas where the soil is poorly suited to crop farming.  Aquaponics is a scalable solution for the simultaneous production of plants and fish in a manner that is cost effective, light on the environment and produces crops with excellent flavour.  Aquaponics stands out as a technology that we will see far more of in the future of aquaculture.