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AQUAPONICS
THE THEORY BEHIND INTEGRATION:
Wilson Lennard
Aquaponics is simply the integration of recirculating fish culture and hydroponic plant production. When fish are cultured only a small proportion of the feed is converted (25 – 30%) to useable energy (Rakocy & Hargreaves, 1993). The remainder of nutrients are excreted in either solid or gaseous form. Recirculating systems are very efficient in removing solid fish wastes, by using solids filtering devices such as drum or screen filters. Gaseous wastes however, aren’t easy to remove with a filtering device and treatment consists of converting harmful ammonia and nitrite to innocuous nitrate with some form of biological filtration. Whilst biofiltration converts the harmful into the harmless, the end point is a build up of nutrients within recirculating systems, principally consisting of nitrates and phosphates. Nitrates and phosphates pose little threat to fish, although may taint the taste of fish cultured in this way. Recirculating aquaculturalists generally account for these nutrient build ups by exchanging a proportion of the system water on a daily basis, which is generally achieved through the back flushing and cleaning of filtering components. Exchange rates of water within recirculating systems are dependent upon the density of fish and the amount of food fed to the system, but approximately 10% seems to be an average daily water exchange rate in most systems. This exchange of water however, doesn’t completely keep nitrate and phosphate build up in check and some systems may run with nitrate levels as high as 500 mg/L. The bottom line for recirculating systems is that new, nitrate free water is required to replace the nitrate rich water removed from the system at times of exchange. This generally poses two additional costs in recirculating systems, the purchase of new, clean freshwater and the disposal of nutrient rich wastewaters.
On the other side of the coin, hydroponic plant farmers purchase nutrients to feed to their plants within the hydroponic system. Hydroponics is a system of farming plant material in an intensive manner. Instead of planting plants within a suitable soil (and fertilising) which supplies the nutrients for plant growth, hydroponic farmers supply the suitable nutrients directly to the water with which the plants are fed. Because the nutrients are already dissolved within the water, it is easier for the plants to uptake and use them and hydroponically grown plants generally grow larger and faster than their soil grown counterparts. As stated, hydroponic growers must purchase the nutrients they require to grow their plants. The nutrients of highest amount used in hydroponics are nitrates and phosphates.
Aquaponics recognises that fish within recirculating systems are producing nutrient wastes which are perfect for hydroponic plant production, so the rationale is to integrate the two systems to take advantage of this fact. If a ratio of plants to fish can be achieved that sees the amount of nutrient waste produced by the fish being removed entirely by the plants in an aquaponic system, then nutrient build ups should be negligible and the need for water exchanges substantially lowered. This equates to substantially lowered reliance on (sometimes costly) new, clean water and nutrient rich waste waster disposal. Research has suggested that aquaponics systems, when balanced correctly, can lower water exchange/replacement rates by as much as 90%. In more practical terms, for example, a 90% reduction in water exchange for a 500,000 L system traditionally exchanging 10% of system water per day (ie: 50,000 L changed per day = 18,250,000 L per year) lowers water consumption to 1% of system volume per day (ie: 5,000 L changed per day = 1,825,000 L per year). With the rising costs of new water and wastewater disposal, this can make substantial differences to the profitability of a system. This decrease in water exchange may also save costs in terms of the energy required to heat water in recirculating systems. A lowered water exchange rate equates to more stable water temperatures and therefore lowered energy costs to heat the incoming water. Thus, aquaponics strives to lower the reliance recirculating systems have upon purchasing new water and disposing of waste water, which also makes it a perfect technology for countries with low freshwater resources, like Australia.
In addition to maintaining water quality and lowering water exchange requirements, aquaponics offers another advantage; what was once a waste product of recirculating aquaculture (nitrates and phosphates) is being turned into a profitable, additional crop. If the vegetable material produced in an aquaponic system is sellable, then compared to traditional recirculating systems, aquaponic systems are realising double the profit. This is because in recirculating aquaculture systems, there is a cost associated with purchasing new water and a cost associated with disposing of wastewaters. Aquaponics removes these costs as the reliance on both new water and wastewater disposal is removed and, additionally, a sellable crop of plants is produced. The removal of a cost and the addition of another profit stream means that profit is potentially doubled.
Another advantage to aquaponic systems is the lowered time frame within which a profit may be realised. Traditional recirculating systems, depending on the farmed species, may take as long as 2 years to begin to produce sales of fish. This means that the profit potential of a recirculating system is something that is realised over a long period of time; maybe years. Aquaponics on the other hand, depending on the plant crop grown, may realise profitable vegetable sales within weeks of start up. Lettuce crops in summer conditions for example, may take as short as 4 weeks to produce and be sellable so at least some sales are already being realised by the business. A faster turnover of profitable material is always better for a business or enterprise.
So, it can be seen that aquaponic technology provides many advantages, both in terms of environmental protection and profitability to the business.
THE PRACTICALITY OF INTEGRATION:
From a theoretical point of view, it is easy to say that fish produce a waste product that is easily used by plants and therefore, if the two systems, recirculating aquaculture and hydroponics, are integrated we get all these great advantages. The reality is, as with most things, that integration, whilst theoretically possible, is practically more demanding.
The amount of nitrate (for reasons of simplification we will concentrate on nitrates as the principle waste product) produced in a fish culture system is directly proportional to two factors; the amount or density of fish in the system and the amount and protein content of the food. If, for example, we take a 1000 L tank containing 100 Kg of fish and feed the tank 2% of the fish body weight per day, we feed 2 Kg of feed per day. Depending on the species of fish however, the amount of waste produced will change depending on the protein content of the food, as different fish species require different protein content in their respective diets. Murray Cod, for example, are presently fed a high protein diet, principally pellets produced for Trout or Salmon farming, with a protein content of approximately 43%. Tilapia spp, on the other hand, are vegetable eaters and are fed a lower protein content feed; approximately 32%. In the same feeding and culture scenario as stated above, the tank containing Murray Cod will produce more nitrogenous waste than the Tilapia spp. tank, as the amount of nitrogenous waste produced is directly related to the feeds protein content; the higher the protein content the more nitrogenous waste. In this example, the Murray Cod tank has the potential to support more plants in the hydroponic component of an aquaponic system than does the Tilapia spp. tank as there are more nitrogenous nitrates produced in the Murray Cod system. So it can be seen that design of the hydroponic component is dependent upon the species of fish to be cultured. Similarly, Murray Cod may eat a feed of similar protein content to say, Barramundi, but Murray Cod may be held in densities as high as double that of Barramundi, so again, for the same size tank there is twice the waste production within the Murray Cod system and the hydroponic component will therefore, be approximately twice the size.
The “Golden Egg” of aquaponic research has been to develop a system where the amount of plant material within the hydroponic component is balanced with the waste nitrate production within the fish culture component, so that overall system nitrate production by fish and nitrate uptake by plants is balanced in a way that see’s no overall increase in nitrate concentration within the system water. The discussion on nitrate production within recirculating systems above demonstrates that this will always be entirely dependent on the species of fish being grown. In addition, the nitrate uptake capacity of the chosen plant species is also critical to this nitrate balance. Research on aquaponics is mainly centred in North America, where Tilapia spp. are principally grown in aquaponic systems, whilst in Australia, Murray Cod and Barramundi are being used. The higher protein content of feeds for these Australian species means that ratios and relationships between fish, nitrates and plants that may be applicable to Tilapia systems in North America are not directly transposable to an Australian aquaponics context.
Research conducted at Melbourne’s RMIT University suggests that, for 1 Kg of Murray Cod standing biomass, 20 – 25 Green Oak variety lettuce plants are required to balance system nitrate concentrations. This number of lettuce plants require approximately 0.60 m2 of hydroponic growing bed area. If we return to the example from before with a 1000 L tank and 100 Kg of fish, we can see that this 100 Kg of fish require approximately 60 m2 of hydroponic bed area. Extending this to a real situation, a 50,000 L fish culture system holds approximately 5,000 Kg of fish and thus, requires 3000 m2 of hydroponic growth bed. This area of hydroponic growth bed equates to an area of at least ten (10) 50 m x 6 m greenhouses and associated hydroponic system. So, it can be seen that in Aquaponics, especially with Australia's high protein requiring fish species, that huge areas of greenhouse production are required to balance what is a relatively small fish production system for nitrate concentration.
In addition to the question of nitrate waste production by fish and utilisation by plants in an Australian context, the question of increasing system complexity arises. A recirculating fish culture system is complex enough by itself, adding a huge hydroponic component with the associated greenhouses, hydroponic systems etc…just increases the complexity of an already complex system. Integrating a large fish production system with a hydroponic plant growth/nutrient reduction component of even greater size is a large job indeed, and it may be argued by some that the complexity outweighs the advantages. Many who know of aquaponic applications argue that aquaponic technologies are likely to be used in the small to mid-scale market of the associated fish culture and hydroponic industries. This is because the problems that may be associated with increased complexity may have the ability to lower the chances of success. In addition to this, the economics of building large-scale recirculating fish culture systems runs into the millions of dollars already, increasing the complexity and size of such systems by integrating them with a very large hydroponic component may be beyond the start-up economics required. Ongoing costs may increase unrealistically as well, as additional staff and hydroponic experts may be required. Of course, in the end, this debate will only be resolved by under taking an in depth economic analysis of the scale-up process.
Smaller-scale aquaponics have proven that integration is possible and work by James Rakocy and associates at the University of the Virgin Islands has shown that larger aquaponics systems can be economically viable. The reality is, in this and many aquaponic systems, that the majority of profit is actually made from the plant species grown, as opposed to the fish cultured. In terms of economics though, the reason for operating any production system, whether fish culture, hydroponic or aquaponic, is the making of profit. The future may see “farmers” who care little whether they grow fish, plants or both and with government legislation and public opinion evolving towards ever more sustainable and water conserving farming practices, it may become a requirement to use aquaponic technologies in some capacity and these requirements may push the introduction of larger-scale enterprises.
Whilst there may be barriers of economics and technology towards true large-scale integration using aquaponics, it must be remembered that there are other advantages of aquaponics that have not yet been discussed. Aquaponics systems, if designed correctly, have the potential to actually allow a system of decreased complexity. Many problems that may occur in a standard recirculating fish culture system stem from the fact that the culture waters contain extremely high levels of nutrients (nitrates and phosphates). These high levels of nutrients lead to high biological loads within the culture water in terms of microbial biomass which, in turn, leads to high B.O.D. (biological oxygen demand) and increased potential for disease. In addition, these conditions also may lead to less stable water chemistry, requiring higher amounts of chemical buffers to counteract nutrient build-ups.
Placing plants in the system has several advantages. The most conspicuous is the management of water nutrient levels to a point where build-up is slow, if not zero. Lowered system nutrient gives a wider margin for error in terms of the parameters discussed above. Plants also produce their own basic buffer molecules when they are assimilating nitrate across their root surfaces. Buffers are released as plants take up nitrate and these act in the same way as the chemical buffers that are added to fish culture systems. This means less is spent on the cost of buffers in integrated systems. Plants also possess the ability to remove tannins and other water staining substances, thus maybe producing water of better clarity. With plants in the system, the ability of other dangerous organisms (such as parasites and disease organisms) to proliferate is lowered and plants may even hold the ability to remove these negative organisms by direct removal or by producing chemicals that may counteract these organisms.
If a gravel substrate is used for plant growth and support, the need for a separate biological filter to process fish wastes is removed. The gravel substrate within a hydroponic component of an aquaponic system acts as the substrate for nitrification bacteria, just as a biological filter would. This lowers the costs of system production, as separate biofilters are not needed. It has even been suggested and proven (Rakocy & Hargreaves, 1993) that hydroponic components without gravel can replace biofilters. Floating hydroponic systems have been shown to have the required biofiltering capacity just because of the surface area provided for nitrifying bacteria upon the submerged plant roots, which have a collective huge surface area. In addition, plants will directly uptake a proportion of ammonia and nitrite and therefore if or when biofiltration may be lacking, the danger period for toxic ammonia effects is lowered in aquaponic systems. Some researchers have even suggested that solids filtering isn’t required within aquaponic systems, as the gravel substrate provides adequate area for mineralisation to useable nutrients.
The question of what type of hydroponic component is most suited to aquaponic systems invariably rises. Gravel culture, floating culture and NFT (nutrient film technique) technologies have all been used in aquaponic situations, with each having its own advantages. Gravel as substrate seems to be able to provide micronutrients to plants because of its extremely slow, but constant break down. Gravel will also exhibit a certain degree of buffering capacity, depending upon its mineral make-up. Work at RMIT University has shown that floating culture has the ability to remove just as much nutrient from the system as gravel culture and because no gravel is involved there is an increased ability for minerals (buffers!) released to act within the water instead of adhering to gravel particles. NFT also has advantages, such as the fact that it is commonly used in standard hydroponics, and is therefore well understood, as well as the cost effectiveness of its installation. To my mind, aquaponics using gravel is well suited to small-scale systems where its inherent weight is not so much a disadvantage to contend with, where as floating and NFT are more suited to large-scale systems where economics become a larger factor.
There are many other aspects to aquaponics and as stated, it is not just a matter of hooking up a hydroponics loop to your recirc system. A complex interaction occurs between the fish, the plants and the chemistry within the system water. That is not to say that aquaponics shouldn’t be looked at, as the advantages do have the ability to far out weigh the costs. Aquaponic technologies are in there infancy and therefore, require many hours of research, but I believe the benefits of this technology in terms of water conservation, environmental sustainability and economic advantage far out weigh the disadvantages and challenges.
As a last note, if you are interested to see if aquaponics really does work, you can do a simple experiment. If you have a recirculating fish system, remove a bucket of water from the system, cut a piece of polystyrene that fits the top of the bucket and place 2-3 lettuce seedlings through some holes in the top. In 3 weeks you should have 2-3 beautiful mature lettuce plants ready to eat.
Wilson Lennard (BSc. Hons.) is a PhD research student at RMIT University, Bundoora Campus, Melbourne. He is researching the practicalities of Aquaponic technologies and their suitability to an Australian Recirculating Aquaculture context. He can be contacted via the phone number and email provided below. He is interested in hearing from anybody who has worked with Aquaponics or is interested in this new and exciting aquaculture technology.
Ph: 9925 7155
Email: S8702774@student.rmit.edu.au
Please note that this email address was changed as at 14 January 2005. Wilson apologises to those who have been seeking to contact him and have been unsuccessful and invites them to try again - Editor |
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