How an Aquaponics System Works
By Brad Todd
In recent years, there has been resurgence toward an alternative form of agriculture known as aquaponics. While the fundamentals of these systems can be traced back to ancient times, the advent of the Internet sparked a large community of do-it-yourselfers who have adapted surplus tanks and equipment into home-based systems.
What people find most appealing about aquaponics is the symbiotic relationship between plants and animals. While this happens around us every day, aquaponic systems provide a great way to view this in real time. In its most elementary form, aquaponics consists of raising plants in nutrient-rich water provided from a source that houses aquatic species, fish being the most popular. Oftentimes, though, the most important organism of the system, beneficial bacteria, is overlooked. Before we delve into the science, we need to first examine the two major systems that make up aquaponics: hydroponics and aquaculture.
Terminology and methods
“Hydroponics” is the production of crops in a nutrient-rich solution. While there are a number of methods in which to cultivate hydroponic crops, the most popular used in aquaponics include media beds, deep water culture (DWC), and nutrient film technique (NFT). Oftentimes a combination of two or more methods may be employed into one system. The choice of which method to use is often dependent upon an individual’s skill level, available time, and financial resources. However, it’s important to note that crop selection should be taken into consideration when deciding upon the method that is right for you. Additional considerations include: media beds should flood and drain at least twice per hour; a 1/2- to 3/4-inch non-limestone gravel can be used for media; a DWC system needs at least 8 inches of water depth and aeration to function properly; and an NFT system should be adjusted to allow for proper drainage from NFT channels.
Nutrients for hydroponic systems are commonly supplied via commercial nutrient concentrates. In an “aquaponics” system, these nutrients are derived from the breakdown of fish waste and nitrification. Aquaponically grown plants require the same macronutrients and micronutrients that their terrestrial-grown counterparts require.
One of the most important parameters to monitor is pH. Nutrient uptake of plants in an aquaponic system relies heavily on proper pH. A range of 6.8 to 7.0 provides the best common ground for plants, bacteria, and fish.
A requirement often overlooked when switching from soil-grown crops to aquaponics is oxygen in the root zone. The delivery method of oxygen depends upon the growing method and delivery of the nutrient-rich solution. When using media beds, the oxygen is supplied by air, which is pulled into the media as the solution drains. If using DWC, the air is provided via air stones submerged in the solution. NFT allows the top of the root mass to be exposed to air, while the remaining roots are submerged in a thin layer of nutrients.
“Aquaculture” is the farming of aquatic organisms. Fish require the same fundamental needs — oxygen, water, nutrition, and a proper environment — as the more typical forms of livestock. Oxygen is commonly supplied through aeration. Provide water from a pathogen-free source, free of chlorination that meets the temperature requirements of the selected species. There are many water qualities that can affect the survival of your fish: dissolved oxygen, total ammonia nitrogen (TAN), pH, alkalinity, hardness, and salinity to name a few.
While there are several types of aquaculture practiced throughout the world, the three most widely used land-based aquaculture methods are raceway systems, pond culture, and recirculating aquaculture systems. Raceway systems take advantage of the topography of continuous flowing water to supply fish with fresh, clean water. Pond culture most often utilizes man-made ponds to raise fish at a lower density. Recirculating aquaculture systems (RAS) consist of a rearing tank, followed by solid filtration, then biological filtration. Oxygen is supplied by aeration in order to maintain adequate levels of dissolved oxygen to support the increased stocking density. The vast majority of aquaponic systems use RAS as their basis for design.
A center draining 3:1 diameter-to-depth round tank is ideal for rearing fish. Incoming water should be directed along the tank perimeter to facilitate self-cleaning of waste. Outlets should be screened to keep fish in tanks but allow waste to exit. It’s important to maintain a stocking density of less than 0.5 pounds fish per gallon of water, and exchange the volume of your fish tank once per hour. Additionally, solid waste filtration is a must, along with utilization of a gravity-flow filtration if possible. Examples of this are radial flow separators, swirl filters, and sock filters.
The most overlooked component of an aquaculture system is biological filtration, the system that converts the ammonia produced by the fish into nitrites, then to nitrates. A biofilter requires oxygen to provide an aerobic environment where nitrifying bacteria can thrive. There are many available designs for bio filtration. One of the most popular methods utilizes buoyant plastic media to increase the internal surface area, thus allowing for the colonization of nitrifying bacteria.
A moving bed biofilm reactor (MBBR) is an example of this. MBBRs are efficient once established, and require very little maintenance. Biofilters require four to six weeks to become properly established. While there are products that can “seed” bacteria in your biofilter, you can also let Mother Nature do the work by routinely adding pure ammonia in small doses, until you see the establishment of nitrifying bacteria. Once ammonia and nitrites have peaked and fell, nitrates should rise and remain steady as long as there is ammonia being produced. Once established, you can add fish.
How it all works together
The nitrogen cycle is the most important process in an aquaponic system; it’s what makes RAS and aquaponic systems work — the efficiency of which determines the system health and production potential. Fish feed is responsible for the majority of the nitrogen in your system. A percentage of nitrogen is converted to fish biomass, while the remaining is excreted into the culture water as ammonia, a byproduct of protein metabolism. Autotrophic bacteria oxidize the ammonia into nitrite, then oxidize to nitrates. While ammonia and nitrites can both be utilized by plants, nitrates are often the preferred form of nitrogen for plants.
The pump is unmistakably the heart of any system. Proper sizing of your pump is essential to energy conservation and filtration efficiency. Too small a pump equals solids collecting in the culture vessel and poor filtration efficiency, while too large a pump results in wasted electricity. Therefore, a great place to start is sizing the volume of your culture tank. Size your pump to exchange culture tank volume once per hour and minimize excess plumbing fittings to reduce pump restriction.
One additional component that is critical to raising fish is aeration. The most common method used on small systems is air pumps, which use atmospheric air to supply oxygen to the system through submerged air stones or diffusers. Generally, dissolved oxygen (DO) should be 5 milligrams per liter, or more, and can be tested with meters or titration kits.
Having looked at the principles, let’s look at the aquaponic system as a whole. A basic design is much like an RAS system with the addition of the plant grow bed. In the majority of RAS systems, 10 percent of the water must be replaced with clean, fresh water each day due to nitrate accumulation. By incorporating plants, which take up the surplus nitrates, we can reuse this water without wasting it. In essence, we feed the fish, which produce the nutrients that grow the plants, which absorb the nutrients, which purifies the water. Grow beds should be located after the filtration system. If unfiltered water enters the grow beds, waste can collect, resulting in anoxic conditions. After the water leaves the grow beds, it can then be returned to the fish tank. There are numerous ways a system can be arranged. As long as all the basic principles are applied, you will have a nice, user-friendly aquaponic system that can provide fresh produce and fish year-round.
Calculations and considerations
Any aquaponic system should be designed for what you are planning to grow. There is a method to the madness when calculating the amount of fish needed for the amount of plants grown or vice versa. If you are interested in growing leafy greens and herbs, you need to feed 0.125 pounds of fish feed each day per 10 square feet of plant grow space, as a general rule. If you are interested in growing fruiting plants or a combination of the two, double that amount.
It is important to note that when raising fish, it is far worse to overfeed than it is to underfeed. Too much uneaten food can compromise the filtration, as well as waste money. A simple calculation to determine feed rates is to periodically weigh your fish and multiply the average weight by the amount of individual fish in your tank, referred to as biomass. On average, feed 3 percent of the biomass in fish feed per day. To operate your system on a long-term basis, a partial harvest of fish is recommended. If you were to harvest all the fish at one time, the feed input into the system would stop, resulting in a lack of nutrients for the plants. Thus, it is best to harvest a few fish, as you need them, over time.
Plant and animal species
Ok, so what kind of fish can I grow? The answer is simple: Any freshwater species you like. However, each species of fish have their own requirements for proper water chemistry and temperature. As long as these conditions are met, you will be growing healthy and happy fish in no time. The most commonly grown food fish for small systems are tilapia and channel catfish. Both species are hardy and can withstand a lower quality of water. If you are interested in growing ornamental fish, koi and goldfish work great.
Finally, when it comes to plants in an aquaponic system, the choice is yours. Again, as long as the proper temperature, nutrients, and aeration are provided, you will grow lush plants at an elevated rate. In regards to pest treatment of plants, remember to only use fish-safe products and minimize direct contact with water.
There are many opinions and thoughts on the inner working relationships between the aquatic species, beneficial bacteria, and plants in an aquaponic system. While they will all work together in a properly designed, symbiotic system, each of these three organisms have certain requirements that must be addressed to ensure proper development and coexistence. Aquaponics is merely a three-legged table, which must be supported by individually meeting the needs of the three legs of support: aquatic species, bacteria, and plants.
As is with everything in life, aquaponics should be designed and operated to mimic nature, rather than manipulate it.
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