Methods of Aquaponics by CCRES
The Raft System
As aquaponic technology continues to develop, there are two primary methods that have evolved. The media-filled bed system, which many backyard enthusiasts use, and the raft system, which most commercial growers use. NFT (nutrient film technique) is a third, less popular method of aquaponics.
In a raft system (also known as float, deep channel and deep flow) the plants are grown in floating Polystyrene boards that float on top of water. The raft system uses filtration components that allow higher stocking densities of fish and, therefore, a higher production of plants.
A media-filled bed system uses a tank or container that is filled with gravel, perlite or another media for the plant bed. This bed is periodically flooded with water from the fish tank. The water then drains back to the fish tank. A media-filled bed system will produce a nice variety of crops. On a large scale, though, raft systems will produce more fish and produce due to the filtration components and the layout of the raft tanks.
In this article, I will discuss the raft method, the advantages and the way it works. In an aquaponic raft system, all of the water is continually recirculated from the fish tanks through filters and large, water-filled, shallow tanks. Plants sit on rafts in these tanks, with their roots dangling down into the water. The solid waste from the fish tanks is removed using a clarifier or other solids removal device.
The Research Scientists at the University of the Virgin Islands-Ag Experiment Station have developed a commercially viable raft aquaponic system that many growers have adopted because it has proven to be viable and it has the greatest base of use, data and research.
One of the unique advantages of the raft system is the high volume of water in the system. This acts as a buffer for the fish. In a typical recirculating aquaculture system (RAS), fish are stocked at a density of up to 1/2 lb of fish per gallon of water. This high stocking rate makes managing the water quality and health of the fish very challenging. In raft aquaponics, all of the water is recirculating all of the time so, even though your fish tanks are a set size, you have 5-8 times more water that is contained in the filter and raft tanks. This means that in a properly designed raft aquaponic system, even when your fish tanks are fully stocked, you are only at about 1/16th lb fish per gallon of system water.
Initially, a raft system takes a fairly large volume of water to fill but, it is one of the most efficient models of water usage in all of food production. Since the largest surface area (the raft tanks) is covered by the rafts, evaporation is minimal. Every day you replace the water that has been used in transpiration by the plants, a small amount used in cleaning the filter tanks and what is lost to evaporation. There is no water lost to weeds because there are no weeds.
The ratios of fish to plants, of fish tanks sizes to filter tank sizes and water volumes have been determined by the research scientists over a nearly 30 year period.
All of these factors contribute to raft aquaponics being the choice for many aquaponic growers. I believe raft aquaponics is currently the most efficient, dependable and developed method of aquaponic food production.
A raft system is typically comprised of fish tanks, filter tanks and the raft tank.
The fish are cultured in tanks that are plumbed to the other components in the system. Often, multiple fish tanks are used so that you can have various sizes of fish in the different tanks. In this case, young fish are stocked in a tank and then raised to harvest size in that same tank. A given period of time later (depending on what fish species you are raising), the next tank is stocked. This pattern is continued until the first tank that was stocked is ready to harvest. Once the fish are harvested from that tank, it is restocked with fingerlings. The provides regular harvests of fish and a steady nutrient supply to the plants. Your system should be sized to function properly at an even bio-mass, which will be the average weight of all of the fish in all of the tanks.
Removing the solids in raft culture allows a grower to stock the fish tanks more densely, resulting in higher production of both fish and vegetables. The solids that have been removed can be composted or dried and utilized to fertilize soil in traditional farming.
Solids capture is one of the most important processes in a raft aquaponic system. If solids are not effectively removed, you will have problems with waste build-up which will cause toxicities in the water, the plumbing will become clogged with waste and the plant roots will become coated with fine solids, which reduces the plant’s ability to uptake the nutrients.
These same factors are an issue in media-filled bed aquaponics. The build-up of organic material, coating of plant roots and fouling of the media result in reduced plant growth. In media filled bed aquaponics, after the system has been operational for a period of time, eventually the media and plants have to be removed and cleaned, causing a disruption in the plant and microbial growth.
The filtration components in raft aquaponics prevent these problems and a complete system clean-out is not required in raft aquaponics. In fact, it is discouraged because it reduces the population of beneficial bacteria and disrupts the planting cycle.
One of the most critical factors in designing a clarifier for your system is the size and required rate of water flow. The flow rate has to be fast enough to meet circulation requirements of the system and yet slow enough to allow the solids to settle out.
A bio-filter is simply a place for the beneficial bacteria to colonize. These bacteria convert ammonia to nitrite and nitrite to nitrate. In raft aquaponic systems, a separate bio-filter is not usually used because the rafts, tank walls and all other surfaces in the system provide sufficient area for the bacteria.
A degassing tank is a tank which allows the release of gasses into the air.
pH Adjustment Tank
In any aquaponic system, it is sometimes necessary to adjust the pH of the water. This needs to be done slowly so the fish or plants are not shocked by a rapid change in pH. In lager systems, there is usually a separate tank where the base or acid is added in small amounts, slowly diluted and then enters the system water stream.
In smaller systems, one of the other tanks, such as the degassing tanks or the raft tanks, can be used for pH adjustment as long as accommodations are made for the slow addition of pH changing additives.
Water Pump(s) and Sump
A water pump is used to continually move the water throughout the system. In many systems, there is also a sump tank. This is the lowest point in the system. The water gravity-flows into the sump from the raft tanks. The water pump, located in or adjacent to the sump, pumps the water into the fish tanks and throughout the rest of the system. The size of the pump you use will be directly related to the size of your system.
Air Blower(s) and Aeration System
Adequate aeration is required in an aquaponic system for the fish, the plants and the beneficial bacteria. Aeration is also critical to the biological filtration and off-gassing processes.
The raft tanks are usually rectangular in shape and are formed from cement blocks, poured cement, metal or wooden frames.
Fish and Vegetable Crops
Plants with low nutrient needs, such as lettuce and some herbs will do fine at the lower amount. Fruiting plants such as tomatoes and melons need to be at the higher feed level.
On a commercial scale, it is essential to maximize production and utilize all of the space in a greenhouse. In this scenario, the raft tanks can be butted together, making one giant lettuce or herb bed.
The raft method of aquaponics has been refined over a 25+ year period and there is more data, baseline info, hands-on operation and overall experience with this system than other methods of aquaponics. In addition, many universities, educators, researchers, commercial growers and hobbyists have adopted this method of aquaponics and the technique continues to be refined.More info at: firstname.lastname@example.org.