USA Student Brennan Cross- Cairns Aquaponics Update

Brennan Cross USA student

Mr Brennan Cross- USA University Student studying Australian Aquaponics systems with Cairns Aquaponic Gardener

When designing an aquaponics system, imagination is the limitation. Structure and appearance are highly variable as long as the basic components are provided. The diagram below (figure 2) indicates these components as the rearing tank (where fish are held), solid removal, biofilter, hydroponic subsystem, and a sump.

As the diagram indicates, certain aspects of the process can be combined structurally. From the rearing tank, solid removal may be required before the solution can continue through the system or the system itself can serve as the solid removal. This depends on the organic loading rate (OLR), or the amount of feed input resulting in fecal matter production. If the OLR is high, meaning lots of fish to not as much plant growth area, then the solid removal process must be efficiently done before the nutrient solution can continue. In this scenario screen filters or other filtration methods are used. In systems with lower OLR, less fish and larger growth area, the solution can be fed directly into hydroponic system.

Nitrifying bacteria, discussed in the previous post, serve as the biofilter. They occur naturally in a film that forms on inert surfaces and biomass. They thrive in neutral pH and warm temperatures and are vital to the process as they clean the water of harmful substances and convert them to usable nutrients. The process of biofiltration often naturally works within hydroponic systems.

As the definition of aquaponics indicates, hydroponics is used in collaboration with aquaculture to produce crops of plants and fish. But what exactly is hydroponics? Hydroponics is an agriculture strategy where plants are grown in soilless systems. Nutrients are provided, and consumed by plants through solutions of controlled concentrations. Highly soluble synthetic commercial fertilizer products are often used to mix solutions of carefully formulated combinations of nitrogen, phosphorus, and potassium as well as other micronutrients. In aquaponics, the process is the same except fish feed is the only input. The ability to control and change concentrations of nutrients for different stages of growth is why hydroponics has been so successful and increasingly popular among greenhouse growers.

So how is it done? How are these finely tuned solutions delivered to the plant roots? There are various structures used by hydroponic growers and, again, no limit as to the ways in which they can be modified for certain settings and parameters. But there are three main structures that serve as the basis for all hydroponic systems: Media filled grow beds, floating raft beds, and nutrient film technique.  Each structure has its drawbacks and positives and none are inherently better, rather, each is suited for particular uses and favor some plants over others.


media beds

media beds

Media Beds:

As the name suggests, this structure consists of beds full of growth media fed by a reservoir of solution in a separate tank. Forms of media commonly used are perlite, vermiculite, gravel, sand, expanded clay, peat, and sawdust. Coarser, aerated materials are best, especially in aquaponics. Often, the media bed structure utilizes flood and drain systems to regulate the application of nutrient solution. As the solution is added to the grow beds, it’s distributed evenly throughout the media. There are different ways in which drainage can occur. The most effective are bell siphons and timed pumps.

Bell siphons are extremely useful pieces of hardware that allow a media bed to flood and drain on a regular cycle. A gravel guard creates a pocket in the media occupied by a standing pipe beginning at the maximum water level and leading to the fish tank or sump. At the bottom of the gravel guard, holes allow the water to fill the pocket. A bell cover is placed over the stand pipe. At the bottom of the bell cover are holes allowing the water to rise within the bell cover at the same rate as in the rest of the bed. An air tube runs from the top of the bell to the minimum water level. When the water fills the bell cover and reaches the standing pipe, the pipe acts as a siphon and the bell creates a vacuum which sucks the water from the tank until it reaches the minimum water level. Hence, the bed is filled and drained on a consistent schedule without the use of external energy.

When using media beds, one must consider the efficiency of solid removal. Media beds are susceptible to clogging. If a finer grained media such as sand gets saturated with waste solids, or if the OLR is too high, the grow beds can become deoxygenated and the waste will decompose anaerobically rather than aerobically. This will result in harmful releases of methane and other gases. However, with good drainage, solid removal systems, and well aerated media, this method is a very effective promoter of biofiltration and hydroponic growth.

Floating Rafts:

Floating raft systems consist of sheets of polystyrene that house net pots. This system is ideal for many garden greens such as lettuce, basil, and spinach. Nitrification occurs in biofilm that accumulates on the bottoms of the polystyrene sheets and directly on the roots. Some solid filtration may be required depending on the OLR of the system overall. Buildup of solids on the bottom of tanks can result in anaerobic processes in floating raft system tanks as in the media beds.

Nutrient film techniqueNutrient Film Technique:

The nutrient film technique uses narrow channels to hold plants as a thin layer of solution is applied. This is a useful technique in vertical growing. However, because the surface area and water volume of NFT is considerably smaller than that of both media bed, and floating raft structures, nitrification processes are not well suited within the system and a separate stage for nitrification must be incorporated before the solution film can be applied.




In the end, the result of any of these techniques will be clean, reusable water which can return to the rearing tank and complete the cycle, allowing for happy plants and happy fish.

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