Aquaponic Tech Talk 133
Aquaponics by Jason Danaher, Ph.D
A recirculating production system requires treatment of solid and dissolved fish waste in order to recycle the water for continued use. In a recirculating system only a portion of the fish diet fed is actually converted to fish biomass. The remainder is discharged from the system as solid waste or remains in the system as dissolved nutrients the fish can no longer use. These dissolved wastes can accumulate and negatively affect fish production. Dissolved nitrogen in the form of ammonia is toxic to fish and has to be controlled. Biological processes in the system help to convert toxic ammonia to less harmful nitrate through the nitrification process. Over time nitrate can accumulate in the water. Traditional methods to control nitrate are dilution of the system water volume or utilizing denitrification processes. Environmental regulations may prevent water exchanges to avoid polluting nearby water bodies and denitrification requires special components to convert the nitrate to nitrogen gas. An alternative option is to use plants to remove dissolved nutrients from the fish culture system. The plant isn’t concerned whether the nutrients come from inorganic fertilizers or aquaculture effluent if nitrogen, phosphorus, potassium and other essential dissolved elements are available for uptake.
Aquaponics combines fish production with hydroponic vegetable crop production in a recirculating aquaculture system and has received considerable attention as a result of the system’s capability to raise fish at high density, sustain water quality, minimize water exchange, and produce a marketable vegetable crop. The dissolved nitrate and phosphorus in the water, along with other essential elements from the fish diet, provide the majority of nutrients for the hydroponic plants. The plants remove and help to maintain appropriate dissolved nutrient concentrations in the production system. The hydroponic plants also help the farmer diversify their production strategy. Small and medium scale aquaponic systems can fit in backyards, on balconies and roof tops. Larger systems are being located in greenhouses or renovated warehouses close to urban settings to supply fresh fish and produce to these areas.
Recirculating aquaculture and hydroponic technology have existed independently from one another for some time now. New technology is not really needed to integrate the two, but appropriate components will ensure successful integration the two production techniques to enable unimpeded flow of the dissolved fish nutrients in the aquaponic system. One of the most important processes to not hinder water flow from fish components to hydroponic components is mechanical removal of solids. The solid fish waste does contain nutrients, but the nutrients are not in an available form for immediate plant uptake. If solids remain in the system they will begin to break down resulting in chronic health issues or acute mortality of fish and plants. Filtration components capable of removing solids as quickly as possible from the aquaponic production system is required to maintain optimal water quality and ensure aquaculture and hydroponics can be integrated successfully. Keep in mind the solid and liquid components discharged from the filters can be used for soil-based gardens, containerized plants, vegetable seedlings and compost.
There are three basic hydroponic technologies commonly used in aquaponics:
1) Raft system: plants are placed into circular cutouts in styrofoam boards and roots are suspended in nutrient solution. Floating rafts are a common hydroponic technology used in large aquaponic systems because of ease of operation and maintenance.
2) Nutrient Film Technique (NFT): plant roots are located in shallow channels and exposed to a thin film of nutrient solution passing through the channel. This technology can be used in aquaponics, but solids filtration is critical and daily maintenance is a must to maintain proper water flow through troughs.
3) Ebb & Flow: plants are supported in inert substrate (i.e. clay pebbles, pea gravel) that is flooded and then allowed to drain multiple times daily. The flood and drain method is typically used for small aquaponic systems. It’s not recommended for large systems because substrate needs to be cleaned to prevent clogging overtime.
Seedling germination will be necessary for whichever hydroponic technology you use to grow your plants in the aquaponic system. There are many variables that need to be taken into account when choosing your growing medium, including type of system, crop being grown, environment, availability, price and personal preference. Below are a few common substrates used to propagate and grow hydroponic vegetable crops in either raft systems, NFT or ebb & flow systems.
1) Rockwool® is derived from volcanic rock. It is heated to very high temperatures and spun into fibers. Those fibers are then spun into cubes, which are used in seedling propagation and long-term plant production. Suited for raft, NFT and Ebb & Flow systems. Net pots are not required.
2) Oasis® Horticubes® are a low density foam. This material drains well and was designed specifically for soilless germination of hydroponic plants. This substrate is well suited for raft, NFT and ebb & flow systems. Net pots are not required.
3) Coconut coir is considered an organic growing medium. It is made from the powdered husks of coconut. It maintains a high oxygen capacity with superior water holding capabilities. Often times it is mixed with perlite to increase air space and allow better drainage. Perlite is a type of growing medium that has been derived from volcanic rock. The rock is heated to very high temperatures where it pops, similar to popcorn. The combination of coconut coir and perlite can be used in either raft, NFT or ebb & flow systems. If used in raft or NFT systems it is recommended seedling plugs be supported in net pots.
Aquaponic systems require knowledge of fish and plant production technologies and management. The operator must have technical knowledge to identify high level issues in system management and form a specific solution to fix the problem without hampering fish or plant production. The following are a few important strategies for an aquaponic system:
1) Feed input: fish feed input should be balanced to the volume and area of the hydroponic grow area. An optimum ratio for raft systems is 60 -100 grams of feed/m2 of plant growing area/day. This equates to approximately 1/8 to ¼ pound of fish diet/yd2 of plant grow area/day. If the fish are being fed one pound of feed daily a then the plant growing area could be 4 – 8 yd2 in size.
2) Solids removal: Removing solids will maintain optimum water quality parameters and prevent hydroponic technologies from clogging with soft organic matter (i.e. fish feces, biological growth). Remember plant roots can only take up nutrients dissolved in the water.
3) Aeration: Plant roots are living cells and require oxygen. Raft systems utilize air stones to aerate the nutrient solution. NFT systems aerate the nutrient sump before it returns to the plants. Ebb & Flow systems rely on atmospheric air getting pulled into the porous substrate when the nutrient solution drains from the grow area.
4) pH control: The fish and nitrifying bacteria prefer pH levels at 7.0 to 7.5 and the plants prefer levels of 6.0 to 6.5. There must be a compromise with aquaponics therefore pH levels are maintained at 6.7 to 7.0. This range allows important biological processes to continue and keeps nutrients available for plant uptake.
5) Nutrient supplementation: Fish feed does not typically supply all the nutrients required by plants. Calcium, potassium and chelated iron often times must be added to maintain optimal levels. Nutrient concentrations can be increased while improving pH level if proper calcium and potassium based chemicals are used.
If you are interested in aquaponics at the hobbyist level or are considering a larger, commercial scale facility Pentair Aquatic Ecosystems can assist in identifying the appropriate equipment (i.e. pumps, filters, aeration, substrate) needed for you system. Additional information on recirculating aquaculture systems and aquaponic technologies can also be found at www.srac.tamu.edu/ in the form of papers and a PowerPoint presentation.