World Aquaculture 23 Aquaponics: The integration of recirculating aquaculture and hydroponics Wilson Lennard1 As the title suggests, aquaponics is the integration of two, separate, established farming technologies - recirculating fish farming and hydroponic plant farming. All aquaculturists know that fish release waste in two forms; solids and dissolved gasses. In a recirculating situation, solids are removed quickly from the system. However, the dissolved proportion remains within the water in the system. If left unchecked, these dissolved wastes will build up in the system to a point where some water must be exchanged. The exchange of system water within recirculating systems is required because, if left to build up, several situations harmful to the fish may result, such as osmotic stress from a high dissolved salt concentration. The release of a waste stream of water from a recirculating aquaculture system (RAS) is probably the number one environmental impact of the system. In addition, in countries such as Australia, water is becoming a commodity that is in short supply and water prices are rising all the time. The obvious way to counteract these two disadvantages, wastewater release and a requirement for new water, is to find a way to utilize the waste nutrients that are accumulating within the RAS. Historically, many RAS farmers have looked to bacterially mediated methods to counteract salt accumulations. This methodology basically consists of creating a chamber in-line to the RAS water flow in which anaerobic bacteria are grown. These anaerobic bacteria utilize nitrate as a food source and metabolize it to release nitrogen gas as an end prduct. Then the nitrogen gas is bled out of the system, thus counteracting waste nutrient buildup from nitrogenous compounds. However, the metabolism of feed by the fish also produces other waste nutrients, which sometimes cannot be converted to a gaseous form by the anaerobic digester and, therefore, remain within the system. A better approach to removing waste nutrients is to expose them to something that will utilize the entire suite of nutrients produced with the RAS. This is where plants and hydroponics come into the picture. Hydroponics is the water-based culture of terrestrial plants. Many people will be familiar with hydroponic tomatoes and lettuce, inasmuch as they are readily available in supermarkets. In fact, many of the vegetables we eat can be produced hydroponically. In some countries, The Netherlands for instance, hydroponics is becoming a principle method for the production of fresh vegetable produce. Hydroponic producers go to great lengths and expense to provide a profile of nutrients perfect for the vegetable crop being grown. The nutrients used in hydroponics are inorganic salts that are purchased and added to the water. Aquaponics simply makes the connection between the fact that RAS fish farming produces waste nutrients and hydroponics utilizes nutrients for plant production. Because both systems are water-based, integration is a logical step. A challenge of this integration is that fish do not produce the perfect nutrient profile for plant culture. This is because fish feed is formulated for efficient fish growth and not for efficient plant growth. Research has been going on with regard to this for approximately 30 years. The approach that has been adopted until recently was that nutrients should still be allowed to accumulate in aquaponic systems because this gives the plants access to the limiting nutrients. I have recently finished a four-year study of aquaponics at RMIT University in Melbourne, Australia. I decided to take a different approach to this problem inasmuch as I wanted the highest efficiency possible from the aquaponic system in terms of removing as much nutrient from the RAS water as possible. My research centered on examining a number of parameters within the aquaponic system and optimizing them to achieve the end point of removing as much nutrient as possible. My optimization studies demonstrated that it is possible to remove close to 100 percent of the waste nutrients within a RAS by utilizing hydroponic plant production as the nutrient removal method. I have now built a small, commercial scale aquaponic system outside of Melbourne to further this research and to try and prove the commercial viability of aquaponic integration. We have now run this system for 18 months in a start up phase situation, meaning that we are only at about 40 percent of system capacity in terms of fish and plant production. This commercial enterprise has allowed me to draw the following conclusions for aquaponics within Australia at a commercial scale: • The system has run for 18 months and we have not released any wastewater; conductivity at present is around 850 μS/cm which is equivalent to a nitrate level of approximately 100 ppm, • Replacement water lost via plant evapotranspiration, amounts to an average of 80 L/day. This equates to 0.4 percent of system volume per day for a 20,000 L system, • Up to 50 kg of plant material is produced per week on a
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