World Aquaculture 15 Geothermal tilapia aquaculture – Pushing the efficiency envelope Mike H. Hosford1 Inland water reuse aquaculture systems (RAS) are an example of cutting-edge technology. Water quality, primarily temperature, and sustainable flow rate dictate the technical complexity and cost of these systems. Where the water supply has a favorable temperature, simple RAS can support large fish density with relatively low recirculating flow and minimal treatment of that flow. At dilution rates of 100 percent or more per day, filters for fine solids capture and nitrification are not required and significant cost-savings in terms of installed and operating costs can be realized. These systems are called “partial” RAS (PRAS). Dilution is the primary solution to pollution accumulation in PRAS, keeping ammonia and suspended solids concentrations in check and providing first-line control of dissolved carbon dioxide (CO2). Circular culture tanks, designed and operated as swirl separators, concentrate and remove heavy solids through center-bottom drains. With effective fish tank design and adequate dilution, dissolved oxygen (DO) supply and CO2 removal are the only remaining water quality challenges. Water recirculation pumps and pumping costs for DO maintenance and CO2 stripping are quite high in traditional systems – too high in fact for most tilapia farms. California tilapia farmers, led by Dean Farrell (19312011), have made significant strides in PRAS design and operation. First, careful fish tank design and management is used to facilitate solid waste removal. Second, miserly water use elevates dissolved CO2, reducing pH and, in turn, the toxic fraction of ammonia. Third, high-efficiency oxygen transfer devices called California Tubes (aka Farrell Tubes) minimize recycle flow requirement, pump size, and operating cost. Finally, radial-flow settling (RFS) devices are being investigated for reducing suspended solids concentration and oxygen demands and for consolidating sludge. Water Conservation and Ammonia Control through Elevated Dissolved CO2 Ammonia and CO2 are the main dissolved metabolic wastes produced by fish. In PRAS, these can accumulate to toxic levels. Tilapia are active and healthy at CO2 concentrations of 40 mg/L and they can probably tolerate greater concentrations without adverse effects. At these relatively high concentrations, dissolved CO2 is readily off-gassed by low-powered paddlewheel aerators. Some California tilapia This is a 440-m3 fish production tank at Kelley Hot Spring Fish Farm, Canby, California. Left to right: 40-m deep California Tube, liquid oxygen cylinder for emergency oxygen supply, paddlewheel aerator, and spray aerator for cooling water prior to harvest. farmers intentionally restrict their dilution water flow rate and paddlewheel operation to affect the balance between toxic and nontoxic forms of ammonia. Allowing CO2 to accumulate lowers pH. At an operational pH of 6.7, the toxic form of ammonia remains at safe levels up to total ammonia concentrations of 20 mg/L. This approach to ammonia control has also been used in European eel and Clarias catfish aquaculture. By minimizing water usage California tilapia farmers get the most out of their water resources. The typical dilution rate is around one fish production tank volume per day. At that rate, farmers can feed up to 1 kg/m3 of tank water volume every day, which is also 1 kg/m3 of water used. Geothermal water is often used for dilution in tilapia PRAS, so farmers must strike a balance between temperature and ammonia control when selecting flow rates. Economical Oxygen Transfer – the California Tube (aka “Farrell Tube”) Dean Farrell was not satisfied with off-the-shelf oxygenation systems because the cost of equipment and op-
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