16 June 2012 Fingerling production tank. In left foreground is the pump sump where water is lifted to a 12-meter deep California Tube (to the right with ladder). eration were just too expensive. When he built his farm in 1996, Dean and his farm manager, Dave Russell, invented a new oxygen transfer device they called the “California Tube.” Tests on a 40-m deep California Tube revealed a field oxygen transfer efficiency of 90 percent. Efficiencies of standard U-Tubes and in-tank oxygen diffusion systems are much, much less. Deep California Tubes, Low Pumping Cost – Pushing the Envelope Ron Ketler, another California tilapia farmer, constructed fish tanks that were 15 m in diameter by 2.4 m deep, much larger than those on Dean Farrell’s farm. Because equipment was not readily available to install largediameter California Tubes, the typical depth was roughly doubled to about 40 m. About 3 mg/L of oxygen are dissolved per meter of California Tube depth. Based on that, the DO in the discharge from a 40-m California Tube would be 120 mg/L! Fortuitously research indicated that such high DO concentrations were fairly stable when water was ‘gently stirred and not shaken.’ Also, the dilution of incoming high-oxygen water with bulk fish tank water resulted in little, if any, DO coming out of solution (effervescence). Operation of the double-deep (40-m) California Tubes resulted in doubled DO concentrations, which allowed a halving of the water recycle flow rate. A 1-kW axial-flow water pump can recirculate 20 m3/hr in this PRAS. At carrying capacity, tilapia in the 440-m3 rearing tanks can be fed up to 600 kg of fish feed per day (1.4 kg/m3). Total electrical power required to operate this PRAS is about 5 kW, including generation and compression of oxygen and paddlewheel operation. This high efficiency cannot be accomplished with other kinds of oxygenation equipment. Some credit for this achievement goes to John Blerot of Oxygen Solutions Inc. for providing high-efficiency oxygen generation systems and service. Whether purchased in liquid form or generated on site, oxygen is expensive and inefficiency is costly. Solid Waste Separation and Consolidation – Radial-flow Settlers (RFS) When the recycle flow rate is reduced through the use of deep California Tubes, solid waste settling devices become more feasible. The first rule for success in RAS water treatment is rapid, gentle and thorough solid waste removal. Radial-flow Settlers (RFS) can capture over 75% of total suspended solids (TSS) in RAS. Particle settling devices are sized by hydraulic loading: the lower the flow rate, the smaller the settler. Ron Ketler has built an experimental tank-side settler and is in the process of building large RFS units to treat farm effluent. Using standard surface loading rates for RFS, the 440m3 fish tank requires a 3.3-m diameter settler. Alternatively, multiple RFS units can be installed in parallel – two at 2.3-m or four at 1.6-m diameter. Smaller settlers are cheaper and easier to install. Multiple banked units allow one RFS to be taken offline at a time and cleaned without interrupting the water recycle flow. Finally, automation of sediment purge and cleaning can be a simple matter in smaller units. Treatment of PRAS recycle flow by RFS allows increased flow through center drains. In turn, this makes for cleaner (lower solids concentration) fish tanks, clean enough for salmonids and other sensitive fish. Fish tank biochemical oxygen demand (BOD), an index of allowed response time in the event of system failure, may also be significantly reduced. The Road Behind, the Road Ahead Dean Farrell spent about ten years studying aquaculture systems before breaking ground on his own farm in Palm Springs, California. Within only eight years, he built a profitable tilapia farm bursting with technological innovation. On this farm, he developed the California Tube, a highly cost-effective oxygen transfer technology. The tubes are fairly simple and are typically fabricated and installed by farmers with the help of local well drillers. Ron Ketler calculates that this equipment paid for itself in two years relative to previous costs for liquid oxygen. This work represents major progress in reducing the costs of geothermal tilapia production. The more than 100 California Tubes installed across the USA offers a glimpse of the potential benefit of using this technology in commercial aquaculture. Notes 1Aquaculture Solutions, 1412 Crescent Ave., Klamath Falls, OR USA +1 541 884 0150 aquaculturesolutions@charter.net
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