Carolina Tips

J A N U A R Y   1 9 9 7

Making a Splash in the Classroom

Clarification works best if solids are removed using gravity. If pumped, they become pureed, making them much more difficult to remove. A clarifier should always be located before the biological filter so the biological filter does not become unnecessarily clogged with waste, thus decreasing the efficiency of nitrogen conversion and restricting the water flow. Several types of clarifiers are available. They include screened filters, sand filters, tube settlers, bead filters, cartridge filters, and gravity troughs. Biological Filter A biological filter contains a medium where attached nitrifying bacteria oxidize toxic ammonia into nontoxic nitrate. Biofilters are typically sized according to the overall poundage or biomass to be grown, or according to how much fish food (percent of protein) is fed to the system at its maximum carrying capacity. Internal media of biofilters consist of plastic bioballs, rings, beads, shredded PVC materials, polyester pads, or reticulated foam. Water Chemistry Water chemistry can seem intimidating at first, but once a few concepts are memorized, it becomes quite easy. The parameters of prime concern are dissolved oxygen, carbon dioxide, nitrification (ammonia, nitrite, and nitrate), pH, alkalinity, and temperature.

Oxygen and Carbon Dioxide Oxygen introduction is a key factor in recirculating aquaculture systems. In conjunction with biofilter size and clarifier efficiency, it determines how much biomass can be supported. Fish require dissolved oxygen to breathe, bacteria require oxygen to break down ammonia, and aeration is responsible for removing carbon dioxide­a by-product of fish respiration. Oxygen is typically introduced through a linear compressor or a small regenerative blower and a series of diffusers (airstones). Other methods of oxygen injection include spray/degassing towers and venturi injection (suction of air under water pressure). Pure oxygen is usually not required for a classroom system. Oxygen is not as abundant in water as it is in air. Oxygen makes up about 20% of the air we breathe, but dissolved oxygen, which fish absorb through their gills, represents only about 0.0001% of water. In fact, the maximum amount of oxygen that water wants to hold is only about 8.3 mg/L at standard temperature and pressure. That is equivalent to 8 red Ping-Pong balls in a classroom filled to the brim with white ones. This maximum level of 8.3 mg/L is referred to as oxygen saturation, which is influenced by temperature, elevation, and salinity levels. As these levels increase, oxygen saturation decreases. Oxygen can be measured in mg/L with a test kit or a dissolved oxygen meter. Usually, the goal for abundant oxygen levels is 80% of saturation. Dissolved oxygen levels of at least 5 ppm are desired for most warm-water species. Carbon dioxide (CO2) is a by-product of fish respiration and can cause disruption in the system for most warm-water species if levels build higher than 25 mg/L. A test kit can be used to measure it. Normally, carbon dioxide is not a problem in classroom systems because the surface disruption caused by aeration normally releases the carbon dioxide into the atmosphere. Small water exchanges also reduce the levels of carbon dioxide.

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