Aeration and Energy
Aeration can be accomplished by mechanical
aerators or underwater air diffusers. Mechanical aerators agitate water to produce liquid/air contact, while underwater diffusers introduce bubbles from a depth to achieve oxygen transfer and mixing. Bubble type aeration systems are replacing many mechanical aerators because of their low maintenance, reliability, safety, flexibility and overall efficiency. They excel where small amounts of aeration are needed in many locations. Bubble aerators are also better at removing gases, such as ammonia and carbon dioxide. Diffusers are made to deliver either coarse (approximately 6 mm), medium (approximately 3 mm), or fine (approximately 1 mm) air bubbles.
Coarse-bubble systems require the lowest air pressure and are very resistant to clogging, but are about a third as efficient as medium-bubble systems in transferring oxygen to the water. The medium-bubble diffuser requires only slightly higher air pressure, but its superior oxygen transfer more than compensates for the
increase in maintenance due to occasional clogging. The fine-bubble diffuser's superior oxygen transfer usually does not compensate for its higher-pressure requirement and much more frequent clogging. Therefore, fine-bubble diffusers are typically chosen for pure oxygen or ozone systems where pressure requirements are less important than transfer efficiency. Overall, however, medium-bubble diffusers are the most popular among aquaculturists.
Diffuser clogging often occurs from the inside. It is caused by dust and dirt particles carried by the air supply or by impurities in the water. Calcium carbonate often forms a deposit that clogs the pore outlet. (This source of plugging is prevalent in hard water and salt water.) Another source of plugging is bacterial slime, which forms on the external surface of the diffuser. Replacing medium and fine-bubble diffusers with coarse-bubble diffusers might seem like a good way to avoid periodic cleaning, but it's not very cost-effective. Let's work out the economics on a 10 horsepower system:
If a 10-horsepower medium-bubble aeration system can support
40,000 pounds of fish, a coarse-bubble system would require 30
horsepower under the same conditions. Electricity currently costs about $60 per horsepower per month in our area. This would make the utility cost rise from $600 a month to $1,800 - that's an extra $14,400 per year paid to the power company. An additional 20 horsepower in blowers would need to be purchased, as well as a larger diameter air-distribution pipe, if coarse-bubble diffusers were chosen over medium bubble diffusers.
Diffuser placement should allow for easy removal and time
should be allotted to clean all the diffusers in one section at one
time. This not only reduces the aggravation created by multiple
individual cleanings, but it also suggests when to schedule the next
cleaning. With our Sweetwater® glass bonded diffusers, frequency of cleaning can range between once per month in very hard water to once per year in soft water.
Did you know that for every pound of oxygen consumed by fish, they exhale one pound of carbon dioxide? Carbon dioxide can, therefore, cause a problem in recirculating systems where aeration or agitation or some other form of degassing is not done. This can be the case, for example, where pure oxygen is used in place of aeration. Somewhere in the system, carbon dioxide must be removed, or it can build up to dangerous levels. Dangerous to the fish, and to you if the fish are kept in a closed building. Here are some numbers to keep in mind.
Oxygen is about 20.9 percent of the air (at all elevations) and, because it is only slightly soluble in water, it becomes saturated at a level of about 9 ppm at 68°F (20°C). Carbon dioxide is 0.033 percent of the air and is saturated in water at about .5 ppm because it is much more soluble in water than oxygen. The comparative solubility of these two gases in blood is similar to that of water. Therefore, if there is a lot of carbon dioxide in the water, there will also be a lot of carbon dioxide in the blood of the fish, so they will be able to hold that much less dissolved oxygen. If there is in excess of 5 ppm carbon dioxide in the water, it is significantly affecting the ability of the fish to breathe oxygen.
If you are conducting intensive aquaculture operations outdoors, a splash aerator or aeration with air diffusers will drive the carbon dioxide into the air. If you are in a closed building, very high levels of carbon dioxide can accumulate in the air (we've seen levels exceeding 4,000 ppm in the air in closed aquaculture facilities). Since carbon dioxide weighs almost twice as much as air, it lays in the bottom portion of the indoor air space similar to the "fog" generated from dry ice used for theatrical purposes. It then has to be removed from the building, which requires air ventilators. In the winter, air ventilators can remove a lot of heat along with the carbon dioxide.
We suggest that carbon dioxide be stripped with a degassing column that is ventilated to the outdoors. To keep from causing a negative pressure inside the building, outdoor air could be drawn into the building directly into the bottom of the degassing tower and forced up through the downflowing liquid, then directed back outdoors separate from the inlet. In cold weather, there will be a significant cooling effect on the water because it is being degassed through cold air.