Air-lifts provide a simple and efficient means of moving large volumes of water. Rising air bubbles inside an air-lift’s tube act like a piston pushing water above it. However, this is efficient only if the water is lifted a small height above the surface. In fact, most air-lifts will not lift water over 3 or 4 inches above the water’s surface.
Air-lifts work most efficiently when they are releasing water at or very near the surface. A single 3-inch air-lift discharging at the surface will move between 50 and 60 gallons per minute if built as described below.
Air-lifts have the added benefit of aerating incoming water when dissolved oxygen (DO) concentrations are much below saturation. In research trials, when pond DO fell below 2 mg/L the DO in the IPRs has been maintained at 3 mg/L even with high biomass.
Because of the mixing action of water and air in the air-lifts, supersaturation is virtually eliminated in the water entering the IPR. Air-lift pumps consist of a battery of single air-lifts. Individual airlifts are constructed from a 36- inch long section of 3- or 4-inch PVC pipe. A 4-foot-wide raceway has room for the attachment of 9 3-inch diameter air-lifts.
A PVC 90o elbow or “L” is glued to the top of each air-lift. Each air-lift is designed so that air from the blower enters the pipe at approximately 32 inches below the center of the PVC “L”. Regenerative blowers are most efficient at powering air-lifts if the air is injected between 30 and 34 inches below the surface of the water.
Optimally the air-lifts are submerged to the halfway point of the “L” or to the top of the “L”. Each air-lift is attached to a plywood or plastic panel. A circular cut-out is made so that each “L” protrudes through the panel and into the raceway area. Silicon sealer can be used around the cutouts to seal the “L’s” to the panel.
This keeps water from escaping the raceway around the “L’s”. Air-lifts are attached to the panel with screws or by pressure straps. Bolts or screws should not extend into the pipe more than 1 /2 inch, as debris can become caught on this obstruction and reduce water flow through the air-lift. Each airlift in an IPR system must be built identically to all others and attached to the same air-manifold and blower in order to work properly.
The panel to which the air-lifts are attached fits into tracks along each side at the front of the raceway. These tracks allow the air-lift pump to be raised or lowered to adjust the height and therefore the water flow through the air-lifts.
The intake of the air lift should be approximately 36 inches underwater. The intake can be moved upward or downward to utilize different water temperatures or conditions. For example, if warmer and more oxygen-rich surface water is desired, the intake could be turned upward (starting at the bottom of the 36- inch vertical section) using elbows and pipe to place the intake closer to the water’s surface. A longer vertical extension could be used if cooler water was desirable. This would depend upon the quality of the deeper water.
Air is supplied to the air-lift pumps by a regenerative blower. Regenerative blowers are highvolume, low-pressure units. The blower is attached to an air-manifold that holds a large volume of air under constant pressure. Without the proper volume in the airmanifold the air-lifts will not function effectively, and the regenerative blower will be damaged due to overheating.
Typically a 1-horsepower blower requires a minimum of 20 feet of 4-inch PVC or 12 feet of 6-inch PVC air-manifold (approximately 2,500 cubic inches). One-half-inch PVC tubing connectors are tapped into the airmanifold and into the air-lifts (at 32 inches as described previously).
A section of garden hose (5 /8 inch), polypropylene, or plastic tubing (1 /2-inch ID) can be used as air-line between the airmanifold and the individual airlifts. The air-line attaches over the tubing connectors from the airmanifold to each air-lift.
The key to making all the airlifts work properly is that they all must be constructed exactly alike, and each requires a constriction orifice at the attachment of the air-line to the air-manifold. The constriction orifice should have a 3 /16- to 1 /4-inch hole in its center.
This orifice can be made from PVC or Plexiglas sheeting (1 /8 to 1 /4 inch thick) and hot-glued to the PVC tubing connector. If constructed in this fashion, a 1-horsepower blower can efficiently power 27 individual air-lifts or enough for 3 separate 4-foot-wide raceways with 9 air-lifts each.
Water flow through the IPR(s) with this air-lift pump design can be regulated by raising or lowering the air-lift pump, or by stopping the air flow to individual airlifts. With all 9 air-lifts functioning properly the flow rate averages about 450 gallons per minute. At this flow rate a 16x4x3.5-foot raceway completely flushes in less than 4 minutes.
At this flushing rate the carrying capacity of the 9 air-lift IPR appears to be approximately 3,000 pounds with warmwater species (e.g., catfish), a stocking rate of 13.4 pounds per cubic foot.
Air-lift pumps have also been constructed in a box or square design. In this type of pump a box is made from plywood or plastic panels 3 inches wide with vertical partitions every 3 inches, resulting in a unit with each individual air-lift a 3-inch square tube, 3 feet long.
Air injection, water discharge, screening, and vertical slide adjustments are similar to those described for the PVC air-lifts above. This design allows as many as 13 air-lifts in a 4-footwide area.
Author:
Michael P. Masser and Andrew Lazur
