Airlifts are simple but efficient devices for moving water. The concept of airlifts has been around for centuries. This article will show you how you can use an airlift pump to operate your Koi pond, including waterfalls. Under some circumstances, using an airlift can help you to realize significant savings on your power bill by using a low power air pump instead of a much higher power water pump. Much has been written about the use of airlift pumps for aquaculture where airlifts are used for aeration and circulation. Koi ponds differ from aquacultural ponds in that they are smaller (500 to 50,000 gallons vs 100's of thousand of gallons) and they usually include ornamental features such as streams and waterfalls. Operating these features requires lifting the water instead of just moving it.

Before delving into the design of airlifts, it is important to understand their limits. Airlifts are extremely good for aerating and circulating water. Airlifts can be good for lifting water if the amount of lift is small. Airlifts will not lift water to great heights. Airlifts are not appropriate for operating tall waterfalls or filters requiring a lot of pressure (such as a pool style sand filter).

Airlift pumps are simple devices. An airlift is simply a vertical pipe. Water enters at the bottom of the pipe and air is injected into the pipe, usually near the bottom. The rising air bubbles create an upward water current. The top of the airlift pipe can be from even with the top to several inches above the pond level. You are probably already familiar with airlifts and don't even know it. Did you ever notice the aquarium filters that have air bubbles going up a vertical pipe to draw water through the filter - that's an airlift. The critical factors in airlift design and the range of values that are typical for a Koi pond are:

· air volume up to 10 cfm

· air pressure (air injection depth) 4 to 12 feet

· water volume 1 to 100 gpm

· lift height (water pressure) 0 to 36 inches

· lift pipe diameter 0.5 to 3 inches

Some generalizations can be made about the relationship of these parameters and the design of an airlift.

· Increasing air injection depth increases water flow.

· Increasing lift height decreases water flow.

· Smaller diameter pipes are capable of higher lift.

· Small air bubbles produce more lift than large bubbles.

The key to producing an efficient airlift is to find the best air flow. Very low air flow will just produce bubbles in the pipe and no water will make it to the top. As the air volume is increased, water will start to flow to the top. Shortly beyond this point, increasing the volume of air will result in the most efficient flow of water (efficiency measured in water flow per air flow). Continuing to increase the air volume will produce increases in water flow, but at reduced efficiency. To efficiently increase the flow of water, you should add more lift pipes operating in parallel instead of putting more air into a single lift pipe.

There are many ways to construct an airlift. The diagram shows the design I prefer which is also the design I used on my previous pond. The best method of injecting air is to use air stones. Air stones provide the smallest bubbles which result in the best lift and aeration. When using air stones, you should increase the diameter of the pipe at the injection area to allow water to flow better since part of the pipe area is blocked by the air stones. When using multiple airlifts that are driven by a common air pump, the air stones provide enough resistance that the air will distribute evenly without having to use any other valves to control air flow.

I have found that gluing air stones inside threaded pipe/hose barb adapters is a convenient way of connecting the air stone. The threaded end of the adapter makes it easy to remove the air stone for cleaning or replacement. For small diameter lift pipes, I use a tee to connect a single air stone. For large diameter lift pipes, I drill and tap several holes in the pipe and then screw the adapters into the threaded holes. To provide more pipe thickness for threading, you can put a pipe coupling in the place where you want the air stones and drill and tap through the coupling and pipe. I like to put some clear pipe just above the air stones so that I can keep an eye on things. If your airlift does not exit directly into your pond or filter, be sure to provide some means to allow the air to escape before piping the water back to the pond. You may be able to incorporate a foam fractionater into the system at this point.

There are many combinations of design parameters that will work. The diagram shows details of the design I used. On my pond I used L-70 linear air pumps from AES. These pumps are efficient, quiet, and capable of producing pressures of over 4PSI. On my old pond, I was using an air injection depth of 8', six airlift pipes in parallel, four air stones per lift pipe, and my pond required around 36" of lift. Recent improvements in normal pump performance combined with the cost of repair kits for the air pumps would cause me to either use a normal pump or design the pond with shorter water falls if I were to use airlifts again.

For a typical pond, I would suggest the following: Lift Pipe - 1.5", Air Injection Depth - 6', Lift Height - 0 to 24". Use the charts to determine an appropriate air flow. Use enough lifts in parallel to achieve the desired water flow rate. To keep wear on your air pump reasonable, I would not use the maximum air injection depth that the air pump is capable of supporting. Running the pump at lower pressure will lengthen the life of the valves which are expensive to replace.

The results of my tests are shown in the charts. The charts show performance of airlifts in a range that is typical of those that would be used in a Koi pond. The chart titled Performance shows the amount of water that flows for given air flows and lift heights. The chart titled Efficiency shows how well the airlift works. You will probably want to design your system to work in the most efficient range possible.

I will assume that you will be using a diaphragm type of air pump (other common types include piston compressor, rotary lobe blower and regenerative blowers). Diaphragm air pumps operate at air pressures of 1 to 5 psi (26 to 130 inches of water). Better results will be had operating the pump at the higher pressure end of its operating range. The reason for this is that the pump's air output volume changes relatively little with changes in the operating pressure over its rated pressure range, but the airlift's performance changes significantly, producing more water volume with greater air injection depths. The water flow rate and lift required will be dictated by the nature of the pond. The lift required is the total of: the drop of all streams and waterfalls, the drag of all plumbing, and the drop across the filter system. The air injection depth will probably be determined by the landscape subject to the limits of the air pump. Many pond keepers will not be willing or able to use the full pressure available from the air pump since doing so would require having the bottom of the airlift at a significant depth below the level of the pond. The air pressure required is the air injection depth plus the resistance of air lines and air stones. Air stones produce a pressure drop of a few inches.

The remaining design parameters to be set are: air pump selection, lift pipe diameter, number of lift pipes in parallel. Setting these parameters is largely a matter of compromise. Some factors to consider are:

Air Pump: Performance (air flow and pressure); purchase cost; operating cost; availability; indoor/outdoor use; operating noise; maintenance. Consider the fail-safe redundancy of multiple pumps vs efficiency of single larger pump.

Airlift: There are no good formulas to find the best design. The best way to develop your design is to use the performance charts to calculate the air requirements for some possible configurations.

Lets consider using an airlift for a pond with the following specs.

· Total pond volume 5,000 gal

· Waterfall height 8 inches

· Filter drag 4 inches

· Plumbing drag 3 inches (each way)

· Total lift required 18 inches

· Flow required 2500 gph - 42 gpm

Looking at the performance chart, you can see that an airlift with 1.5" lift pipe, air injection depth of 6', and lift of 18" will produce the following water flows vs air flows:

7 GPM water with 0.5 CFM air, 11 GPM water with 1.0 CFM air, and 13 GPM water with 1.5 CFM air.

To achieve the desired flow of 42 GPM, you would need one of the following configurations:

    6 lifts at 0.5 CFM each (total air - 3.0 CFM)

    4 lifts at 1.0 CFM (total air - 4.0 CFM)

    3 lifts at 1.5 CFM (total air - 4.5 CFM)

Building a set of airlifts using 6 pipes will allow you to use a smaller air pump. If space is more of a concern, you can use fewer lifts, but they will require a larger air pump.

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