Air lift is far more energy efficient than water pumps in moving water between the pond and the water purification system. Other advantages include: the benefits of aeration and air stripping, no need for ground fault circuiting, no electrical contact with the pond, they can be modified as heating units, fish and eggs can pass through the entire filtration system without being harmed, multiple systems can be operated on one power source they are quiet, they don't leak oil into the pond, an impeller can not get stuck and they are simple to operate.
Disadvantages of air lift powered Koi ponds include a loss of efficiency when used to lift water a significant height, which limits their use with some waterfalls. They are large volume, low pressure systems, and can not be used when the filter has significant resistance. Large piping is needed with broad, sweeping bends, and open flow media with low resistance is also required. They can be adapted to some, but not all, ponds currently powered by water pumps.
Air lift is the main system used for commercial fish farming and for small aquariums. The largest and smallest systems are powered by this method. Why did we overlook it in the mid-sized systems'? One reason is it's hard to find an inexpensive mid-size air source.
The principle of air lift is simple. Air is introduced into a vertical pipe containing water. As the air rises, it imparts energy to the water and forces the water to move vertically up the pipe. In a small plastic aquarium filter, periodic bubbles are released with a column of water in between each bubble. In a large system, the bubble and column of water approach is not used: it is, instead,powered by the force of the rising air. Air lift systems by divers to gently lift lobsters from the ovean floor up to the boat without being harmed in the process. The simplest air lift system is a vertical pipe with an air stone dropped down into the pipe. The air stone however, creates resistance to the escaping air and also creates resistance to the flow of water in the lumen of the pipe. Some of you may recall a system using air stones which I was using about 10 years ago.
A more efficient system employes the injection of air into a series of holes in the circumference of the pipe. This eliminatesnthe resistance to both air and water which is created by the air stone.
I'll describe an example of a system I have used. I powered an Everfiow 500 filter with a Sweetwater 2 cu. ft/minute air pump at 57 watts. This unit filtered my 9.000 gallon pond from October to March. The water was crystal clean (32 fish). In the colder months, f placed the air pump in a styrofoam box and wrapped extra lengths of the air hose in the box to heat the air from the heat of the pump. A 3' vertical 2" Pvc pipe was used, and it moved about 50 gallons per minute through the filter.
To construct this system on the 2" pipe. I drew a circle and drilled 8 holes 1/8" in diameter on this circle. Around this pipe, I placed a 4" PVC pipe about 8" long. I modified 2-4" to 2" reducing bells to fit the 4" pipe around the 2" pipe. Rubber bushings can also be used to connect the 4" pipe around the 2" pipe. A 1/2" hole was drilled into the 4" pipe at the center and a 1/2" PVC pipe was glued in place. This 1/2" pipe was connected to the air line.
Multiple air lifts can be used from the same power source. High effeciency air pumps and air blowers are preferred. The depth of injection is a critical issue to maximize the efficiency of the air pump. Other critical considerations are the size of the pipe, volume of water to be moved, volume of air, and the size and number of the injection holes. If the injection holes are too small, they can clog in hard water conditions. The water can exit from the pipe below the water level. If an elbow is used at the exit, it would preferably be positioned so half the exit flow is below water level and one half is above the water level. Let's save money and make greater use of air lift systems. 'With the greater use of open flow, low pressure filters, it's a good method of powering our systems.