As we all know in water treatment, there are always some mixed liquids and sludge that need to be transported in aeration tanks and sedimentation tanks. If we rely solely on pumps, it will not only consume electricity but also be prone to blockage. At this point, the air lift device comes in handy - without the need for complex mechanical transmission, it can "blow" liquid to high places with just one breath of air, making it a "lazy artifact" in water treatment systems.
Let's talk about the principle of the air lift device first. It's actually quite simple, which utilizes the density difference between air and liquid. You can imagine the air lift device as a vertical pipe inserted into water, professionally called a "riser". When we introduce compressed air from the bottom of the riser, the air forms a large number of bubbles inside the riser. As these bubbles run up, they mix thoroughly with the liquid inside the riser, forming a gas-liquid mixture flow.
The density of gas-liquid mixed flow is much lower than that of pure liquid, while the liquid outside the riser still has a normal density. This "light" and "heavy" creates a pressure difference, and the liquid outside the pipe is pressed into the pipe. The gas-liquid mixture inside the pipe is pushed upwards and overflows from the top of the lifting pipe, thus completing the lifting and transportation of the liquid. There are no rotating parts in the entire process, as long as air is continuously supplied, it can continue to work, making maintenance much easier.
Let's talk about the design points of the gas lift device again, which is the key to determining its usability.
Firstly, the diameter and height of the lifting pipe need to be determined. The diameter of the pipe cannot be too small, otherwise the liquid flow rate will be too fast, the resistance will be high, and it will be easy to block; It cannot be too large, otherwise the gas-liquid mixture will not be sufficient and the efficiency will decrease. Generally, the calculation is based on the flow rate that needs to be increased, and the empirical value is that the flow velocity of gas-liquid mixture in the pipe should be controlled at 0.8-1.5m/s, which is more appropriate. There are also considerations for raising the height. It is not about raising it as high as you want. It is limited by the supply pressure of air, and the effective lifting height is usually 3-8 meters. Beyond this range, using a pump is actually more cost-effective.
Next is the gas distribution method, the quality of which directly affects the gas-liquid mixing effect. Usually, an air distributor, such as a perforated pipe or an aeration disc, is installed at the bottom of the riser to evenly disperse the air into small bubbles. The contact area between small bubbles and liquids is large, resulting in more thorough mixing and naturally higher efficiency. If the air distribution is uneven and bubbles gather into large bubbles and run upwards, the improvement effect will be greatly reduced.
There is also the choice of installation location. The gas lifting device needs to be installed in a place with a liquid level difference. For example, if the mixed liquid in the aeration tank needs to be lifted to the secondary sedimentation tank, the lower end of the lifting pipe can be placed in the mixed liquid in the aeration tank, and the upper end can be extended above the secondary sedimentation tank, using the liquid level difference between the two tanks to assist in lifting. In addition, the lifting pipe should be installed vertically as much as possible to reduce bends, otherwise it will increase resistance and affect the lifting effect.
Finally, we need to have a clear understanding of the advantages and disadvantages of the air lift device. The advantages are simple structure, no mechanical wear and tear, low maintenance cost, and the ability to also serve as aeration and aerate water bodies; The disadvantage is that the lifting height is limited, the energy consumption is slightly higher than that of a pump, and it is suitable for situations with small flow and low head.
In water treatment systems, gas lift devices are often used to lift the reflux of sludge and mixed liquor, or to backwash biofilters, making them a low-key yet practical "old buddy". As long as we understand its principles and focus on the design points, we can make it play a stable role in the system, saving us a lot of trouble.
