Aeration principles and common aeration equipment in water treatment systems

Application of aerators in wastewater treatment
        Biological treatment methods can be divided into two major categories: aerobic and anaerobic methods, depending on the oxygen requirements of the microorganisms involved. Generally, aerobic methods are more suitable for wastewater with lower concentrations, such as wastewater from ethylene plants ; While anaerobic methods are more suitable for treating sludge and wastewater with higher concentrations. Aerobic biological treatment methods can be divided into two major categories: activated sludge method and biofilm method.
        The activated sludge method is an artificial intensification of water self-purification, a method that relies on the activated sludge as the main body to remove organic matter from wastewater. The aerobic microorganisms present in the activated sludge can only function in the presence of oxygen.
        In the aeration tank of the wastewater treatment bio-system, the oxygenation efficiency is positively correlated with the growth of aerobic microorganisms. The supply of dissolved oxygen should be comprehensively considered based on the number, physiological characteristics, substrate properties, and concentration of aerobic microorganisms.
        In this way, the activated sludge can be in the optimal state of degrading organic matter. According to experiments, the dissolved oxygen in the aeration tank should be maintained at 3 ~ 4mg/L It is advisable. If the oxygen supply is insufficient, the performance of the activated sludge will be poor, leading to a decrease in the wastewater treatment effect. To ensure sufficient oxygen supply, it is necessary to rely on a device to complete this, such as an aerator.

The principle of aeration
        Aeration is a means of bringing air into intense contact with water. Its purpose is to dissolve the oxygen in the air into the water, or to expel unwanted gases and volatile substances from the water into the air. In other words, it is a means of promoting mass exchange between gas and liquid. It also has other important functions, such as mixing and stirring.
        The oxygen in the air is transferred to the water through aeration, and the oxygen is transferred from the gas phase to the liquid phase. The theory of mass transfer diffusion currently used more is the two-film theory proposed by Lewis and Whitman.
        The two-film theory suggests that there are gas films and liquid films on the "gas - water" interface, with air and liquid flowing outside the gas film and liquid film, which is in a turbulent state ; The gas film and liquid film are in a laminar state, there is no convection, and under certain conditions, a pressure gradient and concentration gradient will appear. If the oxygen concentration in the liquid film is lower than the saturated concentration of oxygen in the water, the oxygen in the air will continue to diffuse inward through the liquid film into the water body, so the liquid film and gas film will become obstacles to oxygen transfer.
        Aeration mixing is exactly this, the specific method is: reduce the size of bubbles, increase the number of bubbles, increase the degree of turbulence of the liquid, increase the installation depth of the aerator, and extend the contact time between bubbles and liquid. Aeration equipment is widely used in wastewater treatment based on this practice.

Dynamic aerator
        The dynamic aerator is a new type of aerator, which belongs to a fixedly installed micro-bubble aerator. It consists of a dome, a swirl mixing cylinder, a swirl mixing ring, a connecting head clamp, and an air distribution pipe.
        The dynamic aerator adopts the "large-hole air bubble distribution" technology, which introduces the air into the aerator and conducts positive and reverse swirl flow, the positive swirl flow is clockwise, and the reverse swirl flow is counterclockwise. Under the action of two different directions of swirl flow, a momentary continuous local reaction gas-liquid enhanced swirl mixing zone is formed in the sleeve swirl mixing cylinder.
        A large number of bubbles generated by the swirl mixing and swirl flow are then uniformly and densely upwardly generated after being blocked and diffused by the dome. In general, the dynamic aerator uses various structural actions such as large-hole bidirectional swirl mixing, sleeve enhanced swirl mixing, and dome blocking diffusion to make the gas phase collide, shear, and split in the liquid phase, thus forming mixed diffusion.
        Because the dynamic aerator adopts large-hole air discharge, even after the wind and pressure are stopped, the wastewater will not flow back into the aerator and air distribution pipe, which will not cause the air discharge holes to be blocked, thus fundamentally solving the problem of aerator blockage, and the oxygen utilization rate can be maintained for a long time without change. However, because the diameter of the generated bubbles is larger, the oxygen utilization rate is relatively low compared with the microporous aerator, generally between 15 ~ 19% Similar to the structure and performance of the dynamic aerator is the swirl aerator.

Comparison of swing arm microporous aerator and dynamic aerator
        In addition to the differences in bubble diameter, oxygen utilization rate, and whether it is easy to block, there are also the following differences between the two:
1 Installation method
The swing arm microporous aerator is a movable installation. When the aerator needs to be replaced or repaired, a hoist can be used to lift the aerator out of the water, and the construction and repair can be carried out on the pool surface without affecting the operation of other aerators in the same pool, no need to stop the pool for water purification, low repair cost, and less workload.
The dynamic aerator is a fixed installation. Once installed, it cannot be moved. If a certain aeration tank needs to be repaired, the operation of the tank must be stopped, and the wastewater and sludge and other debris in the tank must be removed before construction can be carried out, and the repair cost is higher.
2 Durability
The failure modes of the swing arm microporous aerator are mainly as follows 3 kinds:
① After the steel air distribution pipe rusts, the iron oxide and impurities in the wastewater and air will cause blockage in the air distribution pipe, uneven air flow distribution in the air distribution pipe, causing the air distribution pipe to shake, and causing fatigue damage ;
② The air distribution pipe is installed on the pipe joint. Under the dual action of the air distribution pipe shaking and wastewater corrosion, the pipe joint is easy to break from the root, and the corrosion of the wastewater will also cause the air distribution pipe wall to thin and perforate ;
③ The underwater swing arm activity joint is immersed in water for a long time, and may rust ? Therefore, it cannot rotate, so that the aerator cannot be normally lifted to the water surface. The above 3 With the application of new technologies in recent years, the failure modes have been greatly improved, and the service life of the aerator can reach 5 years or so.
The failure modes of the dynamic aerator are: due to fatigue or corrosion ? cause, the components of the aeration head ( such as the dome, swirl mixing cylinder, swirl mixing ring, etc. ) The connector between them broke or came loose, causing the aeration head to disintegrate or fall off ; Air distribution pipe fracture ; Air distribution pipes generally use UPVC and other non-metallic pipe materials. Pipes and pipes, pipes and pipe fittings are mostly glued together. Once the bonding is not firm, it is easy to fall off and leak from the bonding point.
This 3 type of failure can generally be avoided by reasonable selection, correct material selection, and strict quality control. Therefore, this aerator has a long service life, up to 8 ~ 10 years.
3 、Actual application
Due to the advantages of high oxygen utilization rate and convenient maintenance, the swing-arm microporous aerator is widely used in primary biochemical systems with heavy production load and poor wastewater quality. ; Fixed dynamic aerators, on the other hand, are generally used in secondary biochemical systems with low production load and good wastewater quality due to their slightly lower oxygen utilization rate but long-term maintainability, difficult maintenance but long service life.