Aeration diffusion mechanism

Aeration diffusion is the core technology in wastewater treatment process. This paper presents some preliminary views on the new problems in the application of aeration diffusion mechanism.

1. Analyze the difference of aeration diffusion according to the characteristics of fluid movement

The essence of aeration diffusion is to transfer oxygen in gas phase to liquid phase. Oxygen in the gas phase is transferred to dissolved oxygen in the liquid phase, which is completed by forming gas-liquid contact interface through fluid movement. Therefore, according to the analysis of fluid motion properties, we can see the difference of aeration diffusion technology. If the characteristics of fluid movement are used to distinguish, aeration diffusion technology has the following two basic forms.

1.1 liquid phase fluid active motion type

The surface aeration of impeller and rotary brush (disk) is to form gas-liquid contact interface by hydraulic jump of liquid-phase fluid; jet aeration is to form gas-liquid contact interface by suction of gas-phase fluid by jet liquid-phase fluid, all of which belong to liquid-phase fluid active motion type, and its technical features are: kinetic energy acts on heavy liquid-phase fluid motion; light gas-phase fluid is passive contact; in impeller Or a local continuous gas-liquid contact interface is generated near the stirring part of the rotary brush (disk) and the jet port.

1.2 gas phase fluid active motion type

Blower aeration is to transport gas-phase fluid by fan and form gas-liquid contact interface through the diffusion of aerator in the way of rising bubble movement, which belongs to the active movement type of gas-phase fluid. Its technical features are: kinetic energy acts on the movement of light gas-phase fluid; heavy liquid-phase fluid is in passive contact; rising movement of rising bubble can produce three-dimensional continuous gas-liquid contact interface.

Comparison of the characteristics of fluid movement between air blast aeration and mechanical aeration

Project air blast aeration mechanical aeration

Kinetic energy action gas phase (light) fluid movement liquid phase (heavy) fluid movement

Fluid motion gas phase (light) fluid active motion liquid phase (heavy) fluid active motion

Contact interface gas liquid contact interface solid continuous gas liquid contact interface local continuous

Oxygen charging form, three-dimensional bubble rising, local hydraulic jump

2 "oxygen utilization rate" can not determine the actual operation efficiency of the aerator

The role of the aerator is to promote the mass transfer of oxygen. It seems that the "oxygen utilization rate" should be the index reflecting the technical performance of the aerator. Therefore, there has been a long-term habit of using "oxygen utilization rate" to determine the technical performance of the aerator. However, if the "oxygen utilization rate" is analyzed in depth, it will be found that this index can not really determine the actual operation efficiency of the aerator.

2.1 the essence of "oxygen utilization rate" is a constant value unaffected by variables

2.1.1 oxygen utilization formula

Oxygen utilization rate = [QC / (0.28 × q)] × 100% (CJ / t3015.2-93)

QC - under standard condition, test conditions, oxygen filling capacity of aerator (kg / h);

0.28 - weight of oxygen contained in 1 m3 air under standard state (kg / m3);

Q - aeration volume of aerator (m3 / h) in standard state.

It can be seen from the above formula that the oxygen utilization rate depends on two factors: the capacity of oxygen charging (QC) and the ventilation capacity (q).

2.1.2 there is a direct relationship between the capacity of aeration (Q C) and the volume of ventilation (q), that is, the capacity of aeration (Q C) depends on the volume of ventilation (q). Ventilation capacity is 0, and oxygenation capacity is 0. In a certain range of ventilation capacity, with the increase of ventilation capacity, oxygenation capacity also increases.

The oxygenation capacity (QC) indicated by all aerators is measured according to a certain ventilation capacity (q) under the condition of clear water test.

2.1.3 the formula of oxygen utilization rate can also be written as follows:

(1/0.28)×100%×(qc/q)= 0.0357×(qc/q)

Because there is a direct ratio between the capacity of oxygenation (QC) and the volume of ventilation (q), the result of QC / Q is a constant value, so "oxygen utilization" is essentially a constant value independent of variables. The constant value parameter, which is not affected by the variable, only represents a physical phenomenon, but never shows the technical performance of the efficacy. The constant value parameter of the sound is only a physical phenomenon, but it does not show the actual operation efficiency of the aerator.

2.2 "oxygen utilization rate" does not reflect the efficiency of oxygen mass transfer

2.2.1 if a big bubble is divided into many small bubbles, the area of the "bubble surface film" formed will be larger. "Bubble surface film" is a functional membrane for oxygen mass transfer. If we only look at the problem from the perspective of "oxygen utilization", of course, the smaller the bubble is, the better it is.

2.2.2 in order to obtain a high "oxygen utilization rate", it is necessary to produce as many "bubble surface films" as possible. The more bubbles (one unit of air) are formed by diffusion, the more "bubble surface film" is, and the higher "oxygen utilization" is. It can be seen that "oxygen utilization" is only related to the extent of bubble diffusion, but not to the process of bubble diffusion under the action of kinetic energy. That is to say, the "oxygen utilization rate" only indicates how much a unit of bubble is divided into small bubbles, which has nothing to do with the diffusion process and kinetic energy consumption. Therefore, "oxygen utilization" is not equal to the efficiency of oxygen mass transfer.

2.2.3 according to the principle of pore diffusion, the larger the pore is, the larger the bubble will be. If the air is divided into 1 μ m bubbles through the 1 μ m diameter holes, no matter how much resistance loss, no matter how many holes are blocked, as long as there are holes in the operation, there must be 1 μ m small bubbles. Obviously, at this time, the "oxygen utilization rate" has not changed, but the real operation efficiency has changed a lot.

2.2.4 because the "oxygen utilization rate" is only related to the extent of bubble segmentation and diffusion, as long as the diameter of the vent hole is 1 μ m, whether it is discharged through many pores in a short time or a small number of pores in a long time, because the diffusion result is always divided into bubbles with a diameter of 1 μ m, the "oxygen utilization rate" will remain unchanged 。 It can be seen that only "oxygen utilization rate" is used to explain the oxygen mass transfer efficiency of the aerator, which will obviously mislead.

2.2.5 if the design parameters of the aerator are: ventilation volume = 2 m3 / h, oxygen utilization rate = 25%, the vent hole is designed to use micro holes to ensure high oxygen utilization rate. However, in actual operation, most of the vent holes are blocked, and the ventilation capacity of a single aerator can only reach 0.2 m3 / h, that is to say, the working efficiency has been reduced by 90%. Because the principle of "fine bubbles produced by fine holes" has nothing to do with the degree of pore blockage, at this time, the so-called "oxygen utilization rate = 25%" has no change, but its real oxygen mass transfer efficiency has become very low.

2.2.6 "oxygen utilization rate" indicates the utilization rate of mass transfer of oxygen produced by the "bubble surface film" formed by the separation of air bubbles. The efficiency of oxygen mass transfer should indicate the amount of oxygen mass transfer in unit air through "bubble surface film" in unit time. Obviously, "oxygen utilization" is not the efficiency of oxygen mass transfer.

2.3 comparison of oxygen utilization rate of air blast aerator

Large vent type:

Jet aerator ≈ 5%

Spiral aerator ≈ 5%

Diffused aerator ≈ 7%

Mixing aerator ≈ 21%

Small hole exhaust type:

Hose microporous aerator ≈ 13% (affected by pore change)

Soft membrane microporous aerator ≈ 25% (affected by pore change)

Microporous aerator ≈ 25%

It can be seen from the above "oxygen utilization rate" of all kinds of blast aerators (except for the rotating mixing aerator), the size of the vent hole determines the amount of oxygen utilization rate (the principle of pore diffusion). If the "oxygen utilization rate" is used to evaluate the technical performance of the aerator, it will be concluded that the finer the hole of the aerator, the better.

The "micropore" must be resistant and easy to block, so the "oxygen utilization rate" is high, not the actual oxygen mass transfer efficiency of the aerator. In fact, the prerequisite for determining the oxygen mass transfer efficiency is the reliability of the exhaust structure. If the exhaust structure is unreliable, the real oxygen mass transfer efficiency and technical performance are also unreliable.

Because HS rotating mixing aerator uses large hole to exhaust gas and generates fine bubbles through the diffusion of various structures, it also realizes the excellent technical performance that other types of aerators cannot achieve, such as high "oxygen utilization rate" and real and reliable oxygen mass transfer efficiency. The reliability of the gas structure and the "oxygen utilization ratio" of the aerator are no better. If the exhaust structure is not reliable, its real oxygen mass transfer efficiency and technical performance are also unreliable.

Because of the use of large hole exhaust and the use of bubble floating power to generate fine bubbles through the diffusion of swirling flow, diversion, turbulence, collision, blocking and other functions, the spiral mixing aerator can not be realized by other types of aerators, which not only has a high "oxygen utilization rate" but also has a real and reliable effect of oxygen mass transfer.

3 discussion on the pore problem of microporous aerator

Microporous aerator relies on micropores to diffuse the air flow. The effective ventilation pores on the surface of microporous aerator are the core technology of microporous aerator. Related to the pore physical calculation of micro porous aerator are: ventilation velocity (V), pore space (s), porosity (k) and porosity (n also known as pore unit).

3.1 ventilation flow rate (V)

The velocity of air flow through the vent or pore of the aerator. The micro porous aerator uses the air flow to directly discharge through the micro pores, and only has the diffusion effect of micro pores with large resistance. Therefore, the air flow velocity through the micro pores is as high as the bubble rising velocity generated by the pore exhaust, which is about 0.35 M / s.

3.2 pore space (s)

The size of aeration pore in aerator. Fixed microporous aerator is about 50 μ m, soft membrane microporous aerator is about 100 μ M.

3.3 porosity (k)

The ratio of the sum of the ventilation pore space area to the surface (a) of the aerator. Porosity can be divided into area porosity and volume porosity. The former is used in this paper.

If the surface area diameter of a single microporous aerator is ∮ 250mm, the effective ventilation porosity required for 2 m3 / h ventilation of the aerator is:

K =[(2m3÷3600÷V]/[Am2（∮250㎜）]×100% ＝ [2÷3600÷0.35]／[0.125×0.125×3.14]×100%= 3.24%

3.4 (n is also called pore unit volume)

The number of effective aeration pores on the surface of microporous aerator. If the surface area diameter of a single microporous aerator is calculated as ∮ 250mm, all ventilation pores are regarded as multiple square pores, and the effective ventilation porosity is 3.24%, then:

Fixed microporous aerator:

N  = [Am2（∮250㎜）×K]/Sm2（50 μm） =125×10-3×125×10-3×3.14×3.24×10-2/50×10-6×50×10-6

=636000 (single volume)

Soft membrane microporous aerator:

N = [Am2（∮250㎜）×K]/Sm2（100 μm）

=［125×10-3×125×10-3×3.14×3.24×10-2］／［100×10-6×100×10-6］

=154100 (single quantity)

The vent hole of microporous aerator is not in the form of regular single square hole, but in the form of 50-100 μ M. Therefore, the result of pore volume calculation is actually the connection of multiple and single micropores with pore state.

3.5 discussion on Relevant Issues

3.5.1 for the pores of microporous aerator, the pores that can be vented during operation are effective aeration pores. When the microporosity is blocked, the ventilation will be blocked, which will directly affect the porosity and pore volume.

3.5.2 because micropores are easy to block, it is impossible for a single fixed microporous aerator to maintain more than 600000 units of effective ventilation pores in the long-term operation of sewage treatment.

3.5.3 with a relatively new processing method, more than 100000 units of pores can be opened on a piece of soft film with a diameter of ∮ 250mm. In the long-term operation of sewage treatment, due to the aging of soft film, pore plugging or pore tearing and other reasons, the change of porosity and pore volume will inevitably be affected.

3.5.4 to sum up, although the microporous aerator may have a large pore volume, its technical reliability is very low. In the long-term operation of sewage treatment, it is difficult to guarantee the technical reliability of "microporous ventilation" by adopting the aeration diffusion technology with large pore volume. Therefore, the oxygen filling efficiency of microporous aeration under the condition of new machine and clean water will be seriously degraded in the actual operation of sewage treatment.

4 technical rationality of air diffusion

In the aeration system, the aerator is the key equipment of the terminal. The function of the aerator is to diffuse the air flow.

4.1 rationality of air diffusion

It is impossible for pore diffusion to make air flow diffusion technically reasonable. Theoretically speaking, the higher the diffusion degree is, the better it is. That is to say, the finer the bubble is, the better it is. According to the principle of pore diffusion, "bubble fineness" and "resistance" are a pair of contradictions; the finer the pores are, the finer the bubbles produced by exhaust gas will be, but the finer the pores are, the greater the resistance will be, the more easily the pores will be blocked, and the less gas will pass through in unit time. Therefore, the pore size can only solve the problem of "bubble size", which inevitably leads to the problems of large loss, low reasonable degree of air diffusion technology, and unreliable performance.

4.2 basic requirements for reasonable air diffusion technology

Exhaust resistance should be small, exhaust unobstructed and reliability should be large, under this premise, the more dispersed the air flow, the better. Generally, the aeration air source of sewage treatment adopts the air blast mode. The blower belongs to the low-pressure operation equipment, and the high exhaust resistance will inevitably affect the power efficiency of the blower. The conditions of sewage treatment process are complex and changeable. In order to achieve the technical reliability of small exhaust resistance and no blockage, the exhaust hole can only be a large hole (＜ Φ 5mm =), but according to the principle of pore diffusion, it is impossible to produce fine bubbles by large hole exhaust. Therefore, in order to make the air diffusion technology reasonable, it is necessary to seek other diffusion methods besides pore diffusion.

4.3 pore diffusion with variable porosity

Although the performance of preventing clogging can be improved by using the flexible membrane variable pore air exhaust, the pore size is required to be in a very small range (0-100 μ m); therefore, the flexible membrane variable pore air exhaust is still difficult to solve the problem of easy clogging and high resistance. In the long-term operation process of the soft membrane under pressure, the fatigue and aging problems of the soft membrane inevitably exist, which makes the technical reliability of the pore variable low (pore membrane vulnerable).

4.4 technical rationality of power diffusion

Dynamic diffusion takes advantage of the buoyancy force of gas in the water, and "bumps" and "bumps" occur to obtain fine bubbles. The air flow diffusion is completely separated from the binding effect of small pores. Because the dynamic diffusion adopts the large hole exhaust, the diffusion technology with small resistance and no blockage is reasonable. Only "reasonable technology" is not enough, but also "functional efficiency". The rotating mixing aerator has a variety of "collision" and "collision" functions such as swirl, diversion, turbulence blocking and so on. It realizes both large hole exhaust and functional efficiency. PD rotating mixing aerator solves the technical rationality of power diffusion well.