Views: 889 Author: Site Editor Publish Time: 2024-10-28 Origin: Site
As the key to the dissolved air system, the performance of the flotation equipment directly determines the adhesion efficiency and pollution removal efficiency of the flotation process. Problems include uneven parameters, unstable performance, and lack of evaluation technology in the flotation equipment. To explore the factors affecting the flotation equipment, we conducted laboratory, pilot, and production evaluation studies on the performance and pollution removal effect of the flotation equipment, examined the relationship between the performance parameters of the flotation equipment and the pollution removal efficiency, and provided technical references for users who need to purchase dissolved air flotation equipment to evaluate the flotation equipment.
Table of contents（Click to go to where you want to see）
Current status of air flotation equipment development in China
Main evaluation parameters of dissolved air flotation equipment performance
2.1 Dissolved gas efficiency
2.2 Bubble size
2.5.1 Comparative analysis of investment per ton of water
2.5.2 Comparative analysis of electricity consumption per ton of water
2.5.3 Comparative analysis of drug consumption per ton of water
As shown in Figure 1.1, most domestic flotation equipment manufacturers are located in the eastern coastal areas, mainly small and medium-sized enterprises, and some companies also sell their products overseas. Most manufacturers are small in scale and cannot promote product progress and development; product prices fluctuate wildly, leading to blind low-price competition in the market.
Figure 1.1 Information on Chinese air flotation equipment manufacturers
During flotation, the particle size of microbubbles is a crucial factor affecting the treatment efficiency, and the dissolved air pressure directly affects the size of the bubble particle size.
Through a survey of some manufacturers of pressure-dissolved air tanks in my country, the performance parameters of each manufacturer's equipment vary greatly. The working pressure of the dissolved air tank is between 0.3~30MPa, the working pressure of the dissolved air releaser is between 0.1~5MPa, and the working pressure of the air compressor is between 0.08~33MPa. The working pressure of the equipment produced by each manufacturer varies greatly, and the applicable production scale is also different;
A survey and analysis of domestic water plants that use flotation technology show that the operating pressure of each water plant is between 0.2~0.75MPa, the reflow ratio is between 5~16%, and the operating parameters of the water plant are also uneven. There is a lack of unified evaluation of equipment performance and operating parameters and a lack of basis for guiding actual production.
Due to the difference in the quality of flotation products, the application of flotation technology in water treatment is still immature, and there is a lack of operating experience and design experience, resulting in poor operation of water plants;
The water inlet load of water plants in different seasons and periods is different, and the equipment is not adaptable to the sudden increase in the amount of treated water, resulting in a weakened treatment effect, which has a great impact on subsequent equipment and increases the failure rate and aging rate of equipment;
Compared with the sedimentation process, the power consumption of the flotation process is 17%~21% higher, and the total energy consumption is 7%~8% higher. Therefore, the energy consumption of the flotation process is relatively large, and the economic value it brings to users is not high.
Different types and specifications of fillers in the packing type dissolved air tank, high turbidity of inlet water can easily cause blockage of fillers, which reduces the efficiency of equipment dissolved air;
In the early stage of flotation process operation, there are more impurities in the water, which causes more sediment deposition in the dissolved air water pipeline, which will cause blockage of the releaser;
Different structures of flotation tanks, turbulence in the pool will affect the size of bubbles and the effect of flotation treatment;
Since the flotation process is used discontinuously and the idle period is long, if it is not cleaned properly, it is easy to cause corrosion to the inner wall of the dissolved air tank and dissolved air pipeline;
The air compressor will open and close frequently with the fluctuation of the pressure of the pressure regulating tank, which is easy to cause wear, etc.
The flotation equipment's performance directly determines the flotation process's pollutant removal efficiency. Representative parameters of flotation equipment performance include microbubble particle size, dissolved gas efficiency, bubble stabilization time, dissolved oxygen content, power consumption, etc.
The flotation equipment can dissolve part of the gas in water at a certain pressure, and the ability of the dissolved gas system to dissolve air can be measured by the dissolved gas efficiency.
The dissolved gas efficiency refers to the ratio of the actual gas release amount to the theoretical dissolved gas amount when the gas in the dissolved gas tank contacts the liquid and the gas dissolves in the liquid under certain pressure conditions, and when all the microbubbles are released and separated. The dissolved gas efficiency can be expressed as:
In the formula: 
——dissolved gas efficiency;
V——actual amount of dissolved gas;
V*——theoretical amount of dissolved gas;
Dissolved gas efficiency is a critical parameter for evaluating flotation equipment's performance. Still, it is difficult to measure the amount of gas dissolved in the water directly and must be done under pressure. Therefore, in practical applications, the release efficiency is used, which can indirectly reflect the dissolved gas efficiency. The amount of gas released by the sudden pressure drop really works in the flotation process.
In the research process, it is assumed that all the gas dissolved in the water can be released after the pressure is released; that is, the amount of dissolved gas is equal to the amount of released gas. However, in actual application, due to the difference in the decompression capacity of the decompression device, only a small amount of dissolved gas can be released. Therefore, the amount of gas released can be understood as the ratio of the actual amount released to the maximum amount of gas that can be released theoretically. The release efficiency and the molar fraction of air are:
Where: 
——gas release efficiency;
q——actual amount of gas released;
q*——theoretical maximum amount of gas released;
Where: ω——molar fraction of air dissolved in water;
ρw——density of water;
χΑ——a molar fraction of air at solution pressure;
Μw——molar mass of water;
Because <<1, and according to Henry's law = , we can get:
In the formula: PT——total pressure of gas dissolution;
PW——water vapor partial pressure;
Assuming that the pressure of dissolved gas is , and the pressure of gas released from dissolved water is 0, the mole fraction of supersaturated gas per unit volume of water is:
Therefore, assuming that the gas dissolved in water is a micro-ideal gas, the theoretical maximum amount of gas that can be released under specific temperature and pressure conditions is:
In the formula: ——gas constant;
——absolute temperature;
From formula 5, it can be seen that at fixed temperature and pressure, other factors do not affect the theoretical maximum gas release in the water body.
Therefore, the gas release efficiency depends only on the amount of air the system can release, that is, the gas release amount. The gas release amount directly represents the amount of bubbles generated by the releaser in the flotation system that can participate in the actual operation, so the gas release amount can be used as a parameter to evaluate the performance of the flotation equipment.
The flotation process uses microbubbles to collide with light colloids in the water and adhere to each other to form air-entrained flocs, which float to the water surface for separation and removal.
The average particle size of the alum flocs formed in the coagulation stage is stable at 181μm. If the generated flocs are to be removed, the diameter of the bubbles should be smaller than the particle size of the flocs so that the bubbles and flocs can adhere closely together. If the size of the bubbles formed is too large, the specific surface area of the bubbles themselves will decrease, and the rising speed of the bubbles will accelerate, resulting in violent hydraulic agitation. The inertial impact force cannot make the bubbles and flocs adhere well but will break the air-entrained flocs, and even the tiny bubbles initially attached to the surface of the air-entrained flocs will be desorbed.
Therefore, when the flotation system releases bubbles of appropriate size, the probability of collision and adhesion with light colloids in the water can be increased, thereby improving the decontamination efficiency.
From the point of view of thermodynamics, the adhesion of bubbles and particles occurs spontaneously on the trend of decreasing the interfacial energy of the system.
Assuming that the initial state of the bubble is a sphere with a diameter of 2r, the volume of the floc is more significant than that of the microbubble. In comparison, it can be assumed to be a plane figure with a surface area of A. The interfacial energy when the bubble and the particle are independent of each other is:
Where: , 
——surface tension coefficients of gas-liquid and solid-liquid interfaces.
After the bubbles and flocs adhere to each other, assuming that the bubbles are in a spherical cap shape and the surface area of the bubbles does not change, the radius is changed to R, and the radius of the cap circle is b. The interface energy after mutual adhesion is:
Where: 
——surface tension coefficient of gas-solid interface.
Interface energy changes:
According to the contact angle theory, at equilibrium, the interfacial tension is in equilibrium:
所以，
From b=Rsin , let R=fr, where f is only related to the contact angle. When R and r are changed, f can be regarded as a constant.
Under the condition that the amount of gas released remains unchanged, assuming that the number of bubbles in the two gas release processes are n1 and n2 respectively, and the average bubble diameters arer ̅1 andr ̅2, it can be concluded from 
that: 
,That is 
If 
, then 
That is, under the condition of a certain amount of gas release, the smaller the average particle size of the bubbles, the greater the adhesion ability between the bubbles and the flocs, and the better the adhesion performance of the bubbles.
The bubble stabilization time can be used as an essential parameter to measure the performance of flotation equipment. The smaller the particle size of the microbubble, the slower the floating speed, and the longer it takes for the bubble to float to the water surface. Therefore, the bubble stabilization time characterizes the number density of bubbles in the released water.
Since the particle size of the microbubble is very small, the flow state of the bubble during the rising process is laminar, so the bubble rising speed conforms to the Stokes formula:
（Stokes formula）
In the formula: 
——bubble rising speed, m∙ s−1;
——water dynamic viscosity, g∙ s−1∙ cm−1;
ρω——water density, g∙ cm−3;
ρs——bubble density, g∙ cm−3;
g——gravitational acceleration, m∙ s−2;
d——diameter of the bubble, cm;
It can be seen that the rising speed of bubbles is positively correlated with the diameter of the bubbles. Assuming the height of the measuring cylinder is h, the time required for the bubbles to rise until they ultimately overflow the water surface (i.e., the "white water layer" disappears), i.e., the stabilization time of the bubbles, is t. The rising speed of the bubbles is:
Substituting the above formula into the Stokes formula, we can get the relationship between the bubble stability time and the diameter of the microbubble:
This relationship can be used to verify the relationship between the average bubble particle size measured by image analysis under specific working conditions and the bubble particle size calculated using the measured stabilization time.
The dissolved oxygen content in the dissolved air flotation (DAF) process is an important parameter that directly affects flotation's effect and subsequent treatment's efficiency.
Dissolved oxygen concentration range:
During the DAF process, the dissolved oxygen (DO) concentration is usually between 0.5 mg/L and 5.5 mg/L. In some applications, such as in the Orbal system, the target concentration of dissolved oxygen is at least 3.0 mg/L.
When treating polluted rivers, the dissolved oxygen can be increased from 0.2 mg/L to 2 mg/L to 3 mg/L to 3.5 mg/L.
Factors affecting dissolved oxygen:
The dissolved oxygen concentration is affected by many factors, including air pressure, water inlet flow rate, and injected air flow rate.
In a bioreactor combining pressurized aeration and DAF, the dissolved oxygen concentration increases as the pressure gradually increases from 0.10 MPa to 0.50 MPa.
The total oxygen transfer coefficient (KLa) is the mass of oxygen transferred to a unit volume of water per unit time under a unit mass transfer driving force. It is one of the indicators that characterize the performance of dissolved gas and reflects the transferability of oxygen in the dissolved gas process.
The basic equation of KLa is as follows:
（Formula 23）
Integrating and simplifying gives:
（Formula 24）
Draw a curve from (Formula 24) and obtain KLa(T) by linear fitting. If the temperature and pressure are not under standard conditions, converting the KLa(T) under non-standard conditions into KLa(20) under standard conditions is necessary. The conversion formula is shown in (Formula 25):
（Formula 25）
In formulas 23, 24, and 25:
KLa(20)——Total mass transfer coefficient of oxygen in water under standard conditions (min-1);
KLa(T)——Total mass transfer coefficient of oxygen at water temperature T (min-1);
Cs——Saturated DO value of water body (mg/L);
C——DO value of water body in water pool (mg/L);
t——Oxygenation test operation time (min);
T——Water temperature in water pool (℃);
θ——Temperature characteristic system (take 1.024)
As a conventional treatment process for solid-liquid separation, the economic benefits of flotation are evident to all. To improve the quality of effluent, it is necessary to improve the performance of the equipment and then improve the efficiency of pollution removal. Various energy consumption will also increase accordingly.
The economic efficiency of equipment products significantly impacts the promotion of equipment. For small-scale water treatment plants, if energy consumption is increased, although the water output increases, it is relatively stable, and the increase in water production costs will also bring economic burdens to the water plant. In addition, the financial benefits of different flotation products change with the changes in equipment performance. Therefore, the financial analysis of flotation equipment can improve environmental protection and energy-saving benefits and provide a reference for equipment procurement.
The investment cost per ton of water is the ratio of the total investment amount of the structure to the amount of water treated. Through research, the investment cost per ton of water in the sewage treatment plant is calculated and compared with the cost per ton of water in the conventional treatment process.
After adding the flotation process, the investment per ton of water has increased. Still, with the combined application of the process, the quality of the treated effluent has improved, the water output has increased accordingly, and the scale of treatment has increased. The investment per ton of water in the overall operation of sewage treatment will be reduced accordingly, and the overall economic benefits of flotation are still on the rise.
The flotation process is turned on under specific water quality conditions as a new type of water treatment process. Therefore, when the process is running, it will consume more electricity for dissolved air than conventional water treatment processes. The electricity cost consumed by the flotation process when running stably is about RMB 0.02/m3.
During the regular operation of the flotation system, the high-power electrical equipment that needs electricity includes the reflux pump, the motor for the reflux pump, the scraper, the air compressor, etc. When the reflux ratio and the dissolved air pressure increase, the power consumption of the reflux pump and the air compressor increase accordingly; according to Henry's law, as the dissolved air pressure in the dissolved air tank rises, the concentration of dissolved air in the water will also increase. According to the existing data, at 5℃ and 0.40MPa dissolved air pressure, the concentration of dissolved air in the reflux water is 144mg/L. At 5℃ and 0.50MPa dissolved air pressure, the concentration of dissolved air in the reflux water increases to 172mg/L, increasing the air compressor's power consumption.
Currently, the commonly used coagulants for sewage treatment are mainly inorganic low-molecular flocculants and inorganic high-molecular coagulants.
The traditional process for treating low-temperature and low-turbidity water often uses increased coagulant dosage, but it does not form good alum flowers. After adopting the flotation process, the collision and adhesion of microbubbles are used to remove fine particles in the water, reducing medicine consumption. Therefore, when treating high-turbidity and high-algae or low-temperature and low-turbidity water, the combined use of the flotation process can reduce the number of flocculants, save costs, reduce the consumption of medicines, and improve the water quality of the effluent.
Although the operation of the flotation process brings about higher power consumption and partial agent consumption, it improves the effluent quality, reduces the operating load of the next stage of the filtration process, and reduces the amount of water used for backwashing. Reducing energy and low water consumption during the subsequent treatment process will offset some power and drug consumption. Therefore, the total energy consumption after the flotation process is relatively small, providing an economic basis for promoting the flotation process.
In addition, in future development, environmental protection and energy-saving benefits need to be paid special attention in the research and development of flotation equipment so that the flotation equipment can ensure stable equipment performance, improve pollution removal efficiency, save consumption, provide economic guarantee for promoting the development of flotation technology.
