Views: 526 Author: Site Editor Publish Time: 2024-09-14 Origin: Site
The factors that affect the water quality of dissolved air flotation effluent mainly include the following aspects:
The impact of sewage flow on the treatment effect cannot be ignored. Too large or too small a flow will affect the flotation effect and the effluent water quality. Too large of a flow may cause the flotation tank to overload and the treatment effect to decrease; too small of a flow may cause the equipment to operate inefficiently and the treatment effect to be poor.
In the flotation process, there are contact reactions and separation reactions. The contact reaction is the process of floc formation and continuous enlargement, and the separation reaction is the solid-liquid separation of the formed flocs and the solution. During the contact process, the water flow needs particular turbulence to increase the contact between bubbles and flocs and between flocs.
Appropriate turbulence is helpful for the formation of flocs. Still, when the turbulence is too significant, it will generate substantial shear force in the solution, thereby destroying the formation of flocs and even breaking the already formed flocs again. It will also cause the contact residence time to be too short to form good flocs, thus affecting the treatment effect of the entire flotation process. Therefore, the rising velocity during the contact reaction cannot be too small, let alone too large, so it is necessary to control the increasing velocity of the contact zone. The "Water Supply and Drainage Design Manual" has strict regulations on the growing velocity in the contact zone in the flotation process, stipulating that the ever increasing velocity in the contact chamber is 10~20mm/s.
The flow rate in the separation zone is also significant. The separation zone does not require a certain degree of turbulence to promote the growth of flocs. Still, it is necessary to ensure that the water flow rate is not too fast to cause the destruction of the flocs or to make the formed flocs flow out of the outlet pipe with the water under the action of the water flow without forming scum on the surface of the solution. Therefore, the specified downward flow rate in the separation zone is meager, usually only one-tenth of the upward flow rate in the contact zone. The "Water Supply and Drainage Design Manual" stipulates that the downward flow rate in the separation zone is 1.5~2.5mm/s.
The suspended solid content, organic content, chromaticity, temperature, and other factors in the raw water will affect the flotation effect. For example, raw water with high turbidity, high organic content, and high chromaticity will reduce the flotation effect and lead to poor effluent water quality.
The adsorption process of bubbles is exothermic, so when the temperature is low, the viscosity of the bubbles will increase, which is more conducive to the adsorption process. However, the viscosity of water will decrease as the temperature decreases, thus affecting the speed of bubbles in the water. In addition, when the temperature decreases, the hydration of colloids will increase, reducing the destabilization and coagulation in the water and affecting the treatment effect of the flotation process.
In addition, the temperature directly affects the amount of dissolved air in the dissolved air flotation process.
The settings of the flotation process, such as bubble size, flotation time, gas-solid ratio and other factors, will also affect the effluent quality. Bubble size and flotation time that is too long or too short will affect the flotation effect and thus affect the effluent quality.
The size of bubbles directly affects the number of bubbles that can adhere to the floc surface and the degree of binding after adhesion. In the past, many scholars believed that the smaller the bubble size, the more conducive it is to removing impurities by flotation. They think that more giant bubbles are challenging to adsorb with flocs in water, and even after adsorption, the binding ability is not strong, and it is easy to separate again. In addition, smaller bubbles have a less disturbing effect on water flow, which is more conducive to forming and protecting flocs.
However, in recent years, some scholars have proposed that the smaller the bubbles, the higher the removal efficiency. These scholars believe that the reasonable bubble size needs to be determined according to the particle size of impurities in water. Although more giant bubbles are weaker in adsorption and binding ability, when the particle size of colloidal particles in water is more significant, too tiny bubbles are disadvantageous because the number of small bubbles required to float more significant impurities is much greater than the number of large bubbles, which makes the adsorption of bubbles on flocs more difficult, seriously affecting its buoyancy. At the same time, if there are too many microbubbles in the flocs, it will also make it more challenging to handle the scum. Scholars generally believe the bubble diameter should be less than 100μm, and the best size is 20~50μm.
The rising velocity of bubbles also has an essential influence on the effect of flotation. First of all, the rising velocity of bubbles should be uniform. If the rising velocity of bubbles is not uniform, bubbles will aggregate to form large bubbles during the rising process. Then, the rising velocity of the aggregated large bubbles will be faster, so they will gather and grow more extensively during the rising process, affecting the flotation effect. The rising velocity of bubbles depends on the size of the bubbles. Therefore, the rising velocity is uniform if the bubble particle size is uniform.
The influence of the rising velocity of bubbles on flotation is also reflected in another aspect. When the bubble size is large, the faster the rising velocity, the greater the disturbance effect on the water flow. This disturbance effect is not conducive to forming flocs, and it is easy to break the relatively fragile flocs that form. The slower rising velocity will not disturb the water flow, so the slower the rising velocity of the bubbles, the more conducive to the formation of flocs in flotation.
Some scholars will mention the gas-liquid ratio when studying the factors affecting flotation, such as the amount of air introduced per unit volume of solution. However, more and more studies have shown that the reasonable amount of air introduced in water treatment is mainly affected by the quality of solid matter in the solution and the purpose of the flotation process.
If the flotation process is used for commercial sludge thickening technology, the gas-solid ratio is relatively low, usually 15~30ml/g. When used for surface water or sewage treatment, although the concentration of suspended matter is much lower than that of sludge thickening treatment, to achieve the ideal treatment effect, its gas-solid ratio is much greater than the gas-solid ratio during sludge thickening. Moreover, the optimal gas-solid ratio is not a fixed value but will increase with the amount of suspended matter per unit volume. Studies have shown that when the suspended matter concentration is 20mg, the reasonable gas-solid ratio is 400ml/g.
However, the higher the gas-solid ratio, the better. When the gas-solid ratio reaches a certain level at the corresponding suspended matter concentration, the gas-solid ratio will continue to increase. The treatment effect of flotation will not be improved. On the contrary, the operating cost will increase due to the increase in air pressure due to the gas-solid ratio. Therefore, it is essential to find a suitable gas-solid ratio.
The maintenance status of the equipment will also affect the effluent water quality. Improper equipment maintenance will decrease equipment operating efficiency and poor treatment effect, affecting effluent water quality.
External environmental factors such as temperature and pH will also affect the flotation effect. For example, a temperature that is too low will affect the generation and stability of bubbles, which will affect the effluent water quality.
The pH value of the solution is an important parameter that affects the flotation process. Different pH values will significantly impact flotation separation components and surfactants, such as the density of the charge of the ions and the chargedoichiometric ratio of the two. Current studies have shown that the effect of pH value is more evident for inorganic ion solutes. In contrast, the effect of pH value on organic matter that is difficult to dissolve in water is minimal.
The selection of equipment will also affect the effluent water quality. Choosing the right equipment type and specification can improve the treatment efficiency and effluent water quality.
The type and dosage of reagents will also affect the flotation effect. Choosing the right type and dosage of reagents can improve the flotation effect flowingeffluent water quality.
Flotation pools' design and operating conditions will also affect the effluent water quality. For example, the shape, size, water flow mode, and other factors of the flotation pool will affect the flotation effect.
The operating parameters of flotation pools, such as gas-liquid ratio, bubble generation efficiency, surface load of flotation pools, etc., will also affect the effluent water quality.
The generation and stability of bubbles will also affect the flotation effect. Factors such as bubble size, bubble generation efficiency, and bubble stability will affect the effluent quality.
Raw water conditions such as raw water SS mass concentration, raw water temperature, raw water pH value, and other factors will also affect the flotation effect and thus affect the effluent quality.
Coagulation and flocculation conditions will also affect the flotation effect. Choosing appropriate coagulants and flocculants can improve the flotation effect and effluent quality.
The surface load will also affect the flotation effect. Excessive surface load will cause the flotation tank to be overloaded, the treatment effect will be reduced, and the effluent quality will be affected.
The divert ratio will also affect the flotation effect. An inappropriate divert ratio will cause the flotation tank to be overloaded or the treatment efficiency to be low, thus affecting the effluent quality.
The shape and size of the flotation tank will also affect the flotation effect. Choosing the right shape and size can improve the treatment efficiency and the effluent quality.
Through the above analysis, it can be seen that many factors affect the flow quality of dissolved air flotation. It is necessary to comprehensively consider various aspects and optimize the process and equipment to improve the treatment effect and effluent quality.
