The comparison of dissolved air floatation method and precipitation method

1. The test process and equipment test raw water is Songhua River water.

The size of the rapid mixing tank is 200mm × 200mm × 200mm, the size of the reaction tank is 300mm × 300mm × 800mm, the size of the sedimentation tank of the sedimentation process is 450mm × 1200mm × 800mm, and the size of the air flotation tank is 450mm × 200mm × 800mm, all of which are made of organic glass. The air compressor used in the test is z-0.29/7 type, the gas dissolving tank is 200 × 1500mm steel tank, which is filled with 800mm step ring packing. *

The adopted process flow is shown in Fig. 1 and Fig. 2. *

2100A turbidimeter is used for turbidity detection. UV254 and TOC were detected by 752 UV grating spectrophotometer and TOC detector respectively.

2 results and discussion

2.1 effect of water temperature on turbidity removal

From the middle of November to the middle of April of the next year, the water quality of Songhua River has the characteristics of low temperature and turbidity (water temperature is 0 ℃, turbidity is about 10NTU, pH value is about 6.8), water temperature is about 20 ℃ and turbidity is about 70ntu between June and July. The coagulant added during the test is aluminum sulfate, and the dosage in the low temperature period is determined to be 40mg / L by the small test. The activated silicic acid is used as the coagulant aid, and the dosage is 5mg / L. The dosage of coagulant at higher temperature and turbidity is 25mg / L. In order to compare the effect of air flotation and sedimentation processes on solid-liquid separation, the same operation parameters were used in the two processes for the preceding processes such as mixing tank, reaction tank, dosage and mixing strength. The rapid mixing time in the mixing tank is 2min, the g value is 420s-1, the reaction time in the reaction tank is 20min, the g value is 10s-1, the hydraulic retention time in the air flotation tank is 10min, the reflux ratio is 8%, the hydraulic retention time in the sedimentation tank is 1H, and the surface load rate is 1 / 6 of that in the air flotation tank.

See Fig. 3 and Fig. 4 for the removal effect of two processes on turbidity under different water temperature. *

As shown in Fig. 3 and Fig. 4, during the low temperature and low turbidity period, the removal rate of turbidity in the sedimentation tank is not high, with an average of about 50%, and the turbidity of outlet water is maintained at about 5ntu; while the removal rate of turbidity in the air flotation tank is as high as about 90%, and the turbidity of outlet water can be reduced to about 1ntu When the water temperature is 20 ℃ and the turbidity of the influent is 60-80ntu, the turbidity of the effluent of the precipitation process is about 4ntu, with an average removal rate of 95%; the turbidity of the effluent of DAF is about 2ntu, with an average removal rate of 97%, which is better than that of the precipitation process.

The difference between the two processes is analyzed as follows:

According to Stokes formula [2], if the diameter of a flocculated particle is 50 μ m and the density is 1.01g/ml, the settling speed of the particle is 0.05m/h at 20 ℃, and the surface loading rate of the sedimentation tank is generally 0.75-1.6m3 / (M2 · h), so the particle will not be removed by sedimentation. If the particles want to get a settling speed of 0.75m/h, they need to grow to a diameter of 178 μ m to be removed. The dynamic viscosity coefficient of water at 0 ℃ is 1.8 times of that at 20 ℃, and the settling speed of flocculated particles at 0 ℃ is 0.03m/h. In order to get a settling velocity of 0.75m/h, the particles need to grow to 245 μ m diameter. *

In the investigation of the effect of water temperature on the treatment effect, the effects of temperature on the stability of flocculated particles, the binding properties between particles and bubbles must be considered. At low temperature, because of the high viscosity of water, the thick hydration membrane around the particles, and the enhanced hydrophilicity of the particles, the particles can exist stably in the water. Especially in the case of low turbidity, due to the small amount of colloidal particles in the water, the chances of effectively colliding and destabilizing with coagulant are also small. In the actual production, it is often necessary to increase the dosage to improve the treatment effect. When DAF process is adopted, the probability of collision and adhesion between bubbles and particles increases due to the introduction of air, which makes the density of particles smaller and easier to float to the water surface for removal. A kind of

2.2 effect of reaction time on turbidity removal

No matter at low or high temperature, the reaction time in the reaction tank shall not be less than 20min in order to obtain high turbidity removal rate for the sedimentation process. For the air flotation process, only 5min reaction time is needed to obtain a high removal rate. This is because the sedimentation method relies on the continuous growth of flocculated particles and sinks. When the turbidity is low, the effective collision probability of particles is reduced, so that the growth time of particles is relatively slow. According to relevant data, it is 20-30min. With the aid of the adhesion of microbubbles, the reaction time of air flotation method is shorter than that of precipitation method.

3 conclusion

① for raw water with turbidity less than 100ntu, DAF process is superior to conventional precipitation process, especially for low-temperature and low turbidity water.

② DAF process only needs 5 minutes to get high treatment efficiency, while for conventional precipitation process, it needs at least 20 minutes to give full play to the precipitation capacity of precipitation process.

③ the removal effect of organic matter in water depends on the coagulation process, but not on the solid-liquid separation steps such as DAF or precipitation. *

④ when a new treatment plant with low temperature and turbidity water quality is built or the existing water plant is reconstructed in northern cities of China, the air flotation process can be preferred.