Experimental Study on Filtration of Polymer Containing Sewage by Axial Dynamic Backwashing Filter

With the popularization and application of polymer injection development technology in oilfields, the concentration of polymers in oilfield-produced fluids increases rapidly, which has a great impact on the existing deep bed filtration process of walnut shells. 

The outstanding performance is that the polymer wraps the surface of the filter material, the filter material is partially hardened, the filter material regeneration effect is poor, and the filter material is lost. 

About one billion tons of oilfield sewage need to be treated every year. If the water quality of polymer-containing sewage exceeds the standard seriously, it will affect the reinjection of oilfield sewage and have a huge impact on production.

The walnut shell filtration process will face new and greater challenges.

Aiming at the problems existing in the backwashing regeneration method of walnut shell filter material, a new type of axial dynamic backwashing technology has been researched and developed, which provides a new way and method to solve the problem of backwashing regeneration of filter material for filtering high-polymer-containing sewage. 

The axial dynamic backwash filter test device consists of an axial dynamic backwash filter, a water tank, a submersible pump, a flow meter, a flow control valve, and a backwash control system.

The size of the axial dynamic backwash filter is D 0.5 m × 2.7 m, filled with 0.7-1.3 mm degreasing walnut shell filter material, and the height of the filter bed is 1.25 m. The mass concentration of the polymer was 390 mg/L. The test process flow is shown in Fig. 

Working principle and process of axial dynamic backwashing technology

The filtration process of the axial dynamic backwash technology is similar to the conventional process. The key to the technology is the backwashing process. 

Its core is to apply the cyclone separation technology to the backwashing process of the filter material, through the axial turbine to make the backwashing-filter material mixture rotate in a spiral shape at a speed v. 

In the rotation direction, the front and rear filter particles follow the movement, and the continuous collision between the particles generates a rotation tangential collision force FD. In the radial direction, the radial collision force Fp is generated due to the continuous collision between particles due to centrifugal separation. 

The spiral rotation makes the particles of the filter material collide continuously, which strengthens the scrubbing effect between the particles of the filter material. 

At the same time, the velocity gradient between the water flow and the granular filter material strengthens the effect of hydraulic shear force. 

Under the combined action of scrubbing and water flow shearing force, the wrapping on the surface of the walnut shell can be peeled off, and the walnut shell filter material can be effectively cleaned. 

At the same time, there is a certain density difference between the peeled package and the filter material. The centrifugal separation effect caused by the rotating motion allows oil pollutants with a density lighter than water to be discharged along the central pipe with the water flow, while the filter material with a high density is discharged. It moves along the wall and circulates in the filter, and finally achieves effective separation. 

Analytical method

Determination of suspended solids and oil content: The number of suspended solids in water samples was determined by gravimetric method (Q/SY DQ1281—2009); the oil content in water samples was determined by petroleum ether extraction spectrophotometry (SY/T 0530—2011).  

Determination of oil content of walnut shell filter material: 

  • Petroleum ether extraction. Put the walnut shell filter material into a 250 mL ground-mouth conical flask, add VmL petroleum ether, 100 mL distilled water and 5~10 mL hydrochloric acid in a volume ratio of 1:1, gently shake the conical flask to make the gas reaction complete, then Place the cap of the conical flask tightly on the shaker and shake for 30-60 min until the extraction is complete.  
  • Determination of the oil content of the extract. Use petroleum ether extraction spectrophotometry (SY/T 0530-2011) to measure the absorbance A of the extract and record it.
  • The walnut shells are dried and weighed. The oil-extracted walnut shell filter material was placed in a blast drying oven at (105±1) °C for 2 h to a constant weight, and its weight m was measured with a balance and recorded.
  • Walnut shell filter oil quantity W.

W=A.V.n/1000K.m

With the increase of backwashing duration, the change of oil content of backwashing wastewater has the same trend. Within 0-1 min, the oil content in the backwash wastewater increases sharply and reaches the maximum value. Within 1-7 min, the oil content in the backwash wastewater was significantly reduced. 

Within 7-15 min, the oil content in the backwash wastewater gradually decreased and finally reached a steady state. When the backwash intensity is greater than 8.8 L/(s m2), the oil content of the backwash wastewater after backwashing for 15 minutes is less than 46.5 mg/L, indicating that the trapped oil in the filter has been completely removed, and the filter material has been well backwashed and regenerated. 

At the same time, by increasing the backwashing intensity, the time it takes for the oil content of the backwashing wastewater to reach a state of equilibrium is gradually shortened. 
At the same time, the removal efficiency of the oil retained by the filter (the percentage of the ratio of the amount of oil rejected to the total oil retained by the filter) was evaluated when the backwashing lasted for 15 min. 

The total amount of oil retained by the filter is obtained by multiplying the oil content curve of the backwash wastewater with different backwash durations to the abscissa integral value and the corresponding backwash water flow. 

By analyzing the relationship between different backwashing durations and the oil content of backwashing wastewater and the removal efficiency of retained oil in the filter, the optimal backwashing intensity was determined.   
For different backwashing intensities, the oil content of the backwashing wastewater reaches the highest value when the backwashing lasts for 1 min or 2 min. When the backwashing lasted for 15 min, the backwashing wastewater with higher backwashing intensity had lower oil content, indicating that the walnut shell was cleaned and the backwashing effect was good. 

When the backwashing intensity is less than 8.8 L/(s m2), the backwashing intensity is increased, and the removal rate of retained oil shows an upward trend. 

When the backwash intensity is greater than 8.8 L/(s m2), the removal rate of retained oil is basically stable and is significantly higher than that when the backwash intensity is less than 8.8 L/(s m2). When the backwashing intensity was 8.8 L/(s m2), the removal rate of retained oil was 96.23%, and the walnut shell could be fully cleaned, and the backwashing effect was good. 

Filter backwash regeneration effect

The test investigated the axial dynamic backwashing filter at 7.0, 8.8L/(s m2) different strengths and the same backwashing duration of 15 min after backwashing. The amount of feed oil was further verified to verify the effect of backwash intensity on the regeneration effect of the filter material. The results are shown in Figure. 

It can be seen from Figure 5 that the degree of backwashing and regeneration of the filter media at different positions in the filter layer with a backwashing intensity of 8.8 L/(s m2) is basically the same, and the amount of oil in the walnut shell filter media is lower after backwashing. 

After backwashing with a backwashing intensity of 7.0 L/(s m2), the average walnut shell oil content was 6.1 mg/g, and after backwashing with a backwashing intensity of 8.8 L/(s m2), the average walnut shell oil content was 0.51 mg/g. 

This is mainly due to the increase of backwash intensity, the improvement of the fluidization degree of the walnut shell filter material, the increase of the concentration of the internal circulating particle flow, the strong collision between particles, and the full effect of the stripping of pollutants. 

In conclusion

  1. The axial dynamic backwash filter has better removal efficiency of oil and suspended solids in the sewage containing polymer. The average oil concentration in the effluent was 3.63 mg/L, the average suspended solids concentration was 10.3 mg/L, the oil removal rate was 96.0%, and the suspended solids removal rate was 78.1%.  
  2. In the backwashing process with the backwashing intensity of 8.8 L/(s·m2) in the axial dynamic backwashing method, the oil retention rate of the filter material was 96.23%, and the oil content of the walnut shell filter material was 0.51 mg/g. 
  3. Compared with the conventional hydraulic-assisted mechanical stirring backwashing method, the axial dynamic backwashing method has a better filter material backwashing regeneration effect, and the oil volume on the filter material surface changes significantly before and after backwashing.