Abstract

In the process of oilfield wastewater treatment, the polymer-modified materials with special wettability have been recognized by many scholars for their high filtration efficiency and good adsorption effect. In this paper, we used micro-computed tomography scanning and infrared scanning technology to further explore the internal structure and surface chemistry of polyurethane modified materials and then established an experimental platform for the filtration performance of polyurethane modified materials. The change of suspended solids concentration and oil content in the sewage was tested under different filtration rate, filter layer thickness, and water quality. The results showed that the porosity of the filter material and the oil-absorbing material was 65.85% and 56.03% respectively, and the difference in the number of oxygen-containing functional groups on the surface of these two materials indicated different adsorption force for sewage impurities. And the polyurethane modified materials had good filtration performance. Through these experiments, we demonstrated that the quality of water filtrated by the polyurethane modified materials met the requirements of the ‘National Comprehensive Wastewater Discharge Standards’, and the filtration efficiency for suspended particles and oils in oily sewage was higher than 80%. These materials have important practical significance for the harmless treatment of oily sewage.

HIGHLIGHTS

  • A one-depth filtration device was developed to find a more cost-effective way to treat oily wastewater.

  • Structural filters of highly porous fibre media were prepared by different surface modification with polyurethane to attained different hydrophilic effect.

  • That excellent pore structure of different fillers was measured by Micro-CT technology and infrared scanning.

  • Treatment performance was evaluated by the suspended solids and oil content removal efficiencies.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

With more oilfields entering the later stage of exploitation in recent years and the process of oil recovery from low-permeability reservoirs, the application of the water driver method for oil recovery, which is widely used by many oilfields, leads to the rising of water content in crude oil (about 80% to 90%) and creates more oil wastewater (Carrera et al. 2017; Song et al. 2017). Oilfield wastewater is generally treated and returned to the ground because suspended matters and oil droplets in wastewater breed bacteria, which will cause the corrosion of the pipelines and reduce the permeability of the reservoir (Pandarinathan et al.2013). The difficulty of oil–water separation has increased significantly and the treatment process has been unable to meet the increasingly complex requirements of low-permeability and high-viscosity oily wastewater. For these reasons, the improvements of oilfield wastewater treatment have attracted much more attention (Wambecq et al. 2013; Xiao et al. 2016; Tiwari & Sahu 2017).

One conventional filter uses a surface-treated walnut shell as a filter material (Guo et al.2014; Haines et al. 2015; Li et al. 2017). Since the walnut shell is a concave–convex structure, it has a large surface and a strong adsorption property. The characteristics have a good effect in the oily sewage treatment, but over time, the filter gradually shows low filter energy efficiency and the filter material is difficult to regenerate (Yu et al. 2013; Yang et al. 2015; Yin et al. 2016).

From the existing research results, the concentration of suspended solids and that of oil content are two key factors to evaluate the effect of oilfield water treatment (Gunatilake & Bandara 2017; Zhang et al. 2017; Zhu et al. 2017). At the same time, by modifying the surface properties of the filter material, it can have certain effects on industrial water treatment. Some scholars have done a lot of research on the performance of filter materials. Gunatilake & Bandara (2017) used chemical heat treatment to obtain quartz particles with super-hydrophilic oleophobic effect with their contact angle exceeding 150°. Then the double-layer filter was used for repeated tests, and the suspension removal rate was higher than 90%. Abukhadra et al. (2018, 2019) synthesized a bentonite/polyaniline composite and used it as catalyst support for Ni2O3. The final synthetic product (bentonite/PANI@Ni2O3) showed a higher surface area and lower band gap energy than its individual components. In sewage treatment, the polyurethane sponge is a three-dimensional (3D) porous elastic material with rich pore structure and good hydrophilicity, which can effectively adsorb impurities in the wastewater to achieve purification (Cao et al. 2016). What is more, some people have done a lot of research on the use of polyurethane materials. Wei et al. (2017), from University of Malaysia, obtained polyurethane materials by one foaming and modified materials with a new palm oil-based polyester polyol and they found that the mechanical properties of the materials could be improved for industrial wastewater filtration. Nikkhah et al. (2015), Amir Ahmad of the University of Isfahan, used polyurethane materials inlaid with organic clay as a new filter material for oily wastewater filtration in oilfields and, by a series of tests, they found that the filter material showed ultra-high adsorption capacity, and the effective removal rate of oil particles in sewage was as high as 80.3%. However, the current research on filter media only filter out fine particles in sewage; the oil concentration in the wastewater cannot be removed at the same time.

In this paper, two polyurethane modified materials with different filtering effects on oilfield sewage were obtained by coating polyacrylamine and graphene oxide on polyurethane sponge separately. Polyacrylamide has good hydrophilic and oleophobic effects, and when combined with polyurethane sponge, it can effectively remove suspended solids in oilfield wastewater. Graphene oxide can be a material with a larger specific surface area, which can improve the oil absorption capacity of the material (Kleimann et al. 2005; Choi et al. 2020). In this paper, we have prepared a polyurethane modified material which has the effect of filtering and oil absorption. With respect to the oilfield sewage, we built an experimental setup for the filtration performance test of polyurethane modified material and we found the change of the concentration of suspended solid and oil in the water quality test over time by changing the filtration speed, the height of the filter layer and water quality. Further, we explored the change of filtration effect of polyurethane modified materials on oilfield water treatment.

MATERIALS AND METHODS

Preparation of polyurethane modified material

Preparation of polyurethane modified filter material

The specific preparation method is that an appropriate amount of chitosan having a certain viscosity was dissolved in an acetic acid solution as a dip coating solution, then an appropriate amount of polyurethane sponge was ultrasonically washed with acetone and deionized water for 30 minutes, and then dried in a constant temperature oven at 80 °C for a period of time. After the extraction, weighing of the polyurethane sponge was repeated until the interval was less than 0.01 g. After being taken out, the sponge was immersed in a polyacrylamide (10% by mass) solution until the adsorption was saturated, and the excess polyacrylamide solution was squeezed out; then the sponge immersed in the dip coating liquid for 30 minutes. Finally, the polyurethane sponge was taken out and dried at 80 °C for 3 hours to remove excess dip coating liquid. The obtained material finally was the filter material proposed in this study.

Preparation of polyurethane modified oil absorption material

An appropriate amount of graphene oxide was dispersed by ultrasonication in an appropriate amount of ethanol solution to obtain a dip coating solution, which was ultrasonically dispersed for 45 minutes, and the solution concentration was 2 mg/L. Then, an appropriate amount of polyurethane sponge was weighed and ultrasonically washed with acetone and deionized water for 30 minutes, and then placed in a constant temperature oven at 80 °C for drying. After a period of time, it was taken out and repeatedly weighed to an interval of less than 0.01 g. Then, the polyurethane sponge was soaked with a good dip coating solution until the adsorption reached a saturated state. Finally, the polyurethane sponge was taken out and dried at 80 °C for 3 hours to remove excess ethanol. The obtained material was the oil-absorbing material proposed in this study.

Experimental materials

Water quality

The experimental water used in this study is the raw water and the sewage treated by the first-stage filtration of the walnut shell filter provided by the Hekou Oil Production Plant in Shengli Oilfield which provides the water quality data. In order to ensure the quality of the produced water not affected by the internal microbial reaction, all water was transported from the site. The experimental operation was limited to 3 days. The experimental water quality is shown in Table 1.

Table 1

Water quality samples

NumberParameterUnitRaw waterOnce filtered water
Oil mg/L 166.47 4.96 
Suspended solids mg/L 36.62 18.33 
pH  6–8 6–9 
Temperature °C 50–60 40–60 
NumberParameterUnitRaw waterOnce filtered water
Oil mg/L 166.47 4.96 
Suspended solids mg/L 36.62 18.33 
pH  6–8 6–9 
Temperature °C 50–60 40–60 

Experimental setup

The filtration performance tests of polyurethane modified materials were carried out under ambient temperature conditions. The whole process was carried out by stepwise filtration and purification treatment. The whole process was mainly divided into pre-filtering and removal of de-suspended substances and oil contained in the water. In Figure 1, the No. 1 coarse filter tank was filled with filter material, the No. 2 filter tank was filled with oil absorption material, and the No. 3 filter tank was reserved.

Figure 1

Experimental platform flow chart.

Figure 1

Experimental platform flow chart.

The experiment began as the power was turned on. The oily sewage was first sucked into the pipeline from the sewage storage tank by the centrifugal pump. Then the centrifugal pump provided sufficient power for the water to flow inside the pipeline. The sewage flowed through the turbine flowmeter from the bottom of the No. 1 filter tank. Then it passed through the porous interception filtration material and flowed out from the top of the filter tank into the No. 2 degreasing tank. The internally filled material had a lipophilic hydrophobic effect; thus, it had certain selective adsorption for the oil contained in the water. After the sewage was sucked by the degreaser, it flowed out from the bottom, and finally was collected by the purified water storage tank. We can measure the filtration effect of the materials by detecting the water quality of the two storage tanks.

Methods

Study on structural properties of materials by Micro-CT technology

In this study, two modified materials were scanned using micro-computed tomography (Micro-CT) scanning technology to quantitatively characterize the internal structure of porous polyurethane modified materials. The CT imaging system was powered to operate at a voltage and electric current of 150 keV and 33 µA respectively. The X-rays are absorbed at different degrees in the process of penetrating the sample from different angles; 38,000 projections were recorded over a 180° rotation with a step size of 0.2° (Shah et al. 2016). The porous structure of filter material was shown in the CT image through the noise reduction, threshold setting, color conversion, and image cutting (Song et al. 2017). The resulting 3D image typically consisted of more than 38,000 encoded slices which were encoded in 8-bit precision corresponding to a greyscale level of 0–255 (Haines et al. 2015). The specific parameters of the image are shown in Table 2.

Table 2

Parameters of materials detection by Micro-CT

Filter material typeImage size (mm)Resolution ratio ()Shape
Filter material 224*208*208 29 Black round cake 
Oil-absorbing material 244*256*244 43 White cylinder 
Filter material typeImage size (mm)Resolution ratio ()Shape
Filter material 224*208*208 29 Black round cake 
Oil-absorbing material 244*256*244 43 White cylinder 

Study on surface properties of materials by Fourier transform infrared spectroscopy

In this experiment, KBr was used to make samples because of better ductility than NaCl, which had lower absorption rate of infrared light and the ability to map out the full-band spectrum. The main steps of production were as follows. Firstly, all the vessels used for tableting were rinsed with ethanol and then dried under infrared light. The rest of the steps were carried out under the environment of infrared light irradiation. The sample was weighed at 2 mg and mixed with KBr in a ratio of 1:100, and then ground into a fine powder. An appropriate amount of the ground powder was taken in a mold. The sample was placed on a die press, and the pressure was adjusted to 10 mPa and kept for 3 minutes, after which the oil pressure switch was turned on. At this time, the sample to be tested was evenly transparent. Finally, the film was placed on a magnetic sample holder.

Figure 2 demonstrates the process of infrared scanning. The infrared radiation emitted by the light source was converted into interference light by the interferometer, and an interferogram containing sample information was obtained after passing through the sample. After the infrared radiation was emitted by the light source, it was converted into an interference diagram by the interferometer. The interferogram with the sample information formed after the sample was projected and is collected by the computer. The absorption infrared spectrum was obtained in a short time using Fourier transform. Therefore, we could use the correspondence between the molecular structure and the spectrum based on the vibration frequency and displacement law of a specific group to determine the functional group. The intensity and the shape of the chemical bond represented by the absorption band in the spectrum could be matched with the surface of the material to judge the physical and chemical properties.

Figure 2

Principles of a Fourier transform Infrared spectrometer.

Figure 2

Principles of a Fourier transform Infrared spectrometer.

We took a small amount of the sample to be tested and placed it in a mortar to fully grind it into a fine powder together with KBr, and then compressed it. The compressed sample piece was placed in an infrared spectrometer for measurement, and the spectrum was saved after the baseline was processed on the computer recording end.

Setting methods of filtering speed, filter height, and water quality

The experiment set filter speeds as 10, 15, and 25 m/h respectively. The heights of different filter layers were 100, 200, 300, and 400 mm respectively. The oilfield raw water, pre-filtered water after sedimentation and filtered water once filtered through quartz sand were adopted as samples. Three kinds of water samples in the water after filtration through quartz sand were sampled every 30 minutes, and the experimental data were included in the test. In order to avoid the influence of the sewage on the filter material between the experiments, each group of experiments was backwashed for 30 minutes.

Methods for measuring concentration of suspended solids and oil

One hundred millilitres of a mixed uniform water sample was taken for suction filtration so that all the water passed through the filter membrane. Firstly, 10 mL distilled water was taken out and cleaned three times, and then the excess water of the filter needed to be absorbed. Finally, the filter containing suspended matter was carefully removed and transferred to an oven at 103–105 °C for 1 hour before being removed and dried. The cooling sample was repeatedly weighed until the weighing difference was less than 0.4 mg. The suspended solid content C (mg/L) was calculated as follows:
formula
(1)
where C is concentration of suspended solids in water, mg/L; A is suspension + filter + weighing bottle weight, g; B is filter membrane + weighing bottle weight, g; V is sample volume, mL.

A 260 mL evenly mixed water sample was taken out and poured into a parting funnel, and the measuring cylinder was rinsed twice with 25 mL petroleum ether. The washing liquid was then poured into the separator funnel, shaken thoroughly and allowed to stand for 10 minutes until the mixture was completely separated. The supernatant was obtained by layering and rotating the separating funnel knob to discharge the waste at the bottom, The funnel was rotated to expel waste from the bottom to collect the supernatant. A little bit of petroleum ether was then added to a 100 mL volumetric flask and shaken several times. The cuvette was washed three times with an appropriate amount of distilled water, and the petroleum ether was used as a blank sample. After washing the cuvette three times with the test solution, an appropriate amount was added to the cuvette.

After the blank sample was calibrated, the absorbance of the sample was measured using a wavelength of 430 nm. When the screen data were stable, the measured values were read, and the experimental data were recorded. To ensure the accuracy of the experimental data, each sample was measured in parallel three times, and the average value was recorded. The oil content of the water could be obtained according to the following formula:
formula
(2)
where α is absorbance value, and V is water sample detection volume.

RESULTS AND DISSCUSSION

Study on structural properties of materials by Micro-CT technology

The pore structural features of these two kinds of filter materials were determined by Micro-CT. As shown in Figure 3, the two kinds of materials with different wetting properties showed abundant pore structure, and the pore size was randomly distributed without a fixed value.

Figure 3

Micro-CT images of structural characteristic. (a) Filter material with hydrophilic and lipophobic modified materials. (b) Oil-absorbing material with lipophilic and hydrophobic modified materials.

Figure 3

Micro-CT images of structural characteristic. (a) Filter material with hydrophilic and lipophobic modified materials. (b) Oil-absorbing material with lipophilic and hydrophobic modified materials.

It can be seen from the Figure 3 that pore structure was abundant and had a random arrangement inside of the filter material, which potentially provided abundant storage room for oil in water and the ability to decrease the concentration of suspended solids was improved. Table 3 provides a comparison of the porosity structure parameters and data analysis of the two types of filter materials.

Table 3

Material configuration parameters by Micro-CTD data obtained from a previous study (unpublished)

Material typePorosityPore numberAverage pore radius (μm)Maximum pore radius (μm)Average pore–throat ratioAverage pore volume (μm3)
Filter material 65.85% 3,492 102.379 338.784 2.743 4.2721 × 107 
Oil-absorbing material 56.03% 6,448 123.252 1,090.86 2.982 9.98173 × 107 
Material typePorosityPore numberAverage pore radius (μm)Maximum pore radius (μm)Average pore–throat ratioAverage pore volume (μm3)
Filter material 65.85% 3,492 102.379 338.784 2.743 4.2721 × 107 
Oil-absorbing material 56.03% 6,448 123.252 1,090.86 2.982 9.98173 × 107 

The average pore radius of filter material and oil-absorbing material were 123.252 and 102.379 µm respectively, which obeyed normal distribution in varying degrees as shown in Figure 4.

Figure 4

Pore and throat radius distribution of two functional materials. (a) Pore radius, (b) throat radius.

Figure 4

Pore and throat radius distribution of two functional materials. (a) Pore radius, (b) throat radius.

As shown in Figure 4, the pore radius of filter material was concentrated between 100 and 150 and the peak was 110 . For the oil-absorbing material, the pore radius was mainly distributed from 75 to 175 , and 140 was the peak of this interval. So it could be seen that the pore radius of filter material was less than that of oil-absorbing material. Therefore, filter material was more effective in filtering depending on its tiny pore structure, which was beneficial to filter out the suspended particles in the wastewater. The larger pore–throat ratio of oil-absorbing material meant that its throat radius tended to be a lower level, which decreased the seepage ability and enlarged the local loss of liquid flow in the filter material, all of which was conducive to the collection of oil. According to Figure 4, the throat shape factor of those two kinds of fibre media obeyed a good normal distribution, and the value was very small, which indicated that the pore shape was irregular inside of these filter materials.

Study on surface properties of materials by Fourier transform infrared spectroscopy

The filter materials used in this paper were obtained by modifying the polyurethane. The statistical data processed by the Fourier infrared scanning experiment are shown in Figure 5.

Figure 5

Infrared scanning analysis spectra.

Figure 5

Infrared scanning analysis spectra.

It can be seen from Figure 5 that the intensity of the characteristic absorption bands of the two materials differed in the infrared spectrum of the two functional material components after different surface modification treatments. The filter had a very obvious absorption peak in the range of 3,300–3,550 cm−1. This absorption peak was related to the hydrophilicity of the material. Since the modified filter material was more hydrophilic than the oil-absorbing material, its adsorption of suspended solids was much better, and its absorption peak intensity was greater than that of the oil absorption material. The hydroxyl stretching vibration absorption peak at 3,450 cm−1 and the COC stretching vibration absorption peak near 1,125 cm−1 indicated that the polyurethane material was a polyether polyol, and the 3,600 cm−1 peak, corresponding to the strong absorption peak of amide, indicated that the polyurethane material component after the modification treatment was modified by the amide chemical. In addition, 1,710 cm−1 was the vibration absorption peak of the carbonyl group in the urethane bond and the intensity was high, and 1,530 cm−1 was the bending vibration peak of the N-H bond, and it was known that the amide existed in a free form.

In the fingerprint range of 1,500–400 cm−1, there were many absorption bands, and each peak could clearly reflect the characteristics of the material. This region was also an important region for identifying whether two compounds were the same compound. The difference in the number of oxygen-containing functional groups on the surface of the two materials indicated the difference in the adsorption force of the material for sewage impurities. By comparing the infrared spectra of the two materials, it could be seen that when the materials were subjected to different modification treatments, the functional groups changed the displacement and strength of the absorption bands, thus causing the materials to have different surface physical and chemical properties.

Effect of filtration rate on filtration performance

In order to investigate the effect of filtration rate on the filtration performance of the new filter material, the pre-filtration water was selected as the experimental material. The suspended solid concentration was 17.96 mg/L, the oil content was 8.48 mg/L, and the height of the test bench filter layer was controlled at 40 cm. Experiments were carried out to investigate the effect of polyurethane modified foaming material on sewage treatment at a filtration rate of 10, 15, and 25 m/h. The experimental data obtained are shown in Figures 6 and 7.

Figure 6

Effect of filtration rate on the treatment of sewage suspended solids. (a) Change in suspended solids concentration. (b) Change in suspended solids removal rate.

Figure 6

Effect of filtration rate on the treatment of sewage suspended solids. (a) Change in suspended solids concentration. (b) Change in suspended solids removal rate.

Figure 7

Influence of filtration rate on oil treatment of wastewater. (a) Change in oil concentration of sewage. (b) Change in oil removal rate.

Figure 7

Influence of filtration rate on oil treatment of wastewater. (a) Change in oil concentration of sewage. (b) Change in oil removal rate.

It can be seen from Figures 6 and 7 that the concentration of suspended solids and oil content in the same period of time showed different degrees of change with the increase of flow rate. For the suspended solids contained in the sewage, when the filtration rates were 10, 15, and 25 m/h respectively, after 10 hours of experimental operation, the suspended solids concentrations were 2.67, 3.01, and 3.91 mg/L respectively. The removal rate of the material was higher than 76%. For the treatment of oil contained in the water, when the filtration speed is 10 m/h, it can run continuously for 10 h. The three different filtration rates ensured that the effective oil removal rate of the material was higher than 80%. The three filtration rates had little effect on the filtration performance of the material within 3 hours of the experimental operation. The filtration rate of the suspended solids and oil droplets in the sewage was almost the same for each filtration rate. After running the experiment for 4 hours, the effect of filtration rate on the performance of the filter became more and more obvious.

Comparing the above data, the lower the filtration rate was, the better the filtration effect of the new filter material on the oily wastewater in the oilfield, mainly shown by the higher suspended solids removal rate and oil removal rate. This was because after the increase of the filtration rate, the loss of the head along the inside of the filter layer increased, and the damage of the water flow to the capillary condensation phenomenon in the porous filter tended to be obvious. And the filter layer continued to maintain its interception effect on the sewage flow, but the precipitation of the filter material would be weakened and the retention efficiency would be reduced.

Effect of filter layer thickness on filtration performance

The thickness of the filter layer is a main influencing factor of the filtration efficiency of a filter, and it was one of the main parameters for investigating the filtration performance of the filter material. In this study, in order to examine the effect of filter thickness on the filtration performance of the material, pre-filtration water was selected as the experimental material. The concentration of suspended solids was 17.96 mg/L, the oil content was 8.48 mg/L, and the filtration rate was controlled at 15 m/h. The filtration effect of the filter material on the water before filtration was obtained when the thickness of the filter layer was 100 mm, 200 mm, 300 mm, and 400 mm. The experimental data are shown in Figures 8 and 9.

Figure 8

Effect of filter height on the treatment of wastewater suspended matter. (a) Change in suspended solids concentration. (b) Change in suspended solids removal rate.

Figure 8

Effect of filter height on the treatment of wastewater suspended matter. (a) Change in suspended solids concentration. (b) Change in suspended solids removal rate.

Figure 9

Effect of filter height on the treatment of wastewater containing oil. (a) Change in oil concentration of sewage. (b) Change in oil removal rate.

Figure 9

Effect of filter height on the treatment of wastewater containing oil. (a) Change in oil concentration of sewage. (b) Change in oil removal rate.

According to Figures 8 and 9, it can be seen that the filtration effect of the filter material on the suspended solids and the oil content of the sewage was affected by the thickness of the filter layer. It was manifested that the filtration effect of the filter material on the sewage increased as the height of the filter layer increased. When the thickness of the filter layer was 100 mm, after 4 hours of the experimental operation, the removal rate of suspended solids was obviously reduced. After 10 hours of operation, the concentration of suspended solids in water was 6.05 mg/L, and the oil content was 1.63 mg/L, which did not meet the water quality standards of external drainage. When the thickness of the filter layer was 200 mm, the removal rate of suspended solids was obviously decreased after 6 hours of experimental operation. The final concentration of suspended water after operation for 10 h was 5.87 mg/L, the oil content was 1.53 mg/L, and the water quality was poor. When the thickness of the filter layer was 300 and 400 mm, and the experimental operation was 10 h, the treated water quality could reach the efflux standards. The suspended matter concentration was lower than 5 mg/L, the oil content was lower than 1 mg/L, and the effective treatment of suspended solids and oil was effective. The efficiency was higher than 80%.

According to the above data analysis, the filtration effect of the new filter material on the sewage was affected, to some extent, by the thickness of the filter layer. When the thickness of the filter layer was 100 and 200 mm, the effect of the thickness of the filter layer was small. Because the internal flow time of the filtrate in the filter material was short, the filtrate could not be fully contacted with the filter bed, so the oil droplets contained in the filtrate could not be fully absorbed by the filter bed and long-term work would lead to penetrating of the filter bed due to the limited adsorption capacity of the material and the adhesion capacity of the particles, which was mainly expressed as a significant decrease in treatment efficiency and poor water quality. In addition, since the thickness of the filter layer was too low, the internal flow of the filtrate in the filter material was short, and sufficient contact with the filter bed could not be achieved, so the oil droplets contained in the filtrate could not be sufficiently adsorbed by the filter bed, and the oil concentration of the filtered water was high. Then, if the thickness of the filter layer was too high, the head loss would increase before and after filtration, and the flow resistance of the suspended particles and oil droplets would be enhanced, which would lead to the problem of blocking and pressing in practical applications. But if the height of the filter layer were too high, it would reduce the effective utilization of the filter material and increase the production cost. Therefore, in actual production needs, the filter layer height should be reduced while meeting the sewage treatment requirements.

Effect of raw water quality on filtration performance of new filter materials

In order to control the variables, the filter tank and the suction tank were filled with a filter material height of 40 cm, and the filter speed was controlled at 20 m/h. The filter filtration performance experiments were carried out on three different water qualities. The experimental results are shown in Figures 10 and 11.

Figure 10

Effect of water quality on the treatment of sewage suspended solids. (a) Change in suspended solids concentration. (b) Change in suspended solids removal rate.

Figure 10

Effect of water quality on the treatment of sewage suspended solids. (a) Change in suspended solids concentration. (b) Change in suspended solids removal rate.

Figure 11

Effect of water quality on treatment of oily sewage. (a) Change in oil concentration of sewage. (b) Change in oil removal rate.

Figure 11

Effect of water quality on treatment of oily sewage. (a) Change in oil concentration of sewage. (b) Change in oil removal rate.

It could be found from the above figures that the concentration of oil and suspended solids in the water filtered by new filter material gradually increased with the experimental operation time increasing, and oil concentration in filtered water always stayed at more than 1 mg/L, with the removal rate around 85%. The concentration of suspended solids in filtered water was 7.15 mg/L after 10 h, with the removal rate of 80.40%. It could be considered from the above data that the new filter material was more efficient for raw water filtration, but the quality of filtered water did not meet the discharge standards. In the experiment involving pre-filtration water after sedimentation, the concentration of suspended matter was 3.97 mg/L and the oil concentration was 2.21 mg/L after the experiment had been running for 10 h, at which time the removal rate of suspended matter reached 77.90% and the oil removal rate reached 73.93%, and 10 hours later, the filtration efficiency of the filter material decreased faster. However, when post-filtration water filtered by quartz sand was involved in the experiment, the concentration of suspended matter was 2.66 mg/L and the oil concentration was 0.16 mg/L after the experiment has been run for 10 h, and the quality of filtered water was very good and met the discharge requirements.

As can be seen from Figures 10 and 11, among the three samples of water, the removal effect on the oil content of the pre-filtration water was best, but when the oil content in sewage exceeded a certain content, the adsorption capacity of the filter material reached saturation, unable to further absorb the oil droplets in the water. As a result, the residual oil droplets were desorbed by the current, resulting in the high oil content in the filtered water. However, when the oil content of sewage was too low, the oil content after being filtered was low to some extent, but its removal rate was also low, for if the oil content was low, the relative base of oil was small and the removal rate was not high. As for the removal effect on suspended solids, when the concentration of suspended matter was high, it easily deposited in the filter material and blocked the pore structure in the filter material, thus preventing the continuous interception on the filter material surface. On the other hand, due to the high viscosity of the oil in sewage, the oil droplets adhere to suspended solids, resulting in a synergistic effect between the adsorption of the oil droplets and the filtration of suspended solids. From the analysis of the experimental results, it was found that the filtration performance of the new filter material could not improve with the increase of suspended solids. It was considered that the new modified polyurethane foam should be used in order to improve the filtration performance and filtration efficiency.

CONCLUSIONS

In this paper, we have prepared polyurethane modified materials, including filter materials and oil-absorbing materials, to study the internal structure and surface chemical properties by Micro-CT scanning and infrared scanning technology. At the same time, in order to further test the performance of filtration and oil absorption, an experimental set was designed and constructed, and the filtration and oil absorption performance of polyurethane modified material were tested under different conditions of filtering speed, filter height and water quality. The experimental results showed that the polyurethane modified material had stable filtration performance and higher treatment efficiency. The concentration of suspended matter in the sewage after treatment was less than 5 mg/L, the oil concentration was less than 1 mg/L, and the treatment efficiency was higher than 80%.

  • (1)

    By scanning the internal structure of the two materials, we found that the pores of the material were distributed irregularly. The porosity and average pore cross-section of the filter material were 65.85% and 102.38 µm respectively, and the porosity and average pore cross-section of the oil-absorbing material were 56.03% and 123.25 µm, and the pore volume of the material was an order of magnitude larger than the volume of the throat.

  • (2)

    By comparing the infrared spectra of the two materials, it could be seen that when the materials were subjected to different modification treatments, the functional groups changed the displacement and strength of the absorption bands, thus causing the different surface physical and chemical properties.

  • (3)

    The lower the filtration rate was, the better was the filtration effect of the new filter material on the oily wastewater in the oilfield. But the appropriate increase in the filtration rate could shorten the construction period, so the corresponding filtration speed could be selected according to actual needs.

  • (4)

    The filtration effect of the new filter material on sewage was affected to some extent by the thickness of the filter layer. The higher the thickness of the filter layer was, the better the sewage treatment effect was.

  • (5)

    The concentration of oil and suspended solids in raw water was too high or too low, and the removal effect was not obvious when the concentration of oil and suspended matter was moderate.

ACKNOWLEDGEMENT

This work was supported by The National Key R&D Program of China (2017YFB0603300).

DATA AVAILABILITY STATEMENT

All relevant data are included in the paper or its Supplementary Information.

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