Concentrating emulsified oily wastewater by integrated membrane technology

Membrane separation technology is an efficient and environmentally friendly separation process. Different from traditional filtration, the membrane has a selective separation function, which can realize separation (Zhang et al. 2021), purification (Alias et al. 2019), and concentration (Tanudjaja et al. 2019) of different components of solid–liquid. The separation process is a purely physical process, which has the characteristics of no phase change and energy-saving, and is widely used in the treatment of emulsified oily wastewater. In addition, membrane separation technology is widely used in municipal (Zhao et al. 2021), petrochemical (Ahmad et al. 2020), food processing (Wang et al. 2020), and other wastewater treatment processes and environmental protection industries.

Helali et al. (2020) used polyamide-imide (PAI) to prepare hydrophilic and underwater ultra-hydrophobic microfiltration membranes by the non-solvent-induced phase conversion method, and modified them by polyvinylpyrrolidone and polyethylene glycol, further improving their hydrophilic properties. The experimental results showed that the modified membrane showed a 98% oil rejection rate, and the permeation flux was as high as 210 L/(m²·h). Under the condition of polyethylene glycol (PEG) 6 kDa and polyvinylidene fluoride (PVP) 10 kDa ratio, the underwater oil contact angle of the modified membrane was more than 150°, and the flux recovery rate was as high as 98%. Yuliwati et al. (2011) modified a polyvinylidene fluoride (PVDF) ultrafiltration membrane with LiCl·H₂O and TiO₂ by the phase conversion method in a study. The wastewater used in the experiment was oily wastewater from the refinery. The experimental results showed that when the concentration of TiO₂ was 1.95%, the hydrophilicity of the membrane and the porosity of the membrane reached the highest. It was also found that when pH = 6.9, the maximum flux was 82.5 L/(m²·h), and the retention rate was 98.8%. This study shows that PVDF ultrafiltration membrane can treat oily wastewater from oil refineries.

From the above documents, it can be seen that in the process of wastewater treatment through the single-stage membrane at home and abroad, more research is focused on the preparation and modification of membrane materials by preparing the corresponding hydrophilic membrane to improve the separation performance of the separation membrane, reduce the pollution rate of the separation membrane, and achieve a better separation effect. However, there are few studies on the concentration of oil on the side of the intercepted liquid, the size of oil droplets, and subsequent treatment issues after membrane treatment (Zhou et al. 2008, 2017; Chen et al. 2019; Lee et al. 2020). In the engineering application of membrane technology to treat oily wastewater, the post-treatment of intercepted liquid cannot be avoided. Through the comprehensive consideration and research of membrane separation process parameters, the reasonable integration of membrane process, and membrane pollution, the effective separation of intercepted liquid after membrane treatment can be achieved, so that the permeate can meet the requirements of drainage or recycling and achieve the recovery and reuse of oil, which is of great significance for environmental protection and effective use of energy.

In this paper, emulsified oil is prepared with diesel oil and water, and oily wastewater is simulated. The effects of several aperture separation membranes on the treatment of emulsified oily wastewater under different conditions are discussed. The reasonable combination and optimum process for oil concentration are studied.

The instruments used were electronic balance AE224; pure water machine GT-30; electric mixer JJ-1A; laser particle size analyzer BT-9300H; vacuum drying box DZF-6020; electric heating constant temperature water bath pot HWZ-2; glass dryer 450 mm.

Deionized water (DI water), Tween 80 (Tween80), anhydrous sodium sulfate (Na₂SO₄), hexane (C₆H₁₄), 98% sulfuric acid (H₂SO₄), and sodium chloride (NaCl) were all analytically pure; microporous membrane (PP, pore size: 0.22 μm; 0.45 μm; 0.8 μm); plate membrane module; diesel fuel.

Around 1 g of diesel oil as base oil is weighed by using an electronic balance, 1,000 mL of DI water is added, and 0.1 g of Tween 80 as a surfactant is added and placed in an electric stirrer with 2,500 r/min stirring speed for 50 min. The obtained white emulsion is emulsified oil and stored in a glass dryer at room temperature.

Oil droplet size is one of the important parameters to identify the stability of emulsified oil (Matos et al. 2016; Shi et al. 2019). In the membrane concentration experiment, the concentration effect of emulsified oil under different operating conditions was investigated by measuring the average particle size of oil droplets in the concentrated solution. In this experiment, the BT-9300H laser particle size analyzer was used to measure the particle size of oil droplets. The refractive index of diesel oil is 1.45 when the background value is 1–4; the system background calibration is carried out and then the prepared emulsified oily wastewater is added. When the shading rate is 15–20, the measurement is carried out.

The oil content in oily wastewater can be determined by the gravimetric method, fluorescence spectrophotometry, ultraviolet spectrophotometry, and infrared spectrophotometry (Wei 2011; Padaki et al. 2015; Khorshid et al. 2021). After membrane concentration, the oil concentration in the concentrate will become larger, and the weight measurement method has a wide range of measurements and is not limited by the oil standard, so this experiment chooses the cumbersome weight measurement method.

Figure 4(a)–4(c) shows the effect of operating pressure on the membrane flux, the concentration ratio and the average particle size of concentrated oil droplets. From the figure, in a certain range with the increase of operating pressure, the membrane flux is also increasing. For the membrane with the pore size of 0.22 μm, when 0.1 MPa < P < 0.4 MPa, the membrane flux increased from 229.145 to 330.242 L·m⁻²·h⁻¹. Similarly, for the membrane with a pore size of 0.45 m, when the operating pressure was 0.1 MPa < P < 0.3 MPa, the membrane flux increased from 240.391 to 335.751 L·m⁻²·h⁻¹, and the operating pressure of the membrane with a pore size of 0.8 μm was 0.1 MPa < P < 0.2 MPa. The membrane flux increased from 320.32 to 340.23 L·m⁻²·h⁻¹. It can be seen that within a certain operating pressure range, the membrane flux of membranes with different pore sizes increases linearly with the increase of pressure, which is because there is almost no pollution in the membrane pores at the initial stage of filtration (or when the pressure is low), and the fouling layer on the membrane surface is very thin. The membrane flux increases linearly with the increase of pressure, and the variation trend of oil droplet size and concentration multiple is basically the same as that of membrane flux. With the increase of pressure to a certain extent, the increase of specific resistance of the pollution layer partially offsets the increase of membrane flux caused by the increase of pressure, which slows down the increase of membrane flux and even tends to be stable, and the change of oil droplet size and concentration multiple tends to be flat. With the further increase of pressure, the growth rate of filtration resistance exceeds that of pressure, resulting in the attenuation of membrane flux and the decrease of concentration factor.

Considering the membrane flux, the concentration ratio, the average particle size of oil droplets in concentrated solution and energy consumption, the operating pressures of the three separation membranes were 0.4 MPa (membrane pore size 0.22 μm), 0.3 MPa (membrane pore size 0.45 μm) and 0.2 MPa (membrane pore size 0.8 μm), respectively.

Figure 5(a) shows the effect of feed temperature on membrane flux. The membrane flux of the three separation membranes increased from 221.59 to 261.37 L·m⁻²·h⁻¹ (membrane pore size 0.22 μm), from 240.39 to 296.45 L·m⁻²·h⁻¹ (membrane pore size 0.45 μm), and from 310.32 to 371.56 L·m⁻²·h⁻¹ (membrane pore size 0.8 μm), showing an approximate linear growth. There may be several reasons for the increase in the membrane flux: on the one hand, the viscosity of liquid decreases with the increase of temperature, which improves the flow condition of liquid; on the one hand, the increase of liquid temperature is beneficial to increase the diffusion coefficient of oil droplets, which makes the oil droplets trapped on the membrane surface return to the main fluid and reduces the speed of forming concentration polarization on the membrane surface; on the one hand, increasing the feed temperature increases the membrane pore and reduces the resistance of solvent water through the membrane pore; on the other hand, due to the existence of hydrogen bonds, water molecules exist in the form of association. With the increase of solid–liquid temperature, the movement of water molecules is accelerated, which reduces the association effect and the association size, and makes it easier for water molecules to pass through the separation membrane, thereby improving the membrane flux (Li et al. 2008; Wang 2014). However, it is not that the higher the temperature is, the better the membrane will be. Higher temperatures will cause damage to the internal structure of the membrane and shorten the service life of the membrane. In addition, increasing temperature will increase energy consumption. It is not economical to obtain higher membrane flux by increasing the liquid temperature. In the actual concentration process, with the increase of equipment running time, the liquid temperature will also rise.

It can be seen from Figure 5(b) and 5(c) that the concentration ratio and the particle size of oil droplets in the concentrated solution increase with the increase of temperature, but the increase is small. This is because, with the increase of temperature, the average particle size of oil droplets in emulsified oily wastewater decreases gradually. In addition, the increase of temperature leads to the increase of membrane pores. Smaller oil droplets will enter the osmotic solution through the membrane pores, and the increase of temperature will destroy the membrane pores. Oil droplets will adhere to the membrane pores when passing through the membrane pores, resulting in the blockage of the membrane pores.

Considering the membrane flux, the concentration ratio, the average particle size of oil droplets in concentrated solution and energy consumption, the appropriate temperature range of the separation membrane was 25–30 °C.

Figure 6(a)–6(c) shows the effect of oil content on the membrane flux, the concentration ratio and the average particle size of oil droplets. When the oil content of the feed liquid is low, the membrane flux decreases with the increase of the oil content of the feed liquid. When the oil content of the feed liquid is high, the membrane flux decreases slightly and tends to be stable with the increase of the oil content of the feed liquid (Hu 2021). With the increase of feed liquid, the membrane surface will increase the reservoir cover surface, resulting in the decrease of membrane flux, the increase of membrane fouling and the decrease of concentration ratio. On the other hand, increasing the oil content of the feed liquid will increase the viscosity of the feed liquid, which makes the flow state of the feed liquid worse, and the oil droplets are easier to adhere to the plug hole or to the membrane surface, which accelerates the membrane fouling and the concentration polarization of the membrane surface. The above two aspects lead to the decrease of membrane flux. The lower oil content will increase the chance of collision between oil droplets, resulting in the re-aggregation of oil droplets and the increase of the average particle size of oil droplets. When the oil content of the feed solution is high, a small part of water molecules enters the permeate through the membrane pores, and the increase in the average particle size of oil droplets in the concentrated solution decreases.

Considering the membrane flux, the concentration ratio, the average particle size of oil droplets in concentrated solution and energy consumption, the oil content of feed liquid of the three separation membranes was 2% (membrane pore size 0.22 μm), 3% (membrane pore size 0.45 μm), and 4% (membrane pore size 0.8 μm), respectively.

Figure 7(a)–7(c) shows the effects of membrane surface velocity on the membrane flux, concentration multiple, and the average particle size of concentrated oil droplets. With the increase of outlet flow rate, the membrane flux first increases and then decreases. When the flow rate is low, the oil droplets stay on the membrane surface for a long time, and the oil layer gap on the membrane surface decreases. At the same time, the continuous accumulation on the membrane surface will gradually form an oil layer, resulting in low membrane flux. With the continuous increase of flow rate, the turbulent part in the cross-flow filtration membrane tank increases, which reduces the residence time of oil droplets in emulsified oily wastewater on the membrane surface, and then increases the gap between oil layers on the membrane surface, slows down the membrane pollution, improves the membrane flux, and increases the concentration multiple, which increases the chance of collision and aggregation of oil droplets in the intercepted liquid and increases the particle size of oil droplets in emulsified oily wastewater. With a further increase of flow rate, the particle size of oil droplets in oily wastewater decreases, and some smaller oil droplets enter the membrane pores, resulting in the blockage of membrane pores and the decrease of membrane flux (Padaki et al. 2015; Golshenasa et al. 2020). The water molecules entering the permeate through the membrane hole are reduced, the concentration ratio is reduced, and the chance of coalescence between oil droplets in the concentrate is reduced, which reduces the concentration effect.

In the actual concentration process, the appropriate flow rate can improve the membrane flux, concentration multiple, and the average particle size of oil droplets in the concentrate, but energy consumption should also be considered. Based on the above analysis, the optimal flow rates of the three separation membranes are 200 mL/min (0.22 μm), 250 mL/min (0.45 μm), and 300 mL/min (0.8 μm).

The concentration and purification of oily wastewater were further realized by the reasonable combination of filter membranes with different pore sizes. The initial operating conditions of each combination is shown in Table 1.

In this paper, several membranes with different pore sizes were used to treat emulsified oily wastewater. The effects of operating pressure, temperature, oil content, and membrane surface flow rate on membrane flux, the average particle size of oil droplets, and concentration factor were investigated, respectively. The reasonable operating parameters of membranes with different pore sizes were obtained, and the following conclusions were obtained.

In a certain range of operating conditions, with the increase of operating pressure, feed temperature and membrane surface flow rate, the membrane flux increases, whereas with the increase of oil content of feed liquid, the membrane flux decreases. According to the membrane flux, the concentration ratio, and the average particle size of oil droplets in the concentrate, the optimum conditions for single-stage concentration were determined as follows: the membrane pore size was 0.22 μm (0.4 MPa, 2% oil content, 200 mL/min); the membrane pore size was 0.45 μm (0.3 MPa, 3% oil content, 250 mL/min); the membrane pore size was 0.8 μm (0.2 MPa, 4% oil content, 300 mL/min); the operating temperature range was 25–30 °C.

In addition, the effects of several different membrane combinations on concentrating oily wastewater were investigated. After multi-stage membrane concentration, the average particle size and concentration multiple of oil droplets in the concentrated solution were greatly improved. After three-stage membrane combination concentration, the particle size of oil droplets in emulsified oily wastewater is greater than 20 μm, which exceeds the particle size range of emulsified oil. After a short time of static, a layer of floating oil can be seen on the surface of the concentrated liquid of emulsified oily wastewater. After a short time of static, obvious stratification occurs. It is easy to realize oil–water separation by using a liquid separation bottle (or skimmer).

This research was supported by the National Natural Science Foundation of China (21276195).

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

The authors declare there is no conflict.