Polyvinylidene Fluoride Membrane Modified by PEG Additive for Tofu Industrial Wastewater Treatment

Abstract

This study investigates the enhancement of polyvinylidene fluoride (PVDF) membranes with polyethylene glycol (PEG) to improve their efficacy in treating tofu wastewater through the ultrafiltration (UF) process. PVDF membranes with varying PEG concentrations of 0, 0.5, 1, and 1.5% in the dope solution were produced, characterized via FTIR, mechanical strength, porosity, and contact angle measurements, and evaluated in wastewater treatment at varying pressures of 3, 4, and 5 bar in the UF process. The incorporation of PEG increased the membrane’s porosity from 28.2% for M-0 to 43.5% for M-1.5. The contact angle decreased from 65.3° for M-0 to 53.3° for M-1.5, indicating an increase in hydrophilicity. Elongation increased from 36.0% for M-0 to 113.5% for M-1.5; however, the tensile strength decreased from 11.8 MPa for M-0 to 5.4 MPa for M-1.5. Although PEG-modified membranes demonstrated enhanced flux, with values of 6.3 L∙m⁻²∙h⁻¹ for M-0 and 15.7 L∙m⁻²∙h⁻¹ for M-1.5 at a pressure of 5 bar, pure PVDF membranes (M-0) showed greater rejection rates for chemical oxygen demand (COD), total dissolve solid (TDS), total suspended solid (TSS), and turbidity at 3 bar, achieving values of 66.3%, 41.6%, 99.6%, and 99.1%, respectively. Following ultrafiltration, the pH and TDS levels conformed to Indonesian government guidelines; however, the COD levels were non-compliant, indicating the need for additional treatment. The findings suggest that PVDF/PEG ultrafiltration membranes are suitable for pre-treatment; however, nanofiltrationor reverse osmosis may be necessary to meet the stringent regulatory standards for tofu wastewater treatment. The modified M-1.5 membrane is recommended as the primary ultrafiltration membrane for tofu wastewater treatment due to its superior flux, prior to nanofiltration or reverse osmosis, to comply with the stringent regulatory standards established by the Government of the Republic of Indonesia.

1. Introduction

Indonesia faces the difficulty of regulating soybean consumption, particularly in the form of tofu, due to its substantial consumption rates. In 2022, Indonesia’s per capita weekly tofu consumption was 0.152 kg, contributing to a national soybean demand of 3 million tons, with 38% (1.14 million tons) designated for the tofu food sector. Industries that employ soybeans are often categorized as small and medium-scale enterprises, with over 85,000 small and medium industry (SMI) business units distributed across Indonesia [1]. This rise in tofu production has generated a substantial volume of liquid waste resulting from the manufacturing processes of washing, pressing, and molding, approximated at 20 million m³ annually. The tofu-making industry generates a substantial volume of liquid waste, at around 15–20 L per kg of soybean raw material [2]. Tofu manufacturing enterprises predominantly lack wastewater treatment facilities, resulting in small-scale tofu producers typically discharging their liquid waste into aquatic environments [3].

The wastewater generated from tofu production exhibits a substantial concentration of organic compounds, with biochemical oxygen demand (BOD) values between 6000 and 8000 mg∙L⁻¹, chemical oxygen demand (COD) values ranging from 7500 to 14,000 mg∙L⁻¹, and total suspended solids (TSS) measuring 2000 to 3000 mg∙L⁻¹ [4]. These numbers significantly surpass the environmental criteria established by the Ministry of Environment and Forestry, which mandates that soybean processing waste must maintain a BOD level of 150 mg∙L⁻¹, COD of 300 mg∙L⁻¹, TSS of 200 mg∙L⁻¹, and a pH range of 6–9 [5]. Nevertheless, a significant portion of wastewater from the tofu industries continues to be discharged directly into rivers, resulting in environmental contamination, offensive aromas, groundwater pollution, and health risks to nearby populations [6].

A variety of membrane materials have been used for wastewater treatment. Reverse osmosis (RO), ultrafiltration (UF), nanofiltration(NF), and microfiltration (MF) are among the numerous applications of polymer-based membranes [7]. Polysulfone (PSF), polypropylene (PP), polytetrafluoroethylene (PTFE), polyamide (PA), and polyethylene (PE) are the most frequently employed polymeric materials, with the PVDF membrane being the most prevalent [8,9]. This membrane is optimal for the production of UF and MF membranes due to its exceptional mechanical, physical, and chemical capabilities, as well as its high thermal stability [10,11]. Organic materials, including carbohydrates, proteins, and lipids, can be segregated by the PVDF membrane. Additionally, it exhibits exceptional mechanical strength, resistance to corrosion, and oxidant tolerance [12].

PVDF can be dissolved in a variety of organic solvents, such as N,N-dimethylacetamide (DMAc), which was employed in this investigation. The base material, solvent, and non-solvent are the essential components of the phase inversion process, which is frequently employed for membrane preparation. In recent decades, anti-fouling materials, including poly (ethylene glycol) (PEG), have been extensively investigated and are currently regarded as one of the most promising materials for the fabrication of fouling-resistant membranes. The porosity, hydrophilicity, tensile strength, and flux value of the membrane are all enhanced by an increase in the amount of PEG [13]. PEG is a non-ionic polymer that is chemically stable in both air and solution, soluble in water, and resistant to injury [14,15]. Additionally, it is unsuitable for microbial growth. It is generally recognized that PEG chains have the ability to bind water through hydrogen bonds, thereby establishing a “barrier” that impedes the adsorption of non-specific proteins [16]. This study aimed to enhance the hydrophilicity of PVDF membranes by incorporating PEG to treat tofu wastewater through the UF process; this is evaluated based on the rejection of COD, TDS, TSS, and turbidity.

PVDF ultrafiltration membranes are particularly effective for treating wastewater in the tofu industry, owing to their superior material characteristics, structural adaptability, and demonstrated ability in eliminating elevated levels of organic pollutants, suspended solids, and emulsified fats found in this effluent. The primary factors encompass PVDF’s exceptional mechanical strength, chemical resistance, and thermal stability, rendering it resilient in demanding operational environments and impervious to corrosion and oxidation from organic and inorganic substances present in tofu effluent. The inclusion of PEG can markedly boost its hydrophilic characteristics, hence improving water permeability. The membrane structure intended for this purpose is often a porous and asymmetric membrane, optimized for high flux and retention ability.

2. Materials and Methods

2.1. Materials

Solvay Chemical USA (Houston, Texas, TX, USA) provided polyvinylidene fluoride (PVDF) Solef 6012 with an average molecular weight of about 530,000 g/mole, and Merck Indonesia (Jakarta, Indonesia) offered analytical grade N,N-dimethylacetamide (DMAc) and NaOH, while Sigma-Aldrich Indonesia (Jakarta, Indonesia) supplied polyethylene glycol (PEG) with a molecular weight of 600. Ethanol with a purity of 96% and deionized water were procured from PT. Dwinika Intan Mandiri Indonesia (Depok, Indonesia).

2.2. Membrane Preparation

Synthesizing the membrane involved dissolving PVDF in DMAc and adding PEG. With PEG concentrations of 0, 0.5, 1.0, and 1.5, the formulation consists of 15% PVDF + PEG and 85% DMAc. Subsequently, PVDF and PEG were gradually added to an Erlenmeyer flask containing DMAc, with stirring maintained at 200 rpm and a temperature of 25 °C. By gradually increasing the temperature to 60 °C and controlling the stirring speed at 300 rpm during the dissolution phase, the polymer and additives were completely dissolved [17]. The solution was homogenized after about three hours of stirring at 500 rpm. Placing the homogenous PVDF/PEG solution, also known as the casting solution, at room temperature allowed the bubbles to escape and the foam to dissipate. After the glass film was coated with the casting solution using a casting-glass roller, it was submerged in deionized water for around 24 h. For characterization, the flat sheet membrane was first immersed in a solution of 96% ethanol for 30 min, then in a solution of 50% ethanol for 1 h; it was then dried in room-temperature ambient air.

Table 1. The composition of the dope solutions.

Membrane	PVDF (g)	PEG (g)	DMAc (mL)
M-0	15.0	0.0	85
M-0.5	14.5	0.5	85
M-1.0	14.0	1.0	85
M-1.5	13.5	1.5	85

shows the composition of the casting solution for the membrane preparation.

2.3. Membrane Characterization

Membrane characterization was performed using Fourier Transform Infrared Spectroscopy (FTIR) (Diamond Nicolet IS 5 (ThermoFisher Scientific, Waltham, MA, USA)) to verify the presence of PEG in the membranes. Water contact angle and membrane porosity measurements were also performed, in addition to mechanical strength tests using a Universal Testing Machine (10 kN) (Infinity Machine, Dongguan, China).

2.4. Coagulation–Flocculation Process

The tofu wastewater that has been characterized is subsequently treated prior to the UF process. In order to serve as the feed for the UF process, the pre-treatment process is implemented to reduce the parameter levels that are still exceedingly high. Fouling in the UF membrane is unavoidable at elevated parameter levels, such as TSS. In the UF process, using wastewater with extremely high impurity levels will result in persistent damage to the membrane and fouling, which will result in costs that are significantly higher than the pre-treatment costs [18].

The tofu solid suspension is reduced by filtering the initial tofu wastewater through a filter fabric. The initial liquid refuse is subsequently diluted twice. In order to obtain optimal conditions, the pH of the tofu wastewater is elevated to 7 by adding 5 M NaOH, which has an initial pH of 3.7. pH adjustment is implemented to satisfy the relevant quality standards. Additionally, in the coagulation–flocculation process, the wastewater is stirred at a high speed (120 rpm) for 2 min using a jar tester (Biobase, Qingdao, China) to ensure that the particles are suspended in the solution. The wastewater with a pH of 7 is then mixed with 300 ppm polyaluminum chloride (PAC). After rapid stirring, gradual stirring (40 rpm) is conducted for 10 min to facilitate the formation of large flocs by agglomerating unstable solids and coagulants. The solution is stirred and then allowed to settle for 30 min to allow the large flocs to settle to the bottom of the beaker. Subsequently, qualitative filter paper is employed to filter the solution.

2.5. Ultrafiltration Process

This work employed a high-pressure agitated filtration cell (HP4750, Sterlitech, Washington, WA, USA) with a dead-end configuration to determine the membrane flux. Measurements were conducted every 30 s for 90 min and repeated three times to obtain the experiment’s average. The membrane’s cross-sectional area was 0.00146 m², and the feed operating pressures employed for this measurement were 3, 4, and 5 bar. The membrane flux (J) and rejection (R) were calculated using the following:

$$J= {\displaystyle \frac{Q}{A_m}}$$

(1)

$$R\left(\%\right)=100\ast (1- {\displaystyle \frac{C_{out}}{C_{in}}})$$

(2)

where J is wastewater flux (L∙m⁻²∙h⁻¹), Q is wastewater flow rate penetrating the UF membrane (L∙h⁻¹), and Am is membrane area (m²). Meanwhile, R is the parameter rejection (%), $C_{in}$ and $C_{out}$ are the parameter concentration (mg∙L⁻¹) at the inlet and outlet UF membrane, respectively.

In the ultrafiltration (UF) process, Q represents the volume of water passing through the membrane over a specific period, while $A_m$ denotes the membrane area. Additionally, $C_{in}$ and $C_{out}$ refer to the concentrations of parameters at the inlet and outlet of the membrane, respectively.

3. Results and Discussion

3.1. Membrane Characterization

FTIR was employed to investigate the interaction between PVDF and PEG. The effectiveness of PVDF synthesis and modification can be assessed by assessing the suitability of the functional groups that are generated. The FTIR spectrum can be employed to determine the chemical composition of pure PVDF and PVDF/PEG membranes with varying quantities of PEG (0.5 to 1.5 g), as illustrated in Figure 1. PVDF is the main membrane-forming material, and as a result, the FTIR spectrum of M-0 (pure PVDF) to M-1.5 is typically identical. The wavenumbers 840 cm⁻¹ and 1200 cm⁻¹ are characteristic of vibrations, specifically for CF₂ bonds and CH₂ bonds that indicate the presence of PVDF. The PVDF composition in the four membrane compositions is indicated by the dotted line in Figure 1. The vibrational stretching of the functional groups in PEG is demonstrated by a new peak that appears following the addition of PEG to the PVDF membrane. The alkyl vibrational stretching (R-CH₂) was observed at approximately 2850–3000 cm⁻¹ [19]. The peak at 2877 cm⁻¹, as illustrated in Figure 1 is linked to the stretching vibration of the aliphatic functional group -CH₃, suggesting the presence of PEG in the PVDF mixture. The successful incorporation of PEG into the membrane is confirmed. Nevertheless, the FTIR results did not exhibit other characteristic absorption peaks of PEG, such as ether groups in the range of 1050 to 1150 cm⁻¹ and O-C=O groups at 1728 cm⁻¹ [20]. In addition, the membrane that was modified with PEG additives exhibited a broad peak at a wavelength of approximately 3406 to 3424 cm⁻¹, which suggests that the hydroxyl group (O-H) in PEG is in a stretching vibration. The O-H vibration peak also suggests that the PVDF/PEG membrane has the capacity to interact with water and function as a hydrophilic membrane [21].

The sessile drop method was employed to conduct the contact angle test on the four membrane types, with ten repetitions conducted at distinct locations on each membrane. Angle meter software (Angle Meter 360 v1.9.1) was implemented to quantify the angle magnitude.

Table 2. The average water contact angle of PVDF/PEG membranes.

Membrane	Contact Angle (°)
M-0	65.3 ± 2.1
M-0.5	62.3 ± 2.1
M-1.0	57.3 ± 2.5
M-1.5	53.3 ± 2.9

compares the water contact angle of different membranes. The hydrophobic properties of the membrane are chemically represented by the contact angle between the membrane surface and water. A contact angle greater than 90° for water indicates that the membrane is hydrophobic, while a contact angle less than 90° indicates that the membrane is hydrophilic. The outer surface layer of the membrane interacts with the water when it comes into contact with the membrane surface. A hydrophobic surface with a low free energy will generate a high contact angle with water. Conversely, a surface with a high free energy will permit water molecules to disperse, resulting in a reduced contact angle [22].

M-0, a pure PVDF membrane, exhibits the most significant contact angle, namely 65.3° ± 2.1°. PVDF is classified as a hydrophobic polymer due to its chemical composition, which comprises a substantial number of fluorine atoms. These fluorine atoms form hydrophobic solid interactions with oxygen atoms in water molecules due to their electronegativity, which is comparable to that of the F atom [7]. This prevents the penetration of PVDF by water and other polar solvents. The membrane’s performance can be enhanced by its inclination toward hydrophobicity or hydrophilicity, contingent upon its application or utilization. For instance, gas absorption membranes, such as those used for CO₂ removal, necessitate a high degree of hydrophobicity. This is because the hydrophobicity of the membrane is crucial in mitigating or eradicating the problem of wetting, which arises during the absorption of CO₂. Membrane hydrophilicity is essential for the treatment of effluent in the tofu industry, as it reduces the risk of fouling by preventing proteins from adhering to and accumulating in the membrane pores [23]. Furthermore, the membrane’s hydrophilic properties facilitate the passage of water molecules through its channels during the ultrafiltration process. The addition of PEG to the PVDF membrane results in an increase in the membrane porosity. Upon affecting the membrane surface, water droplets instantaneously fall into the micro pore holes, thereby decreasing the water contact angle on the membrane surface. Additionally, the modified membrane’s elevated polarity results in faster adhesion of water to its surface than on the pure membrane surface [24].

The gravimetric method was employed to quantify the membrane porosity. The results indicated that the porosity of the membrane increased as the PEG concentration in the membrane increased, as illustrated in Figure 2. The porosities of membranes with additives, such as M-0.5 at 30.5%, M-1 at 39.5%, and M-1.5 at 43.5%, are higher than those of M-0, a PVDF membrane without additives, which is 28.2%. The results of the membrane morphology, which show the distribution of pores on the membrane surface and their tendency to increase as the PEG concentration in the membrane increases, are corroborated by the fact that the porosity increases from M-0.5 to M-1.5. It can be attributed to the hydrophilic PEG segments in the dope solution, which facilitate the movement of solvent and non-solvent molecules inside the membrane structure, leading to increased porosity [25]. The membrane porosity significantly affects the membrane’s permeability. Water penetration is facilitated by the membrane’s ample space, which can be interpreted as high porosity.

The mechanical properties of membranes are described by two critical parameters: tensile strength and elongation at break. The tensile strength and elongation test results for the four membrane varieties are depicted in Figure 3. The membranes exhibited a comprehensive improvement in mechanical properties with the addition of PEG. Elongation increased as the amount of PEG in the membrane increased, as illustrated in Figure 3. Concurrently, the membrane’s tensile strength decreased as the amount of PEG increased, as presented in Figure 3.

The mechanical properties of membranes are significantly influenced by the intermolecular forces along the membrane polymer chain and the membrane microstructure [26]. Incorporating PEG as an additive in PVDF membranes increased the porosity and resulted in a pore structure with larger spaces. It is directly correlated with the weakening of the resulting tensile strength and the increase in elongation. A high density of pores, which results in their inability to withstand strain, characterizes pure PVDF membranes (M-0). Increasing the polyethylene glycol (PEG) content results in a decrease in membrane tensile strength, which, in industrial contexts, leads to a shorter membrane lifetime and reduced operational safety [27]. The membrane’s susceptibility to mechanical failure under pressure is increased by its lower tensile strength, which may result in costly shutdowns, equipment damage, and process breaches. It requires more stringent pressure limits, which can result in a higher likelihood of premature replacement or decreased efficiency, thereby increasing operating costs and maintenance expenses. However, this density also enhances the structural integrity of the membrane matrix. In addition, PVDF particles and PEG interact on a molecular level. PVDF and PEG are immiscible, which means that the polymer chain bonds are weakened when blended compared to the bond strength of pure PVDF polymer chains. Consequently, the membrane tensile strength decreases due to the development of weakly bonded structures [28].

3.2. Tofu Wastewater Treatment

Figure 4 illustrates the findings of the pure water flux using deionized water on the PVDF/PEG membrane, prepared as a function of operating pressure. The filtration procedure was conducted for around 90 min using a dead-end experiment, constrained by the permeation cell’s size of 15 cm². During this period, the flow findings remained relatively stable, with just modest fluctuations at the beginning of the experiment. By increasing the amount of PEG present in the PVDF/PEG membrane, its porosity increases, resulting in a higher water flux through it. For the same reason, elevating the operating pressure increases the driving force, which in turn leads to an increase in the water flux. The pure water flux on the PVDF/PEG membrane was shown to increase with increasing PEG composition, as also reported by Song et al. [29], who observed a similar pattern between the two variables.

The initial characteristics of the tofu effluent that were tested were COD, pH, TSS, TDS, and turbidity. The initial waste characterization revealed parameter values that were significantly higher than those observed in previous studies [3,6].

Table 3. The characteristics of the initial tofu wastewater, subsequent to the coagulation–flocculation process, and following the ultrafiltration (UF) process.

Parameters	Initial Tofu Wastewater	After Coagulation- Flocculation Process	After UF Process	Government Regulation [5]
pH	3.7 ± 0.2	6.9 ± 0.3	7.0–7.8	6–9
COD (mg∙L−1)	11,880 ± 255	4698 ± 199	1580–3800	150
TDS (mg∙L−1)	740 ± 14	1300 ± 32	759–1232	2000
TSS (mg∙L−1)	1640 ± 40	271 ± 18	1–81	50
Turbidity (FAU)	900 ± 54	346 ± 24	3–93	25

shows the parameter levels of the wastewater produced by the tofu industry after the coagulation–flocculation process. In the coagulation–flocculation process, the rejection values for COD, TSS, TDS, and turbidity are 11.1%, 25.8%, −97.0%, and 19.0%, respectively. Due to the addition of NaOH during the pH adjustment stage of waste, the TDS rejection value is negative, as the TDS level of waste increases following pre-treatment.

The permeate flux increases in conjunction with the mass of PEG incorporated into the PVDF membrane, as presented in Figure 5. The pore structure generated during membrane formation closely correlates with the flux. The membrane-containing PEG additives exhibited morphological results greater than those of the pure PVDF membrane, including distribution and pore size. Furthermore, an increase in membrane porosity was observed as the PEG content increased in the porosity test. This generally enhances the membrane properties or flux in fluids streaming through the membrane pores. The nature of PEG addition enlarges the total pore area in the membrane contact area with the waste stream, increasing flux, as presented in Figure 5. The increase in membrane hydrophilicity is another factor that supports the increase in permeate flux as the PEG mass increases. The contact angle test in the preceding section increased as the PEG content increased. Water is more easily transported through the membrane due to its hydrophilic properties, which bond to water. The membrane flux increased with higher PEG concentration, primarily due to the pore-forming impact of the PEG addition, which enhances membrane porosity [30].

The water flux from tofu wastewater in this investigation was lower than that reported in earlier studies utilizing polyvinylpyrrolidone (PVP) as an addition [6]. In comparison, at an operating pressure of 5 bar, the study utilizing PEG as an additive yielded a flux of roughly 15.7 L∙m⁻²∙h⁻¹. In contrast, the previous study employing PVP resulted in a water flux of approximately 40 L∙m⁻²∙h⁻¹. The membrane porosity resulting from the incorporation of PEG, roughly 43.5%, is significantly lower than that created by the inclusion of PVP, which is around 80%.

A higher operating pressure enhances the driving force that pushes particles through the membrane pores. However, this increase in pressure also significantly affects the separation performance of the feed particles. As the pressure compels particles to pass through the membrane more quickly, it can result in the unintended escape of particles that the membrane should retain. Consequently, the overall effectiveness of the ultrafiltration process may decline.

The COD rejection results for the four types of membranes at three different trans-membrane pressures are shown in Figure 6. The findings suggest that COD rejection decreases with an increase in PEG in the membrane mixture and the operating pressure. The micro-pore structure of the resulting membrane is closely linked to these characteristics of COD rejection [31]. As the PEG concentration increases, the membrane pores expand, allowing more particles and substances to pass through the permeate, raising the COD level. The pure PVDF membrane, labeled M-0, achieved the highest COD rejection; this was approximately 66.3% at 3 bar. As the pressure increased, the rate of change in the COD rejection performance among the four membranes decreased. However, there was no significant decrease in COD rejection with an increase in pressure. Increased pressure facilitates the transport of particles; nevertheless, this also includes larger particles with impurities. As a result, the influx of these larger particles can lead to higher COD in the permeate side. The membrane pores may mildly deform or constrict as a result of high pressure. Although UF membranes are relatively resilient, they are not entirely rigid. The membrane material is compelled to compress to a certain extent by the applied pressure.

Figure 7 shows that TDS rejection diminishes as the PEG concentration increases. The reduction in TDS rejection is conceptually analogous to a decrease in COD rejection; this is attributed to the formation of larger pores and the increased distribution of membrane pores that occurs with higher PEG concentrations in the membrane matrix [32]. The TDS rejection exhibited by membrane variations is notably low, particularly in M-0.5, M-1, and M-1.5, which incorporate PEG additives. Ultrafiltration membranes, specifically pure PVDF or PVDF/PEG, are generally considered ineffective for TDS removal. This is due to the relatively larger pore size of ultrafiltration membranes (0.02–0.05 µm) in comparison to the size of TDS, which is similar to that of ionic compounds (0.005–0.01 µm). Reverse osmosis is more effective in reducing TDS [33]. A comparable gradient is observed when TDS rejection is reduced from M-0 to M-1.5 at operating pressures of 3 and 4 bar. In addition, the TDS rejection at 5 bar operation decreases as the PEG concentration increases, with a slight gradient difference in the decrease from M-1 to M-1.5. M-1 demonstrates its capacity to degrade TDS by approximately 11.9% and 9.2% at 3 and 4 bar, respectively. Nevertheless, the TDS rejection of M-1 is significantly reduced to approximately 6.1% at a pressure of 5 bar. The elevated feed pressure can result in the deformation or widening of the M-1 pores. M-1.5 exhibits inadequate TDS rejection capabilities and is typically unresponsive to pressure fluctuations. This results in a stable TDS rejection performance within the 5.2–6.5% range. Figure 7 also illustrates the impact of pressure variations, as the rejection of TDS decreases as the operating pressure increases in all membrane compositions. A higher feed pressure enables particles with a size that is comparable to the membrane pores to pass through the membrane and penetrate the permeate due to the membrane material being compelled to compress to a certain extent by the applied pressure [34,35].

All membranes significantly improved the membrane’s performance by reducing TSS levels compared to TDS rejection, as illustrated in Figure 8. The membrane with a pure PVDF composition, M-0, exhibited the highest rejection rate, ranging from 98.2 to 99.6%. The effective removal of the pure PVDF membrane can be attributed to its pore size, which, according to the removal results, is relatively smaller than that of TSS particles. Furthermore, a significant factor is the increased hydrophobicity of M-0 relative to other membranes. The hydrophobicity of the membrane contact surface with wastewater can create a water-repellent layer, which prevents solids from traversing the pores, thereby enhancing TSS rejection in M-0. High hydrophobicity in the membrane, however, elevates the risk of fouling, potentially leading to a decrease in the durability of the membrane [36].

The rejection of TSS decreases as the composition of PEG increases, as the membrane material is compressed to a certain extent by the applied pressure [35]. The formation of larger pores in the membrane with PEG additives may increase the potential for more particles to escape. The reduction in TSS rejection across the four membranes at each pressure exhibits a consistent gradient. Furthermore, each membrane experienced a reduction in rejection rates as the pressure increased, suggesting that the total suspended solids in tofu wastewater were sufficiently large to impede passage through the membrane pores [37]. A more significant reduction in rejection is observed at M-1.5 when comparing pressures of 3 to 4 bar. The phenomenon observed in membrane micro pores occurs at 3 bar, which serves as a critical threshold. Beyond this point, increased pressure facilitates the passage of additional TSS through minor deformations in the membrane pore structure.

Figure 9 illustrates the results of the turbidity rejection test in relation to the PVDF/PEG mass ratio and operating pressure. Turbidity is linearly correlated with TSS. High TSS indicates the presence of suspended particles in a solution, which can lead to increased turbidity, especially if the particles are darker than the water [38]. The turbidity rejection rates are nearly identical to the TSS rejection rates for each membrane’s composition and comparable pressures, as shown in Figure 9. This indicates that membranes that effectively filter suspended particles are also effective at reducing turbidity. These two parameters are closely linked, as they both help characterize the clarity and quality of water produced through the ultrafiltration process [39].

The impact of PEG mass and pressure on pH is illustrated in Figure 10, which shows that an increase in PEG concentration in the dope solution results in a minor decrease in pH. Additionally, the pH levels rise as pressure decreases. The wastewater from the tofu industry is rich in organic materials, including proteins, carbohydrates, and lipids, which can result in a low pH [40]. During pre-treatment, the pH of the wastewater increased from 3.6 to 7.1 following the addition of NaOH. The introduction of NaOH also led to a rise in TDS, which was not entirely removed. Nevertheless, the removal of TSS efficiently eliminated organic substances, including suspended acids [41]. As a result, compounds that lower pH (acids) were removed during the ultrafiltration process, while those that raise pH (specifically NaOH) were not eliminated. This situation leads to an increase in the permeate pH when the PEG content in the membrane decreases.

The actual appearance of the initial tofu wastewater after pH adjustment, pretreatment, and the ultrafiltration process is presented in Figure 11, while a comparison of their characteristics is presented in Table 3. The image demonstrates variability in the clarity and color of the liquids. Tube (a) contains initial tofu wastewater, characterized by high turbidity and elevated TSS, COD, and TDS concentrations. Tube (b) demonstrates cloudiness after pH adjustment. The tube (c) indicates that the product of the coagulation-flocculation process yields a cloudy liquid, which implies a substantial concentration of suspended solids. The tube (d) containing the product of the UF process is a clear liquid, indicating a significant rejection of TSS and turbidity. Table 3 demonstrates that the pH and TDS levels of UF water meet the standards set by the Indonesian Government [5]. Only the M-0 and M-0.5 membranes comply with the standards for TSS and turbidity in UF water. Meanwhile, none of the UF water meets the standards set by the Indonesian government concerning COD; therefore, further treatment may be required to meet COD regulatory standards, in which case nano-filtration or reverse osmosis processes may be proposed.

4. Conclusions

This study examined the effectiveness of PEG-modified PVDF membranes in the treatment of tofu industrial effluent. Alterations in the PEG concentrations within the dope solution modified the membrane’s porosity and hydrophilicity, as evidenced by porosity and contact angle assessments. Subsequent ultrafiltration (UF) studies indicated a compromise: although water flux increased, enhanced PEG loading diminished the membranes’ capacity to reject TSS, turbidity, COD, and TDS under higher pressures. The addition of PEG elevated the membrane’s porosity from 28.2% for M-0 to 43.5% for M-1.5, while the contact angle diminished from 65.3° for M-0 to 53.3° for M-1.5, signifying an enhancement in hydrophilicity. Furthermore, it exhibited improved flux, measuring 6.3 L∙m⁻²∙h⁻¹ for M-0 and 15.7 L∙m⁻²∙h⁻¹ for M-1.5 at a pressure of five bar. Nonetheless, unmodified PVDF membranes (M-0) exhibited superior rejection rates for COD, TDS, TSS, and turbidity at 3 bar, attaining 66.3%, 41.6%, 99.6%, and 99.1%, respectively. These findings emphasize the importance of the M-1.5 membrane in the treatment of tofu wastewater, since it facilitates the achievement of a high flux. It is a crucial preliminary stage in the treatment process because of its remarkable flux capabilities. The M-1.5 membrane is optimal for the preliminary reduction in pollutants during the tofu wastewater-processing phase, serving as a precursor to nanofiltration or reverse osmosis, hence enhancing the effluent purification process. It provides a means for tofu industries to comply with environmental requirements by incorporating these membranes into a multi-stage treatment process. The broader impact is mitigating water pollution from tofu making, fostering sustainable practices within food businesses, and enhancing environmental health and regulatory adherence. For future studies, it is recommended to emphasize a comprehensive examination of the anti-fouling characteristics of PEG-modified PVDF membranes. It will involve performing tests mainly aimed at quantifying and characterizing membrane fouling, to verify the validity and viability of the membrane’s long-term performance under various testing conditions.