The increasing industrialization and urbanization have led to a significant surge in industrial wastewater production, resulting in severe environmental pollution. Shockingly, approximately 80% of the wastewater generated by communities are released into the environment without any treatment or reuse. Industrial wastewater accounts for about 28% of this untreated wastewater and contains hazardous components such as toxic substances, persistent organic pollutants, volatile organic compounds, and hazardous organic compounds (Mao et al. 2022). Vegetable oil refinery wastewater (VORW), especially sunflower oil refinery wastewater (SORW), are classified as heavily polluted industrial wastewaters, containing a diverse range of contaminants, including free fatty acids, total suspended solids (TSS), chemical oxygen demand (COD), oil and grease (O&G), and other substances (Yu et al. 2018). However, traditional methods for treating these complex wastewaters have proven inadequate in effectively removing organic matter and other pollutants. As a result, there is a need to develop a more cost-effective and technically feasible intensive process for the removal of organic matter (Suganthi et al. 2013).
Membrane bioreactors (MBRs) have become increasingly popular for wastewater treatment in both municipal and industrial wastewater due to their numerous advantages, including a compact design, high biomass concentration without sludge settling issues, extended sludge retention time (SRT), and production of high-quality effluent (Abdollahzadeh Sharghi et al. 2014). To the best of the authors' knowledge, there are a few studies on the application of MBR technology for the treatment of VORW which is presented in Table 1. However, despite these benefits, membrane fouling remains a major challenge in the widespread implementation of MBR technology.
To address this issue, electrocoagulation (EC) has emerged as a promising technique to reduce membrane fouling, which involves the use of an EC system to form larger particles through aggregation and/or adsorption effects (Liu et al. 2012). In recent years, the EC process combined with MBRs (EC-MBR) has attracted a lot of attention due to its environmental compatibility, high removal rate, zero added chemicals, no production of secondary pollutants, excellent effluent quality, and better dewatering ability (Hasan et al. 2014; Liu et al. 2019). This system has been applied to treat various types of wastewater, including municipal wastewater (Hasan et al. 2014; Liu et al. 2019), gray water (Bani-Melhem &Smith 2012a, b), produced water (Al-Malack &Al-Nowaiser 2018), tannery wastewater (Keerthi et al. 2013; Suganthi et al. 2013), and hospital wastewater (Djajasasmita et al. 2022).
The EC process is a method that involves destabilizing suspended, colloidal, and dissolved particles in an aqueous medium through the application of electric current (Derakhshesh et al. 2022). This process reduces the surface charge of the particles, allowing them to overcome the van der Waals force and coagulate into larger particles (Othmani et al. 2022). In the EC process, the following two reactions are carried out simultaneously (Eqs. 1 and 2).
$$\text{M} \to {\text{M}}^{+}+ {\text{n}\text{e}}^{-1} \left(1\right)$$
$$2{\text{H}}_{2}\text{O}\left(\text{l}\right) \to {\text{O}\text{H}}^{-} +{ \text{H}}_{2}\left(\text{g}\right) \left(2\right)$$
According to Eq. 1, the current passes through a metal electrode and oxidizes the metal (M) to its cation (M+). Simultaneously, according to Eq. 2, water is reduced to hydrogen gas and hydroxyl ion (OH−). In more detail, during the EC process, metal electrodes, typically made of aluminum or iron, are used as anode and cathode. When an electric current is applied, metal cations, such as Al3+ or Fe2+, are released from the anode into the wastewater. These metal cations neutralize the negative charges on the particles, reducing the electrostatic repulsion between them. As a result, the particles start to come closer together and form aggregates. Additionally, at the cathode, water molecules are reduced to form OH− and H2. The hydroxyl ions contribute to the coagulation process by reacting with the metal cations, producing metal hydroxide precipitates. These precipitates further enhance the aggregation of particles, forming larger and heavier flocs that can be easily separated from the water (Kabdaşlı et al. 2012).
The treatment of VORW has been studied in a few studies and the results are given in Table 1.
Table 1
Previous studies on the treatment of VORW in MBR and EC systems.
Type of system | Type of wastewater | Operational conditions | Finding | Reference |
| Unspecified type of VORW | HRT = 16–23 h OLR = 1.22–2.03 kg COD m− 3 d− 1 Infinite SRT | The COD and O&G removal efficiency were 86.1% and 94.8%, respectively. Predominant mechanism of membrane fouling was the formation of a gel layer on the membrane surface caused by the accumulation of EPS and the presence of inorganic and oily components. | Ma et al. 2015 |
| SORW | HRT = 48 h OLR = 0.35 kg COD m− 3 d− 1 SRT = 20 days | The COD and O&G removal efficiency were approximately 85% and 83%, respectively. | Abdollahzadeh Sharghi et al. 2016 |
MBR | High oleic acid SORW | HRT = 18 h OLR = 1.26 kg COD m− 3 d− 1 SRT = 10 days | The COD and O&G removal efficiency were over 70%. Membrane fouling was negligible due to the use of a short SRT and a small increase in MLSS concentration during 52 days of MBR operation. | Ghasemian et al. 2017 |
| Mixture of SORW and corn oil wastewater | HRT = 12–48 h OLR = 0.2–3.79 kg COD m− 3 d− 1 Infinite SRT | The COD and O&G removal efficiency were 90.6% and 83.3%, respectively. Dominant membrane fouling mechanisms was gel layer formation and membrane pore blocking at the highest OLR value. | Abdollahzadeh Sharghi et al. 2020 |
| High oleic acid SORW | HRT = 18 h OLR = 1.26 kg COD m− 3 d− 1 SRTs = 10 and 40 days, and infinite | At 40-days SRT, the sludge flocs compressibility and bioflocculation improved and COD and O&G removal efficiency were 79.2% and 86.4 respectively. The result presented that under the industrial conditions of MBR operation in treating SORW with high oleic acid optimal operating conditions in terms of system performance and membrane fouling are predicted to be at 40-days SRT. | Abdollahzadeh Sharghi et al. 2023 |
| OMW | Electrodes = aluminum current density = 15–120 mA cm− 2 Electrolysis time = 10–60 min | A current density of 75 mA cm− 2 was selected as an optimum that allows fast and low-cost treatment. The COD, polyphenols and dark color removal efficiency were 76%, 91% and 95%, respectively. | Adhoum 2004 |
| olive oil mill wastewater (OOMW) | Electrodes = Aluminum and iron Current density = 10–40 A m− 2 Electrolysis time = 2–30 min | In the electrolysis time of 30 min, COD removal efficiency was 52% by aluminum anode and 42% by iron anode. In addition, in the electrolysis time of 10 min, in the current density of 10–40 A m− 2, the color removal efficiency was 90 to 97%. | Inan et al. 2004 |
| Olive mill wastewater (OMW) | Electrode = iron or aluminum Current density = 20–75 mA cm− 2 Electrolysis time = 0.5–3 h | The COD removal efficiency was in the range of 62–86% whereas O&G and turbidity removal was 100% at the current density range of 20–75 mA cm− 2. | Ün et al. 2006 |
EC | VORW | Electrodes = aluminum Current density = 25, 30, 35 A m− 2 Electrolysis time = 15–90 min | At current density of 35 mA cm− 2 and electrolysis time of 90 min, the COD removal efficiency was 98.9% at, which was completely in accordance with the direct discharge standards. | Un et al. 2009 |
| Palm oil mill wastewater | Electrodes = aluminum Current density = 10–80 A m− 2 Electrolysis time = 2–15 min | At current density of 20 A m− 2 for 5 min, the O&G, COD and suspended solids removal efficiency were 72%, 64%, and 53% respectively. | Phalakornkule et al. 2010 |
| OMW | Electrodes = Aluminum Current density = 10–40 A m− 2 Electrolysis time = 2–30 min | In the electrolysis time of 15 min and current density of 250 A m− 2, discoloration of the OMW and reduction of the COD and polyphenols exceeded 70%. | Hanafi et al. 2010 |
| OMW | Electrodes = aluminum, stainless steel, and RuO2/Ti Current density = 5–40 mA cm− 2 Electrolysis time = 5–30 min | The COD, polyphenols, color, turbidity, suspended solids, and O&G removal efficiency were 96%, 94.4%, 91.4%, 88.7%, 97%, and 97.1% respectively. At this study current density of 40 mA/m2 and 30 min reaction time were optimal conditions. | Esfandyari et al. 2015 |
| Olive debittering wastewater | Electrodes = aluminum, stainless steel, cuprum and iron Current density = 3–15 A m− 2 Electrolysis time = 15–120 min | The lowest operating cost and the highest pollutant removal efficiency were obtained using two aluminum electrodes at a distance of 1 cm from each other. Also, in the electrolysis time of 60 min and the current density of 15 mA cm− 2, the COD and turbidity removal efficiency were 90.44% and 97.92%, respectively. | Niazmand et al. 2019 |
| Olive oil factory wastewater | Electrodes = aluminum Current density = 2.5–17.5 A m− 2 Electrolysis time = 15–67 min | In the electrolysis time of 60 min and the current density of 12.5 mA cm− 2, the COD removal efficiency was 99% and the removal efficiency increases with time. | Ghahrchi et al. 2021 |
The results obtained from the aforementioned review of studies on the application of MBR and EC systems for VORW treatment purposes confirmed the good potential of these systems for VORW treatment. However, there is a lack of sufficient information on the use of hybrid systems in treating VORW. Furthermore, the implementation of hybrid systems combining two of the most promising treatment technologies, namely EC and MBR, for VORW treatment is completely absent in the published literature. Therefore, there are several gaps in this field, including the lack of information on the physicochemical and morphological characteristics of the mixed liquor, system performance, the relative biodegradability of VORW components, and membrane fouling control in the treatment of VORW using a hybrid system.
The present study is the first investigation whose main focus was to study and compare the system performance and membrane fouling in two MBR and EC-MBR systems for SORW treatment at a sludge retention time (SRT) of 25 days and a hydraulic retention time (HRT) of 8 h. The study includes comparison of the two systems in terms of treatment performance such as chemical oxygen demand (COD), oil and grease (O&G) and turbidity removal efficiency as well as sludge biological properties such as mixed liquor suspended solid (MLSS), mixed liquor volatile suspended solids (MLVSS), specific oxygen uptake rate (SOUR), sludge volume index (SVI), supernatant turbidity (ST), particle size distribution (PSD), soluble microbial products (SMP), extracellular polymeric substances (EPS), scanning electron microscope (SEM) micrographs and X-ray analyzer (EDX) spectroscopy. The study also examines the behavior of membrane fouling in both the mixed liquor and cake layer, considering relevant parameters identified in previous studies as potential influences on membrane fouling in MBRs. Additionally, the energy consumption of the EC-MBR system was measured.