1. Introduction
Water scarcity is currently a serious problem in most parts of the world, owing to freshwater depletion, increasing infrastructure costs, global warming, population growth, and massive waste productions [
1]. Approximately 2 million tons of industrial and agricultural wastes are discharged every day into the water and up to 80% of industrial and municipal sewage are deployed into the environment without any treatment [
2,
3]. Over 2 billion people already live in areas subject to water stress. In total, 45% of the world’s population i.e., approximately 3.4 billion people, lack access to safely managed sanitation facilities. According to independent assessments, the world will face a global water deficit of 40% by 2030. This situation will be worsened by global challenges such as COVID-19 and climate change [
3].
Conventional wastewater treatment plants (WWTP) comprise mechanical-, biological-, and/or chemical-based purification processes. The conventional treatment of especially surface waters for drinking water purposes in drinking water treatment plants (DWTP) comprise mainly chemical-physical processes followed by disinfection prior to distribution. Both commonly use chemical coagulation (CC) in order to form larger aggregates being retained subsequently. Chemical coagulation typically involves the addition of coagulants such as iron or aluminum salts to the water in order to destabilize particles and colloids and facilitate their agglomeration to form larger flocs. Depending on process setup, organic matter can be co-precipitated and entrapped within the flocs. Therefore, coagulants and coagulant aids (e.g., acid or base for pH adjustment) are dosed and rapidly mixed in a coagulation tank, and afterwards, the flocculation aids are added and gently mixed in the flocculation tanks. The formed flocs can be effectively separated in the next process, such as sedimentation, flotation, or filtration. For instance, pressure-driven membrane filtration, such as microfiltration (MF) and ultrafiltration (UF), is state of the art technology for particle retention and widely used to treat fresh waters, waste water for recycling purposes, as a membrane bio reactor (MBR), and as a pretreatment in desalination [
4].
CC is a conventional method to reduce membrane fouling, ideally being achieved by a hybrid process, combining CC with membrane filtration. The integration of coagulation with membrane filtration has two main advantages: enhanced removal of NOM molecules and reduction of membrane fouling [
4,
5,
6,
7]. For instance, the hybrid process has become a common method to comply with the legal, chemical, and microbiological requirements for drinking water [
5] and in the treatment of wastewater and wastewater effluent [
8]. The most recent mode in this hybrid process is to add the coagulant and coagulant aids directly into the feed stream immediately prior to the membrane process or to submerge the membrane into the water to be treated. The advantages of this in-line coagulation or submerged reactor systems are the reduced footprint, reduced retention time, and lower coagulant dose, as settle able flocs are not needed [
5,
8,
9,
10]. Generally, fouling layers formed by flocs will affect layer density, porosity, and thickness and, therefore, resistance to flow, limit pore blockage, and increasing backwash efficiency. Coagulation followed by tangential UF should gather the beneficial effects of particle growth and cross-flow velocity [
6,
11,
12,
13].
Considering CC, the main disadvantages of this treatment method are sometimes longer retention times, especially in cases where in-line coagulation is not possible. Furthermore, larger quantities of sludge and the addition of chemicals for coagulation, flocculation, and pH adjustment are to be considered, which might make the process costly. A “chemical addition free” process alternative to chemical coagulation is an electrochemical technology known as electrocoagulation (EC) [
14]. EC is commonly applied in the treatment of water and industrial effluents because of its capability to remove a very wide type of pollutants, e.g., colloidal and suspended particles, heavy metal ions, and toxic organics. Moreover, EC has gained a popular attention being applicable in different water sources, not only in drinking water treatment, for instance, in produced water treatment [
15], ground water [
16], wastewater treatment [
17], and industrial waters [
18]. One of the advantages of the EC method is that coagulant chemicals are supplied electrochemically in situ and on-demand instead of a direct chemical supply [
19]. One disadvantage when combined with membranes might be insufficient degasification leading to gas holdups within the membrane system. This needs to be avoided in order to maintain a stable filtration process and leads to the work presented here.
Generally, the EC process includes four steps: anode dissolution, formation of OH
– ions and H
2 at the cathode, adsorption/absorption of colloidal pollutants on coagulants, and floc removal by sedimentation or flotation [
20]. The metal ions produced by electrochemical dissolution of a consumable anode (Fe and/or Al anodes) directly undergo hydrolysis in water, simultaneous cathodic reaction yields for pollutant removal either by deposition on cathode electrode or by flotation (evolution of hydrogen at the cathode) [
21]. By imposing electric current between the cathode and anode, made of metals such as iron (Fe) or aluminum (Al), tiny contaminants are removed (even radioactive components) in the wastewater. The anode, e.g., iron, oxidizes and generates polyvalent metal cations directly into the water [
21]:
On the cathode, the water molecules are reduced into hydroxyl anions and hydrogen gas [
21]:
Metal hydroxides are formed with poor solubility and readily precipitated in order to remove pollutant as a ligand (
L) by surface complexation [
21]:
Most of the important parameters that affect the EC efficiency are electric current, pH, electrode material, conductivity, treatment time, and hydrodynamics of the flow. Although some experimental works have been performed in the literature to investigate these parameters, recently, CFD has become an important tool to study fluid flow and current density inside EC reactors and to estimate complicated inherent phenomena, particularly in the case where an experimental method is restricted by technical limitations [
21]. Safonyk et al. [
22] numerically showed the impact of the rate of heat formation from the electrodes on the efficiency of the formation of coagulant. Song et al. [
23] numerically investigated the effect of flow rate on the removal efficiency of As and Sb in an EC reactor. They found that although the increase of flow rate is advantageous to the mass transfer, it does not promote the formation of flocs and the effective combination between flocs and pollutants, leading to the decrease of the removal efficiency of As and Sb. Lu et al. [
24] simulated the generation and mass transfer of soluble and insoluble hydroxides in an EC channel. They found that though (Al
3+), H
+, and OH
− are generated and accumulated at the direction of streamline, the concentration of these species increases and reaches its maximum around the inlet area, and after that, they decrease gradually to a much lower value. Al-Barakat et al. [
25] numerically investigated the impacts of rotating speed of an electrode on an EC unit. They found that the increase in the rotation of an electrode enhances the removal efficiency, which reaches the maximum at 100 rpm, while at higher rotation, i.e., 150 rpm, the removal efficiency decreases gradually due to the destabilization of flocs that are formed. Vázquez et al. [
26] studied the current distribution and cell hydrodynamics for the design of electrocoagulation reactors. They found that the cell geometry arrangement generates low velocity profiles between the electrodes. Delgadillo et al. [
27] studied the performance evaluation of an electrochemical reactor used to reduce hexavalent chromium Cr(VI) from industrial wastewaters. The outcome of the work was that by increasing the angular velocity of the rotating ring electrodes, the Cr(VI) removal rate increases and also improves the mass transfer rate between the electrodes and the liquid.
Multiple hydrodynamic phenomena, such as channeling, internal recirculation, and dead zones, which may influence the formation of coagulations, are required to be characterized in EC reactors. Vázquez et al. [
28] performed simulations to hydrodynamically characterize the EC cell performance. They showed that a higher velocity may support the removal of mud. Vázquez et al. [
26] reported that when the volumetric flow increases, the area corresponding to stagnant zones decreases since the fluid starts to circulate preferentially along the cell center, and new channeling zones and recirculation regions begin to form.
Although the EC technique has many advantages compared to CC, such as less sludge generation, fewer disinfection by-products (DBP) formation, providing more efficiency in color and turbidity removal, and no chemical addition, the technique also has disadvantages, such as higher maintenance requirement for fouling control and electrode replacement. In addition, it is hard to apply this technique for low-conductivity wastewaters and complex two-phase flow hydrodynamics [
21,
26,
27,
29,
30]. Considering the reactions inside the cell, two-phase flow occurs due to the hydrogen gas bubbles that are produced along the cathode and accumulate inside the electrolytic cell, which increases electrical resistance and reduces the efficiency of the process. The bubble density influences the flow hydrodynamics, which in turn affects mass transfer between pollutants, coagulant, and gas micro-bubbles, and finally directs the collision rate of coagulated particles, which results in floc formation [
21]. In this context, degasification is required to get rid of gas collected inside the reactor.
The two-phase flow containing water liquid and hydrogen gas can be modeled to determine the flow characteristics. However, this phenomenon has been less investigated in ECs via CFD due to the complexity of two-phase modeling. One of the very few studies was performed by Villalobos-Lara et al. [
20]. They simulated two-phase flow in a continuous rotating cylinder electrode reactor. They found that depending on the electrochemical kinetics, the conductivity of the electrolyte varies due to the formation of bubbles.
In the literature, few papers use CFD simulations of EC process and even fewer deal with its application in degasification. Therefore, this work aims to model the two-phase flow of the water and gas fluid in EC-reactor by means of CFD-tools with the Euler-Euler approach in order to describe the degassing effects at various water throughputs. Different flow rates of water and gas were investigated in two operational modes. Subsequently, the stored bubbles are modeled during watering and degassing for each case. Based on the results, the degassing period can be optimized for more effective EC operation. This is very important because it will further influence the formation and transport behavior of flocs formed in the real process. This third phase is not considered in the model as a third (solid) phase yet. This was done on purpose to focus on the area of interest. Embedding the third phase as solid particles in the model will be done in future research as well as determining the resulting fouling of the membranes.