Metallurgical Technology: A new process for removing sulfates using a two-phase anaerobic reactor and a microelectrolysis cell

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　　Abstract: This paper proposes a new process for treating sulfate wastewater, which mainly consists of a two-phase anaerobic reactor and a microelectrolysis cell. Sulfate-reducing bacteria (SRB) are used to reduce SO42- to sulfides, which then combine with Fe2+ in the microelectrolysis cell to form FeS precipitates, thereby removing most of the sulfates and preventing inhibition of the methane production process in the subsequent anaerobic reaction. 　　0. Introduction In recent years, the development of industries such as light industry and pharmaceuticals has generated large amounts of industrial wastewater containing high concentrations of sulfates, which requires urgent treatment, such as wastewater from sulfate-based papermaking processes and citric acid production. The main problems arising from the presence of sulfates in industrial organic wastewater include: high concentrations of sulfates exerting a strong inhibitory effect on methanogenic bacteria (MPB), thereby hindering the digestion process ； Secondly, large amounts of sulfate-containing wastewater are discharged into already heavily polluted water bodies, which not only generates hydrogen sulfide with a foul smell and corrosive properties, but also poses a direct threat to human health and disrupts the ecological balance. This paper proposes a new process for treating sulfate wastewater, which mainly consists of a two-phase anaerobic reactor and a microelectrolysis cell. Sulfate-reducing bacteria (SRB) are used to reduce SO42- to sulfides, which then combine with Fe2+ in the microelectrolysis cell to form FeS precipitates, thereby removing most of the sulfates and preventing inhibition of the methane production process in the subsequent anaerobic reaction. 　　1. Comparison and evaluation of processes The traditional treatment methods for wastewater containing sulfides and sulfates include: (1) Controlling pH value The pH value of the digestate affects the degree of dissociation of H2S. The sulfide that plays a inhibitory role in anaerobic digestion is mainly unionized H2S. As the pH value increases, the concentration of unionized H2S decreases, and accordingly its toxicity also decreases ； It is generally believed that a pH value in the range of 7.5 to 8.0 is appropriate. 　　(2) Two-stage anaerobic digestion process The two-stage anaerobic digestion process is employed: in the first stage, appropriate environmental conditions are maintained for acid-producing bacteria, with the resulting products being mainly short-chain fatty acids and H2S; the effluent is then treated using a H2S removal device to eliminate H2S. In the second stage, methane fermentation takes place, with methane as the main product. 　　(3) Addition of SRB inhibitors – mainly to inhibit the activity of SRB, thereby reducing the number of bacteria that are normally involved in the hydrogen and acetate production processes. 　　For the first method, it is difficult and cumbersome to control the pH value, as this requires constant monitoring and very precise control. This method is difficult to scale up, requires large amounts of chemicals, and incurs high operating costs. The second method aims to remove sulfates prior to the second stage of anaerobic treatment, and this depends on the reduction capacity of the previous anaerobic system as well as the operating conditions of that system. Due to the complexity of the H2S removal equipment, practical operation is difficult, and the treatment efficiency cannot be guaranteed. The third method, the addition of inhibitors, although it reduces the amount of H2S produced, also inhibits the activity of MPB, thereby decreasing methane production. 　　All of the above processes have their own drawbacks, such as practical implementation difficulties, and it is necessary to propose a more practical new process. This process combines a two-phase anaerobic reactor with microelectrolysis; it makes use of sulfate-reducing bacteria (SRB) to reduce sulfates to sulfides, which are then combined with Fe2+ in the microelectrolysis reactor to form FeS precipitates that remove most of the sulfates. This prevents any inhibition of the methane production process in the subsequent anaerobic reactor. Additionally, reflux facilities are employed to increase the conversion rate of sulfates. The process of the new technology is shown in Figure 1. 　　(1) Coarse and fine grids (2) Coagulation and sedimentation tank (3) First micro-electrolysis reactor (4) Sedimentation tank (5) First anaerobic reactor (6) Second micro-electrolysis reactor (7) Second anaerobic reactor 2. Characteristics and principles of the new process 2.1. Characteristics The purpose of the entire process is to carry out the anaerobic reaction in two stages, thereby effectively removing sulfates, improving biodegradability, and reducing COD and BOD levels. The first anaerobic reactor converts sulfates into sulfides, which are then removed in the second microelectrolytic cell. The removal of sulfides from the effluent eliminated the secondary inhibition on MPB, creating favorable conditions for the anaerobic digestion of organic matter in the second anaerobic reactor. In addition, a recirculation facility has been added to the process, mainly to reduce the sulfate concentration when the influent water contains high levels of sulfates, while also increasing the reduction rate of sulfates. 　　2.2. Principles 2.2.1. First microelectrolysis cell According to the theory of corrosion of metal materials in aqueous solutions, any form of corrosion occurs between electrodes, with an electric current flowing between them. Cast iron is an alloy of iron and carbon; therefore, when cast iron shavings are immersed in water, a complete circuit is formed, and an electric current flows across its surface. Current flows through thousands of tiny micro-batteries; pure iron acts as the anode and is corroded, while carbon serves as the cathode. Under acidic conditions, the main reactions are as follows: Anodic reaction: Fe – 2e⁻ → Fe²⁺; Cathodic reaction: 2H⁺ + 2e⁻ → H₂↑. In this process, aeration is applied to the first microelectrolysis cell in order to oxidize Fe²⁺ to Fe³⁺. Thus, oxidation-reduction reactions occur: 4Fe²⁺ + O₂ + 2H₂O → 4Fe³⁺ + 4OH⁻ (oxidation by aeration); Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ (neutralization and flocculation). The newly formed Fe³⁺, after being neutralized with lime, yields Fe(OH)₃, which acts as a colloidal coagulant. Its adsorption capacity is higher than that of Fe(OH)₃ obtained through conventional hydrolysis methods; as a result, the suspended particles present in the wastewater, as well as the insoluble substances generated by microelectrolysis and the organic compounds responsible for color, can be adsorbed and aggregated.