Anaerobic Technology in Sulfate Wastewater Treatment

Sources of Sulfates in Industrial Wastewater
1、 From mining wastewater
In China's mine resources, coal mine, pyrite and polymetallic sulfide ore are the main forms of existence. During the mining process, sulfur and sulfides in the ore form sulfates through oxidation. The concentration of sulfate in this type of mining wastewater is usually greater than 1000mg/L, but due to the low organic content in the wastewater, biochemical treatment may not be suitable.
2、 From industrial production wastewater
Some fermentation, pharmaceutical, and light industry industries usually add auxiliary raw materials such as sulfuric acid, sulfite, and their salts in the production process. This results in production wastewater often containing a large amount of sulfate ions. These wastewater not only contain high concentrations of sulfate ions, but also typically contain high levels of organic matter. This type of wastewater usually requires biochemical treatment, and anaerobic biochemical treatment processes are often used.

3、 Control techniques for sulfate and H2S in anaerobic treatment
1. Physical and chemical methods
*Diluting sulfate ions in wastewater: By increasing the volume of wastewater, the concentration of sulfate ions is reduced.
*Increase pH value: The ionization constant of H2S is approximately 6.8-7.0, which is similar to the operating pH value of anaerobic reactors. Increasing the pH value will significantly alter the ionization of H2S to HS -. For every 0.3 pH unit increase, the ratio of HS - to H2S doubles, thereby reducing the concentration of undissociated H2S in gases and liquids, ultimately reducing inhibitory effects.
*Gas stripping method: Utilizing the property that most sulfides will exist in the form of H2S when the pH value is low. Researchers have found that installing a circulating gas stripping device in a single anaerobic treatment system can remove sulfides and reduce the inhibitory effect on methane production. There are two main blow off processes:
-Internal blow off method: The biogas produced in the anaerobic reactor is purified by removing sulfides through gas extraction. Its biggest drawback is that the amount of gas blown off is difficult to control, making it difficult to maintain normal blowing off.
-External blow off method: Only blow off the effluent of the reactor, remove H2S, and reflux some of the treated water, which can dilute the influent. The effluent is more effectively circulated through an external blow off column, and the addition of iron salts is very effective in removing sulfides from the solution. From an economic perspective, trivalent iron salts should be added to remove an additional 50% of sulfides. However, it should be noted that after the addition of iron salts, sulfur is converted into FeS precipitation, which can cause inorganic accumulation in anaerobic filters, UASB, anaerobic contact and other processes. However, using the addition of iron salt and precipitation in the external blowing tray can alleviate this problem by circulating the effluent.
*Oxygen introduction: Reports have shown that introducing oxygen (equivalent to 10% of biogas production) into anaerobic effluent can effectively remove 90% of H2S from biogas at a low cost. However, this method requires high requirements for the design of equipment and air ducts.
*The anaerobic desulfurization effluent gas separation process is greatly affected by the pH of the solution: when the pH condition of the wastewater is controlled below 6.6, the sulfide separation effect in the wastewater can reach over 84%; When the pH of the solution is maintained between 7.0-7.5, the gas extraction effect is still less than 65%. Due to the neutral nature of anaerobic effluent, it is not practical to adjust the pH value by adding acid. Biogas rich in CO2 after purification and desulfurization treatment can be used as the stripping gas source, and CO2 can be used to form a buffer system to maintain the pH of the system in a relatively ideal environment. Under experimental conditions, the removal efficiency of sulfide in wastewater by gas extraction can reach over 80%.
2. Biological treatment method - two-phase anaerobic process
Anaerobic reactions can be divided into two processes: hydrolysis acidification and methane production. Based on the differences in microbial populations between the two reactions, two independent reactors are established. By controlling the operating conditions, the optimal growth conditions of the two groups of bacteria in their respective reactors can be ensured, resulting in higher processing capacity and operational stability. On this basis, some researchers have proposed to control the sulfate reduction process in the acid production stage and complete it simultaneously with the ordinary acid production process, and then remove all sulfides in the effluent and enter the methane production reactor for methane production reaction. This idea has been proven feasible by the experimental results of multiple researchers. Under acidic conditions, both acid production and sulfate reduction can occur simultaneously; Controlling sulfate reduction during the acid production stage has the following advantages:
*Fermented bacteria can tolerate higher sulfide concentrations than methanogenic bacteria (MPB), so acid production can occur simultaneously with sulfate reduction without affecting the acid production process.
*Sulfate reduction and methane production are carried out separately in two reactors to avoid matrix competition between SRB and MPB. The final product of sulfate reduction, sulfides, can be removed between the two phases without direct contact with MPB and will not be toxic to MPB. Moreover, most of the sulfides have been removed in the acid producing phase, while there are sufficient methane precursors to produce methane, ensuring a high methane production rate, The formed biogas has a low H2S content and is easy to recycle and reuse.