Effects of Sulfates on Anaerobic Systems

In industrial production, sulfuric acid is often used. For example, fertilizer plants produce phosphate fertilizers and ammonium sulfate, daily chemical plants produce detergents, and food processing industries use it for soaking and extraction... These production processes will produce high-sulfate wastewater and flow into sewage. processing system. Experienced sewage treatment technicians all know that when high-sulfate wastewater enters an anaerobic system, it will cause toxicity to anaerobic bacteria. So what is the critical concentration of poisoning and what is the root cause of poisoning? Let’s talk about this today question.

01

reaction principle

In fact, sulfate itself does not have a serious inhibitory effect on methanogens among anaerobic bacteria. However, the anaerobic reaction process and the anaerobic products of sulfate can cause toxicity to methanogens.

First of all, when the sulfate concentration in wastewater is very high, even higher than the concentration of COD, then during the anaerobic reaction process, the reduction reaction led by sulfate-reducing bacteria will gradually take the dominant position, and the methanogenesis reaction of organic matter will gradually weakened; due to the emergence of sulfate-reducing bacteria The generation cycle is shorter than that of methanogens, and it has strong tolerance to the environment and inhibitory substances. If it is run for a long time, the sulfate-reducing bacteria in the anaerobic sludge will become the dominant species, and the methanogens will become the weak species, thus As a result, the COD degradation capacity of the anaerobic reactor decreases and then fails.

Secondly, in an anaerobic environment, sulfate-reducing bacteria will reduce sulfate to hydrogen sulfide, and free hydrogen sulfide will cause toxicity to methanogens in anaerobic bacteria. According to research, when the concentration of free hydrogen sulfide in wastewater reaches 250 mg/l, the activity of anaerobic granular sludge decreases by about 50%.

At the same time, because the free hydrogen sulfide contained in the water can also be oxidized by oxidants, it is characterized as COD; therefore, when testing data, it will show that the COD of the anaerobic effluent will increase and the removal efficiency will decrease.

Of course, the hydrogen sulfide produced in the anaerobic reaction will also cause some problems, such as odor in the anaerobic device area, serious corrosion at the air-water interface in the anaerobic system, and reduced biogas quality. We will explain these separately in other articles. .

02

Operation precautions

In the anaerobic treatment system, we should try to avoid the entry of sulfate. However, in actual production, we cannot avoid the sulfate entering the anaerobic system along with the production drainage water due to objective reasons. At this time, the following three points should be paid attention to during operation: :

(1) Under ideal conditions, the ratio of COD to sulfate should be maintained above 10:1, or at least 5:1, to ensure that the methane-producing reaction in the anaerobic reactor is dominant. If the ratio is out of balance, pretreatment or other wastewater with a lower sulfate concentration needs to be introduced for dilution.

(2) During normal operation, the free hydrogen sulfide concentration should account for less than 20% of the total hydrogen sulfide concentration. Therefore, when the anaerobic reactor is running, it is necessary to control the sulfate concentration in the anaerobic influent water below 1000 mg/l to ensure that the concentration of toxic free hydrogen sulfide in the reactor is well below 250 mg/l.

(3) For wastewater with relatively high sulfate concentration, the pH value of the incoming water can also be appropriately increased to keep the pH value in the anaerobic reactor neutral or weakly alkaline to reduce the concentration of free hydrogen sulfide.

Source: Environmental Protection Platform