The effect of sulfate on anaerobic systems!
In industrial production, sulfuric acid is often used. For example, chemical fertilizer plants are used to produce phosphate fertilizers and ammonium sulfate, daily chemical plants are used to produce detergents, and food processing industries are used for soaking and extraction... These production processes will produce high-sulfate wastewater, which will eventually enter Sewage treatment system. Experienced sewage treatment technicians all know that high-sulfate wastewater entering the anaerobic system will cause toxicity to anaerobic bacteria. So what is the critical concentration of poisoning and what is the root cause of the poisoning? Let’s talk about it today. problem.
1. Reaction principle
In fact, sulfate itself does not have a serious inhibitory effect on methanogens in anaerobic bacteria, but the process of anaerobic reaction and the anaerobic products of sulfate can cause toxicity to methanogens.
First, when the sulfate concentration in the wastewater is very high, even higher than the COD concentration, then in the anaerobic reaction process, the reduction reaction led by sulfate reducing bacteria will gradually take the lead, and the methane production reaction of organic matter will gradually Weakening; because the generation cycle of sulfate-reducing bacteria is shorter than that of methanogens, and they are more resistant to the environment and inhibitory substances. If they are operated for a long time, sulfate-reducing bacteria in anaerobic sludge will become the dominant species. Methanogens become a weak strain, which leads to a decrease in the COD degradation capacity of the anaerobic reactor and ultimately fails.
Secondly, in an anaerobic environment, sulfate-reducing bacteria will reduce sulfate to hydrogen sulfide, and free hydrogen sulfide can cause toxicity to methanogens among anaerobic bacteria. According to research, when the free hydrogen sulfide concentration in wastewater reaches 250mg/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 the oxidant, it is characterized as COD; therefore, in the laboratory data, it will be manifested as an increase in the COD of the anaerobic effluent and a decrease in the removal efficiency.
Of course, the hydrogen sulfide produced in the anaerobic reaction will also bring some problems, such as odor in the area of the anaerobic device, serious corrosion of the gas-water interface in the anaerobic system and degradation of the quality of biogas. We will explain these separately in a later article. .
2. Operation precautions
In the anaerobic treatment system, the entry of sulfate should be avoided as much as possible, but in actual production, it may be due to objective reasons that we cannot prevent the sulfate from entering the anaerobic system with the production drainage. At this time, the operation and operation should pay attention to the following three points :
1. In an ideal state, the ratio of COD to sulfate should be maintained above 10:1, and should be controlled at least above 5:1 to ensure that the methane production reaction in the anaerobic reactor is in a dominant position. If the ratio is out of balance, pretreatment or other wastewater with a lower sulfate concentration is needed 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, the sulfate concentration in the anaerobic influent must be controlled below 1000 mg/l to ensure that the toxic free hydrogen sulfide concentration in the reactor is much lower than 250 mg/l.
3. For wastewater with relatively high sulfate concentration, the pH value of the influent 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.