Generally speaking, for the anaerobic process with the main purpose of methanogenesis, the pH value is required to be between 6.5 and 8.0. When the wastewater alkalinity is low or the operation load is too high, it will cause the accumulation of volatile acid in the reactor, resulting in the loss of methanogenic bacteria activity and the proliferation of acidogenic bacteria. If it continues for too long, it will cause the loss of methanogenic bacteria activity and the proliferation of acetic acid producing bacteria, causing the degradation of the reactor system“ Acidification ". After serious acidification, it is difficult for the reactor to return to its original state.
"Acidification": ① decrease of biogas production; ② decrease of methane content in biogas; ③ increase of VFA in digested liquid; ④ decrease of organic matter removal rate; ⑤ decrease of pH value of digested liquid; ⑥ obvious increase of difference between carbonate alkalinity and total alkalinity; ⑦ lightening and tarnishing of washed granular sludge; Ⅷ obvious peculiar smell in reactor effluent; Ⅸ increase of ORP value, etc.
Causes of acidification in anaerobic reactor
1. Overload operation of anaerobic reactor
As we all know, sludge load is a very important control parameter in various process control conditions of anaerobic reactor. Sludge load refers to the amount of organic matter applied to anaerobic sludge per unit mass in unit time, expressed in kgscod / kgvs. D. For a certain wastewater, anaerobic sludge has a limit value. When the operating load exceeds the limit value, it means overload operation.
Although the limit value from the concept of sludge load is for the whole anaerobic sludge, in fact, the real object is for methanogens in anaerobic sludge. In fact, overload operation means that the load exceeds the methanogenic capacity of methanogens in anaerobic sludge, while the load does not exceed the hydrolytic acidification capacity of anaerobic sludge. So the VFA of the reactor began to accumulate, the concentration increased, the pH value of the effluent decreased, and the removal efficiency decreased.
Therefore, understanding the total amount of sludge in anaerobic reactor and maintaining reasonable operation load is one of the important means to prevent acidification in anaerobic reactor.
2. Serious deviation occurred in pH value, temperature and other operation control conditions
Because the requirements of anaerobic sludge methanogens on their living conditions are much more stringent than those of hydrolytic acidification bacteria, when the pH value or temperature control range of the reactor deviates greatly, the methanogenic ability of methanogens will be seriously affected, while the influence of hydrolytic acidification bacteria is far less than that of methanogens, which will also lead to the acidification of anaerobic reactor Elephant.
3. Inflow of toxic substances
Compared with aerobic activated sludge, anaerobic sludge is more easily inhibited by toxic substances. In fact, methanogens in anaerobic sludge are more susceptible to toxic substances than hydrolytic Acidifiers. When the wastewater contains some or more toxic substances, the concentration of which is not enough to seriously inhibit the hydrolysis acidification bacteria in anaerobic sludge, the methanogens have been inhibited, and the sludge acidification phenomenon will occur.
Therefore, it is an effective measure to investigate the toxic inhibitors that may exist in the pollution source, establish the accident discharge notification mechanism between the pollutant discharge source and the sewage station, and establish the daily monitoring mechanism of potential toxic substances.
4. There is a serious shortage of nutrients
It is necessary to add enough nutrients to some waste water which lacks n, P or other trace elements. Both methanogens and hydrolytic Acidifiers need these elements to metabolize and synthesize cell substances in anaerobic sludge.
The activity of methanogens will be seriously affected when one or more nutrients are deficient in wastewater. This is because for anaerobic sludge, especially for anaerobic granular sludge, methanogens are located in the center of granular sludge, while hydrolytic acidification bacteria are wrapped around methanogens. Hydrolytic acidification bacteria are more likely to obtain these elements for metabolism than methanogens. In addition, the reproductive rate of hydrolytic acidification bacteria is much higher than methanogens, which makes the waste water insufficient The nutrient elements are used up by hydrolytic acidification bacteria, but the activity of methanogens will be greatly inhibited if they can not get these essential elements for life activities. As a result, acidification of the reactor is inevitable.
Recovery measures of acidification
1. Reduce the load
The main reason of "acidification" is the inhibition of methanogens. The volume load of anaerobic reactor is determined by the sludge load. The decrease of methanogens activity directly reflects the decrease of sludge load. Therefore, the influent should be controlled in time when acidification occurs, and the influent should be stopped completely when the situation is serious.
2. Adding alkalinity
When the anaerobic reactor is acidified, VFA with high alkalinity neutralization can be added to the reactor to maintain the stability of pH value, ensure the living environment of methanogens and prevent serious acidification. NaHCO3, Na2CO3, NaOH, Ca (OH) 2 are commonly used to adjust the alkalinity of the chemical agents. Although the addition of NaOH or Ca (OH) 2 and other strong alkaline substances can quickly improve the pH value in the reactor, but the hydroxide will consume the CO2 required in the process of methanogenesis, destroy the process of methanogenesis, and is not conducive to the recovery of methanogenic bacteria, so NaOH and Ca (OH) 2 should not be used.