Regarding the effect of pH on methanogens and other issues
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During the anaerobic experiment phase, the pH of the influent water kept rising (the reason for this is unclear; the raw water tank is an iron tank that allows air to enter). Once the pH of the influent water exceeded 7.2, gas production was significantly suppressed – in other words, it dropped from 25 cubic meters to less than 10 cubic meters. Is the impact of pH changes that rapid? Of course, once the pH of the water decreased, gas production returned immediately. Also, are various salts included in VFA? When the pH is above 7, does acetic acid partially turn into acetate? When measuring VFA, does this portion of acetate count as VFA? (Don’t laugh; I come from the chemical industry – I’ve never measured wastewater data myself; everything is sent to the laboratory for analysis.) Can methanogens utilize these acetates? Or rather, which is faster in terms of the utilization rate: acetic acid or the other one?
It is only known that a high pH has a fatal impact on anaerobic bacteria; at high pH levels, their activity is reduced, preventing them from carrying out hydrolysis and acidification, and thus methane is not produced.
Anaerobic bacteria produce methane through a biochemical reaction that requires hydrogen; their activity is inhibited when the acidity decreases or under alkaline conditions. In the actual process of anaerobic treatment to produce biogas, it is advisable to add online monitoring of pH, ORP, and DO in order to adjust the conditions of the anaerobic tank at the appropriate times.
What kind of wastewater is this? It’s best to look at examples of how similar projects were handled. Could an increase in PH lead to factors such as air entering? A pH of 7.2 is normal under anaerobic conditions. Methanogens that utilize acetate cannot use it directly, but if it is hydrolyzed, it can still be utilized indirectly, depending on the specific internal environment
Generally, methanogens require a strictly anaerobic environment, but based on my experience, the entry of air into the raw water tank – where the water is already anaerobic and pre-treatment usually does not involve sealing – does not have such a significant impact on anaerobic gas production. pH 7.2 is within the appropriate pH range for methanogen growth. If gas production drops significantly and returns to normal immediately after water is added, it can be inferred that the water input load is too low; the organic matter available is no longer sufficient to support the growth of large numbers of methanogens, so the load can be increased further. VFA refers to volatile fatty acids, and salts are not included; methanogens cannot utilize salts. One of the posts seems to be yours as well – you mentioned that the substances that methanogens can use are limited; methane production is possible only after these substances are broken down into forms that they can utilize. For reference.
Anaerobes are quite sensitive to changes in pH and water temperature, and they also have strict requirements regarding pH values.
There are two ranges for the optimal water temperature for anaerobic processes: one is at moderate temperatures of 35–37°C, and the other is at high temperatures of 55–57°C. The optimal pH range is 6.8–7.2, but depending on the material conditions, many microorganisms can function normally even at values above 7.2 after being acclimated. :)