A functioning anaerobic reactor requires the production of fatty acids. To increase the pH above 7 would require the elimination of those acids, and prevent anaerobic treatment. So I'm not sure that this would be possible without external dosing of alkali, and that dosing would interfere with operation.

If your wastewater has a high alkalinity and you have a denitrification process based on external carbon as the last step in the wastewater treatment it is possible to reach high pH. Perhaps that is not an ideal design, but such configurations exist e.g. in Malmo, Sweden.

if it change from 8.5 to 9.5 and 4.5 in same day or in less that 5 days i think that was something wrong with your pH meter or your sampling or maybe someone add something to your reactor,,

It is not posible to see the fluctuation from pH 4.5 to 9.0 through a day even if other electron acceptors like nitrate enter the system. Make sure pH probe calibrated!

slight increase in pH in anaerobic reactors is possible due to the conversion of N-NO3- to gaseous nitrogen - a process which consumes hydrogen ions. Though the increase in pH in your study is very high, this could be due to a very high concentration of nitrates in your effluent. Nevertheless the answer to this can be found by looking at your study in entirety.

The anaerobic reactors operate in a pH range of 6.5 to 7.5. Movement to either side of this range quickly affects their metabolic rates and slows or stops methane production.

Even if the pH is 4.5, depending on the methanogenic consortia and the temperature of operation (thermophilc), the pH can increase and reach a stable value around 7. further increase can occur because of presence of alkanity, when methanogenic as well as acid producing bacteria are directly affected by increase in pH and thus are not able to decrease the pH. Incrase in pH up to8.5 is also seen when formate has been the main interspecies electron transfer and/or used as a substrate. But in this case the methanogenic activity should also be active.