Dear Alok R Singh,

It is bit difficult to maintain pH exactly at 7, But you can maintain around 6.85 using the mixture given below. But you have to think about the impact of PO4 to your system. if there is a problem with that mixture, you have think about other mixture.

A. G. Piyal Aravinna

Increasing pH means you have increasing ammonia concentration. If you don't check the increase, you might upset the system resulting in failure.

https://www.researchgate.net/post/How_to_neutralize_VFA_in_a_biogas_digester