I am working on a biogas plant, digester volume 3000 liter, right now pH condition is acidic at 4.00 my requirement is to maintain pH to 6.50 to 7.20 what should I do? some suggestions are there like, add NaOH or CaCo3 but I am confused about the how much amount should I add to increase pH
Add a base (whichever is suitable for your application) and then use pH paper (eg. https://www.coleparmer.in/i/hach-ph-strips-ph-paper-test-strips-0-to-14-ph-range-100-strips-pk/0554642) to test the pH in a sample of a specific volume of the mixture. Keep adding base (keeping track of the total amount added) and measuring the pH using pH paper until you are in the desired pH range. Just be careful not to add too much base and end up at higher pH than you need. Also make sure to give enough time to the solution to stir after each base addition before testing the pH. Once done, scale the amount of base used to the total volume in the digester. Ideally i would use an inline pH meter to continuously monitor the pH if possible (could contact mettler toledo who has really good quality products but wont be cheap, eg https://www.mt.com/int/en/home/products/Process-Analytics/pH-probe.html).
It seems that here: https://www.biogasworld.com/biogas-plant-monitoring/ a procedure is described that addresses your question where the ratio of volatile organic acids to alkaline buffer capacity is determined and therefore the risk of acidification of a biogas plant.