Urea or ammonium shall only be used as NUTRIENT (nitrogen source) but NOT for pH control.

pH shall be controlled by alkalinity preferably by exploiting the natural H2CO3 H+ + HCO3- 2H+ + CO3- - pH 7 - 7.5 buffer in anaerobic digestion.

Firstly, it is alkalinity and VFA concentrations (VFA/Alkalinity ratio) that you need to control to prevent digester failures. pH is only an indication of what has already happened. For instance, a low pH in you digester will indicate high VFA concentrations which may have already affected your methanogens! 

To control pH in your digester, you may use NaOH, NaHCO3, Na2CO3, CaCO3. Urea or ammonium chloride are used as sources of nutrient, as rightly pointed by Mr. Peeters. I normally use NaHCO3 (sodium bicarbonate) since for 1 unit of pH increase, it increases your alkalinity by 2 units as opposed to NaOH (Check Deffeyes' Diagram). How much to add will actually depend on your actual pH. However, for the substrates you are using (tend to digest rather slowly as compared to food wastes), initially setting your pH to 7.5 should be okay. As you may be aware, alkalinity is also produced during digestion and this will eventually increase your pH if the system is operating well.

Alternately, you may use an automatic pH control (feedback control system) such that whenever your pH drops below the set value, acid or alkali is added accordingly to bring the pH to the set value.

Kind regards,

Zumar

Dear Junaid, 

In one of my study I had synthetic wastewater COD 6000 mg/L and as a general rule, I added NaHCO3 half of the COD. it results in pH around 7.2-7.4. you can try by adding half and then adjust by slightly adding or reducing. But keep in mind, excessive Na can inhibit the anaerobic process.  

Ammonium chloride is one of the chemicals needed in your basic medium as explained by Peeters and Ali Bundhoo. This basic medium is also needed for optimum function of anaerobic microorganisms except if available in your substrate or inoculum.

Check the article attached.

Dear Muhammad

according to Standard methods, Alkalinity for properly operating anaerobic digester is about 2000-4000 mg CaCO3/L.

You can add carbonate, bicarbonate or hydroxide to reach the optimum value. 

Yes... Got it.. Urea or ammonium chloride is used to adjust C/N not for pH control. How much dose is required to adjust the C/N ratio of these substrates (Wheat straw, rice straw and bagasse?

To adjust the C/N ratio of your substrates, you will first need to determine the C/N ratio of the substrates. Lignocelluloses like wheat straw, rice straw and bagasse can have C/N ratios higher than 60:1 (some even above 100:1). Depending on the C:N ratio of your substrates, you calculate how much N you need to add to bring the C:N ratio to the required value (25:1-30:1) for effective digestion. Based on this and the ratio of nitrogen in NH4Cl (14/53.5=0.262), you then calculate how much NH4Cl you need to add... (Amount of NH4Cl=Amount of Nitrogen/0.262)...