Dear Arun Babu Vathsalan

You have to adjust Ph by Ca(OH)2 and addition alkalinity of the wastewater.

Regards

Generally, the optimum pH for methane-forming bacterial growth lies between 6.5 and 7.5 (Metcalf and Eddy, 1991). The lower the pH and the longer the pH is maintained at that level, the more likely an upset can occur. It is very important that the pH be maintained above 6.5 and that sufficient alkalinity is present to ensure that the pH will not drop below this point. As a result, an alkali such as magnesium hydroxide

slurry can be introduced into the digester to neutralize any excess acid that can not be consumed by the methane-forming bacteria.   

Was taken from:

http://magnesiaspecialties.com/wp-content/uploads/The-Use-of-MgOH2-Slurry-for-Treatment-of-Wastewater-010215.pdf

You may use calcium hydroxide or carbonate very finely divided.

Hi

I think this publication will be helpful to you

pH adjustment in anaerobic digestion

M.G. Capri and  G.v.R. Marais 

doi:10.1016/0043-1354(75)90052-4

soda ash is also used to adjust pH of the Anaerobic digestion process

I think  use Calcium carbonate (not very soluble) when bicarbonate alkalinity 500-1000 mg/l for pH 6.5 to 6.6.

Why don't you use NaOH for adjusting pH to neutral as required in anaerobic digestion process

I agree with Dr Bertello and Dr Swarnakar.

It is true that CaCO3 is not very soluble; a moderately stirred basin, though, can give you the results you aim at. Even a polyethylene - lined pond could meet your needs; the unit power input for solids suspension could be as low as 6 W / m3. 

A properly sized wind-driven mixer could do the work.

Alternatively, a reactive filter bed can be used. You could even avoid the use of pumps, provided that you have a ground level drop available to allow for the head losses.

Granular or coarsely powdered calcium carbonate can be got for free from marble quarries, if there are any in your district.

I would not recommend Dr Vathsalan neither NaOH nor soda ash for his purpose.

Not only since soda ash (Na2CO3) is expensive and NaOH is hazardous to handle, but because both give no buffer reserve to the leachate to be treated.

More, errors in dosing a strong alkali like these are sadly very easy. And the dosing station should be a complex one, fitted with machinery and instruments ... and therefore expensive.

Las but not least, all chemicals exert an environmental impact for their production. Is a base stuff - like waste landfill leachate - worth that?

In pH adjustment of wast water many factors are important such as: The type of sulfide mineral, Oxidation process, water content, Alkaline rocks, acidity intensity and etc.

It is important to you use some materials for pH adjustment to considering the operating conditions included in: Calcium carbonate, sodium hydroxide and etc, , 

you can also adjust the pH by using inoculum having alkaline in nature.

u can also use Sodium bicarbonate, 

It is important to you use some materials for pH adjustment to considering the operating conditions, do not disturb the growth of bacteria also consider the case of hydrogen sulfide control.