Typical control strategy in methanogenic anaerobic reactors is to maintain a relatively low concentration of volatile fatty acids (VFA) and a pH range of 6.6 < pH < 7.4. Normally in such reactors the carbonate system forms the main weak-acid system responsible for maintaining the pH around neutrality, while the VFA systems (acetic, propionic, and butyric acids) are the major cause for pH decline. Under stable operating conditions, the H2 and acetic acid formed by acidogenic and acetogenic bacterial activity are utilized immediately by the methanogens and converted to methane. Consequently, the VFA concentration is typically very low, carbonate alkalinity is not consumed and the pH is stable. Conversely, under overload conditions or in the presence of toxins or inhibitory substances, the activity of the methanogenic and acetogenic populations is reduced causing an accumulation of VFA which in turn increases the total acidity in the water, reducing pH. The extent of the pH drop depends on the H2CO3 alkalinity concentration. In medium and well-buffered waters (typically the case in anaerobic digestion), high concentrations of VFA would have to form in order to cause a detectable pH drop, by which time reactor failure would have occurred. Therefore, pH measurement cannot form the sole control means, and direct measurement of either (or both) VFA or H2CO3 alkalinity concentration is necessary.

The most used technique for the determination of alkalinity for the control of the system anaerobic is described below:

25 mL of sample are taken and placed on a plate with stirring to a solution titrated with 0.02 N sulfuric acid, it initial pH is measured and the acid is added until the pH changes to 5.75 volume of spent acid, followed by titrating until the pH changes to 4.3 and the volume of spent acid is taken and is determined the alpha value.

Alpha = acid vol. (5.75)/ acid vol. (4.3) if this value is greater than 0.55 the bioreactor is acidified and must add a buffer, on the contrary, if it is less, acid must be added [35].