Fractality in pH time series of continuous anaerobic bioreactors for tequila vinasses treatment

Author-Highlights
• pH time series from continuous AD could be associated to the current conditions governing the process behavior.
• Rescaled range fractal analysis can be used to identify correlations in pH time series from continuous AD.
• Fractal analysis of pH signals from continuous AD allows the identification of three characteristic regions.
• Fractal indexes in these three regions provide useful information about key process variables in continuous AD.
• Fractal analysis can be used as an alternative inexpensive and easy method that provides useful information in continuous AD.

pH measurements taken from anaerobic digestion (AD) processes show fluctuations with an apparent random behavior that is usually associated to noise. However, it is well-known that the AD process is carried out on the basis of electron interchange from donors to acceptors. Therefore, such a random behavior could be associated to the current conditions governing the process behavior and then, a suitable analysis can provide important insights into the dynamical behavior and operational stability of AD processes. In this work, fractal scaling properties of pH signals from different continuous anaerobic bioreactors (applied to the treatment of tequila vinasses) are analyzed by using R/S analysis. Results indicated that fractal dimension is directly related to key process variables (influent COD, VFA and biogas production). This findings are helpful to achieve better insight into the operational aspects of continuous anaerobic digesters.