Dynamic characterization of an anaerobic digester during the start-up phase by pH time-series analysis

• The start-up of anaerobic digestion processes is by nature critical and complex.
• The monitoring of key process s variables is difficult and expensive.
• pH fluctuations contains relevant information about the start-up phase evolution.
• Fractal analysis of pH signals provides valuable information about key variables.
• Fractal analysis is shown as an inexpensive and easy qualitative tool for digester diagnosis.

In this paper, an alternative methodology for the online monitoring of key variables during the start-up phase of continuous anaerobic digestion processes is presented. The proposal is based on the application of fractal analysis to pH time-series obtained from a lab-scale fixed-bed reactor (FBR) used in the tequila vinasses treatment. Results showed that fractal parameters exhibit a dynamic behavior closely related to that of key process variables such that, chemical oxygen demand (COD), volatile fatty acids (VFA) and biogas production, whose on-line monitoring is difficult and expensive in practice. Thus, the industrial implementation of the here proposed ideas is quite encouraging, since it is shown that fractal analysis of conventional pH signals can be used as an inexpensive and easy to implement tool for anaerobic digesters diagnosis, which could drastically reduce the time and problems currently associated to the start-up phase.