On-line titration of volatile fatty acids for the process control of anaerobic digestion plants

The on-line titration of volatile fatty acids (VFA) was found to be a reliable method to measure the substrate (metabolite) concentration without the use of expensive analytical devices. A measurement cell was designed that ensures stable long-term operation, high throughputs and copes with both very low and very high VFA concentrations. Using synthetic textile wastewater, a recalibration of the pH probe was necessary only twice a week. A good reproducibility of the VFA concentrations was determined and standard deviations were below±1% of the measured concentration. Changing salt concentrations of the wastewater in the range of 2.5–150 g l−1 NaCl did not influence the determined VFA concentration significantly. The method was suitable to control the hydraulic retention time in an acidic phase reactor and to determine the acidification rates of wastewater compounds in batch experiments.