Accurate on-site volatile fatty acids (VFA) measurement in anaerobic digestion – verification of a new titrative method

This paper reports on the development and verification of a new simple, rapid and accurate titration method suitable for on-site measurement of volatile fatty acids (VFA) and carbonate alkalinity in anaerobic reactors. The method takes into account the effects of the phosphate, ammonium, sulfate, and sulfide weak acid subsystems on titration results. The method involves eight pH observations and typically the full procedure takes 15 minutes. To evaluate the accuracy of the method three industrial anaerobic digesters effluents were tested. The technique was verified by adding known aliquots of acetic acid to effluents previously measured for VFA and carbonate alkalinity using the method. After the addition, the VFA concentration was re-measured and the difference between the two results was compared with the amount of acetic acid added. Very high accuracy for VFA measurements (> 97%) plus very good repetition was obtained with all three effluent types. Generally, VFA measurement accuracy increased with VFA concentration. In the typical case of low VFA concentrations accompanied by very high carbonate alkalinity, the raw sample should be “spiked” with a known amount of acetic acid in order to increase measurement accuracy.