Online headspace chromatographic method for measuring VFA in biogas reactor

A headspace chromatographic method has been applied to measure volatile fatty acids in anaerobic digesters using gas phase extraction at pH less than 2 and temperature higher than 65 °C. The concentration of volatile fatty acids in liquid and gas phase can be correlated in the form of Henry's coefficient. Analysis of different factors in the batch indicated that pH, temperature, and salt addition had a strong impact on apparent solubility, while liquid/gas volume ratio and organic acid concentration had little impact. Larger liquid volume had a positive impact on extraction efficiency, while increased gas headspace had a negative impact, indicating that the system was total mass-limited. The best conditions were at pH < 2.0, and temperatures above 75 °C. The advantage of an automated gas-extraction system for organic acid analysis, compared to an automated liquid system is that it contains no filter element, which is susceptible to fouling in manure samples or samples with high solid content. An automated system which included a valerate pulse to the parent reactor was implemented, and operated for a short period.