Evaluation of titration methods for volatile fatty acids measurement: effect of the bicarbonate interference and feasibility for the monitoring of anaerobic reactors

The analysis of volatile fatty acids (VFA) is of utmost importance for monitoring the stability of anaerobic reactors. For routine control, titration methods are advantageous because of their simplicity, speed and low cost. However, bicarbonate, which is usually present in relatively high concentrations in anaerobic reactors, influences the potentiometric titration. In this paper, the methods of Ripley, Kapp, DiLallo, DiLallo with modified calculations, and DiLallo with sonication were evaluated regarding the effects of bicarbonate interference and their suitability for monitoring an anaerobic reactor containing high concentrations of organic matter and bicarbonate. Standard solutions with various combinations of acetic acid and bicarbonate, ranging from 100 to 4,000 mg/l, and anaerobically treated stillage containing VFA in concentrations between 5,500 and 30 mg/l were analyzed. The methods of DiLallo and Kapp were little affected by sodium bicarbonate interference, unlike Ripley's and DiLallo's methods with sonication, which overestimated VFA concentrations as the relative concentrations of bicarbonate were high compared to the acids. When the anaerobic reactor was starting, the Ripley, Kapp, DiLallo and modified DiLallo methods were statistically equal. As the system achieved stability, with VFA levels up to 200 mg/l, Ripley's method gave significantly higher VFA values. Among the methods evaluated, Kapp and original DiLallo presented the closest results, indicating their feasibility for analysis of VFA over a wide concentration range. Kapp's method stands out due to its greater simplicity and standardization.