Using Volatile Fatty Acid Relationships to Predict Anaerobic Digester Failure

Abstract

ABSTRACT VOLATILE fatty acid (VFA) relationships are important in the anaerobic digestion of animal wastes as they (acetic, propionic and butyric) are direct precursors of methane, either through direct conversion of acetate or through the intermediate formation of hydrogen and carbon dioxide. Thus, they are essential compounds in the biological conversion of heterogenous wastes to useable products. VFA's are also known inhibitors in the biological conversion process if their concentrations are sufficiently high. Thus, VFA's are simultaneously essential for the process and can be toxic agents should they be present in excess quantities. This relationship makes quantifying VFA*s in modeling studies essential to accurately predicting digester failure or success. A highly correlated realtionship between the level of acetic acid and/or the propionic to acetic acid ratio in digesters that were successful and in digesters that failed has been shown. These data have been used to calibrate an original comprehensive methanogenesis model and along with the addition of dual-use substrate kinetics for the simultaneous catalysis of propionate and butyrate, have produced a much improved prediction of the VFA relationships observed in operating anaerobic digesters. This manuscript describes the addition of the dual-use substrate kinetics and the modification of the kinetic parameters of the original methanogenic model and compares the simulated output of the original and modified models to demonstrate the improved predictive ability.