MODELING PREDICTIVE INDICATORS IN ANIMAL WASTE METHANOGENESIS

The methanogenesis model developed by Hill (1982) and subsequently mod ified by Hill and Bolte (1987) has been updated to incorporate the inh ibitory effects of longer-chain (C4 - C5) volatile fatty acids (VFA). These changes to the model are based upon the work of Hill and Holmber g (1988), Cobb (1989), and Cobb and Hill (1988, 1989, 1990, 1990a, 199 0b, 1991) which demonstrated that interconversion of C2 - C5 VFAs play s a significant role in the operation of anaerobic fermenters. These w orks also demonstrated that monitoring the levels of long-chain VFAs c an be a valuable tool in predicting and preventing process failure in the operation of anaerobic fermenters. The previously validated model has been revised to include the longer-chain VFAs so that the fermenta tion process can be more accurately predicted. Specifically, the model has been improved by: 1) modification of the model stoichiometry to i nclude valeric acid production via acetogenesis and valeric acid hydro genogenesis; 2) addition of triple-use substrate kinetics for hydrogen ogenesis; and 3) incorporation of fractionation functions for butyrate and valerate which divide these acids into their iso- and n- forms to make the model more useful for predicting future stress and failure o f fermentation systems. This manuscript includes a description of the modified model and the rationale for the included modifications, as we ll as the results of validation testing of the model based upon compar ison of model output with experimentally obtained data.