Modeling Volatile Fatty Acid Concentration in Livestock Manure–Based Anaerobic Digesters by Simple Titration

Abstract

Abstract

There are several methods to determine volatile fatty acids (VFAs) in anaerobic digesters, the simplest and cheapest method so far being titration. A very simple empirical model using titration for determination of VFAs in livestock manure–based digesters has been developed and validated with samples from manure-based commercial codigestion plants and pilot digesters. The method uses two endpoints and can be used by both manual and automatic titrations. The method was used to determine the total VFA concentrations in digesters from cattle manure solely and digesters codigesting livestock manure, maize, and organic waste. Total VFA concentration measured by the titration correlated well with those measured by a gas chromatograph and a new model was developed with an R² value of 0.98 for the entire VFA range from 38 up to 30,056 mg total VFA/L and a slope close to 1. No other models have been developed for such a large span in VFA concentration. For lower VFA concentrations <5,000 mg/L, an adapted model might be used, but the correlation (R²=0.8) is lower than in the global model, indicating that the model is best over a higher range.