Characteristics of volatile fatty acids in stored dairy manure before and after anaerobic digestion

Volatile fatty acids (VFA) are among the most abundant volatile organic compounds in dairy manure and are associated with odour nuisance. This paper presents research results of VFA production during a three-month storage of dairy manure from four different sources: a dairy barn (raw), the inlet of an anaerobic digester (influent), the digester outlet (effluent), and the effluent after solid separation (effluent SS). Manure from each source was studied in two lab-scale reactors that were continuously ventilated with fresh air in the manure headspace to simulate manure storage conditions. Two manure samples were taken weekly in the top and bottom manure layers from each reactor for VFA analysis. Five VFA (formic acid, acetic acid, propionic acid, butyric acid, and 2-methylbutyric acid) were identified in all reactors using high performance liquid chromatography (HPLC). The dominant VFA was formic acid for the influent and acetic acid for the other three manure sources. The overall average concentrations of the five VFA were 1963 ± 685 (mean ± standard deviation), 14,175 ± 4825, 286 ± 98, and 169 ± 80 mg l−1 for the raw, influent, effluent, and effluent SS, respectively. The “pre-consumer” organic wastes mixed with dairy manure in the influent significantly increased the total VFA concentrations and the proportion of individual VFA. Concentrations of VFA demonstrated highly temporal and spatial variations. Anaerobic digestion significantly reduced formation of VFA in the effluent and effluent SS. However, the complexity of VFA characteristics made it difficult to reliably model and predict the concentrations and compositions of VFA in dairy manure.