Effect of Anaerobic Digestion on Volatile Fatty Acids in Dairy Manure

Abstract

This paper presents study results of volatile fatty acids (VFA) production during three months of storage of dairy manure collected from four different sources: (1) a dairy barn, (2) the inlet of an anaerobic digester (influent), (3) the outlet of the digester (effluent), and (4) the effluent after separation of biosolids. Manure from each source was tested in two lab-scale bioreactors of 61.0 cm high and 38.1 cm in diameter. Each bioreactor was initially filled with manure to as depth of 25.4 cm. The bioreactors were continuously ventilated with 6.5 L/min of fresh air in the manure headspace during the entire test to simulate manure storage conditions on dairy farms. Two manure samples were taken weekly from each bioreactor for VFA concentration analysis: one in the top manure layer within 2.5 cm below the surface and another in the bottom manure layer within 5.0 cm above the reactor bottom. Five VFA including: formic acid, acetic acid, propionic acid, butyric acid, and 2-methylbutyric acid, were identified in manure samples from all reactors using HPLC (High Performance Liquid Chromatography). Significant differences in VFA concentrations between the digested and undigested manure were observed. Results showed that the dominant VFA in the stored manure from the dairy barn was acetic acid followed by propionic acid. Concentrations began to decline around the fifth week of storage. The stored influent manure had initial formic acid concentrations as high as 27 g/L, but the concentration declined within 4 wks of storage. The acetic and butyric acids were the dominant VFA after the decline of formic acid. For all digested manure, the dominant VFA was acetic acid but its concentrations only reached a maximum of 0.9 g/L compared with a maximum concentration of 5 g/L in influent manure. This study provides a better understanding of the environmental effects of anaerobic digestion on the dairy manure storage.