Chemical and Decomposition Characteristics of Anaerobic Digester Effluents Applied to Soil

A study was conducted to determine the elemental concentrations of an effluent that resulted from the anaerobic digestion of rum distillery by-products (also called molasses residue) and decomposition characteristics in soil as measured by CO₂ evolution. For comparison, two additional effluents from the anaerobic digestion of swine wastes and bovine wastes were included (Susscrofa and Bos taurus, respectively). The carbon content of oven-dry molasses residue and swine waste effluents were both approximately 250 g kg⁻¹, whereas the bovine waste was 152 g kg⁻¹. The molasses residue effluent sample had a greater K concentration on an oven-dry basis than the swine or bovine waste materials. However, the concentrations of most other elements were higher in the swine waste effluent. In contrast, higher solute concentrations were found for most elements in the molasses residue effluent than in the swine waste effluent. An analysis of variance performed on total CO₂-C evolved after 20 d as well as at the end of the experiment (117 d) indicated an interaction between effluent source and rate. This was caused by the low amount of CO₂ evolved from soil amended with the bovine waste effluent regardless of application rate. At day 20, cumulative CO₂ evolution was higher with the molasses residue effluent than with the swine waste effluent; however, by day 117, only at the high application rate was CO₂ evolution higher with the molasses residue. Lower application rates allow for proportionally more short-term decomposition of the molasses residue and swine waste effluents than at higher rates. Although effluent sources result in different amounts of total evolution, between 40 and 50% of the total CO₂ evolved may occur in the first 20 d. Contribution of the Dep. of Soil Science, Univ. of Florida, Gainesville, FL 32611. Agric. Exp. Stn. Journal Series no. 8481.