COMPARATIVE EVALUATION OF THE CHEMICAL COMPOSITION OF BIOGAS DIGESTATE SLURRIES UNDER VARYING FEEDSTOCKS

Abstract

Biogas production is increasingly considered a good source of clean energy and Biofertilizers in the form of digestate slurry. This study sought to characterize biogas digester slurries under varying Feedstocks and provide information useful for assessing the potential impact of their usage as organic fertilizer on crop productivity and environment. The study was informed by the need to allow agricultural scientists determine how individual biogas feeds affect the composition of the final digestate and hence the impact the digestates could have. Digestates from human waste, livestock waste, and abattoir waste feedstock were sampled for the study. The pellets from the samples were then analyzed using Energy Dispersive X-ray Fluoroscopy (EDXRF) spectroscopy for elemental components. After the requisite statistical analysis, the human waste digestate had the highest concentration values for most elements. In human waste, essential elements were determined at 40600 ± 2000, 19000 ± 1140, 1300 ± 400, 200 ± 30, 900 ± 260 ppm for Ca, Fe, Mn, Zn and Cu respectively, as compared to Ca (26400 ± 1400), Fe (9500 ± 440), Mn (820 ± 190), Zn (180 ± 40), Cu (360 ± 70), in livestock waste. In abattoir waste, the mean content was 49500 ± 4100 for Ca, 15220 ± 1350 for Fe, 1090 ± 90 for Mn, 200 ± 50 for Zn and 140 ± 50 for Cu. Potentially toxic elements Hg and Cd were determined below detection limits, while Pb concentrations were highest in human waste at 20.81 ppm. The high amounts are associated with the micro industrial activities in Kibera. Therefore, human wastes digestates might be the best nutrient supplement but is the most probable threat to environment. Indeed, an unchecked Bioaccumulation of the other elements might be counterproductive as well. For agronomists, crops need matching with digestates from which they would benefit the most, given unique crop requirements.