Anaerobic digestion of cattle offal: protein and lipid-rich substrate degradation and population dynamics of acidogens and methanogens

Abstract

Anaerobic digestion of cattle offal was investigated in batch reactors at 35 A degrees C to determine the feasibility of using cattle offal as a feedstock. The organic content [i.e., volatile solids (VS)] of the cattle offal was mainly composed of protein (33.9 %) and lipids (46.1 %). Hydrolysis along with acidogenesis was monitored to investigate the substrate degradation and generation of intermediate products (e.g., volatile fatty acids, ammonia). Acetate (2.03 g/L), propionate (0.60 g/L), n-butyrate (0.39 g/L), and iso-valerate (0.37 g/L) were major acidogenesis products (91 % of total volatile fatty acid concentration). Overall protein and lipid degradation were 82.9 and 81.8 %, respectively. Protein degraded first, and four times faster (0.28 day(-1)) than lipid (0.07 day(-1)). Methane yields were 0.52 L CH4/g VSadded and 0.65 L CH4/g VSremoved, indicating that anaerobic digestion of the offal was feasible. A quantitative QPCR assay was conducted to understand the microbial dynamics. The variation patt erns in the gene concentrations successfully indicated the population dynamics of proteolytic and lipolytic acidogens. A fourth-order Runge-Kutta approximation was used to determine the kinetics of the acidogens. The molecular biotechnology approach was appropriate for the evaluation of the acidogenic biokinetics. The maximum growth rate, mu (m), halfsaturation coefficients, K (s), microbial yield coefficient, Y, cell mass decay rate coefficient, k (d), of the proteolytic acidogens were 9.9 day(-1), 37.8 g protein/L, 1.1 x 10(10) copies/g protein, and 3.8 x 10(-1), respectively. Those for the lipolytic acidogens were 1.2 x 10(-1) day(-1), 8.3 g lipid/L, 1.5 x 10(9) copies/g lipid, and 9.9 x 10(-3) day(-1), respectively.