Aerobic degradation kinetic of the effluent derived from the anaerobic digestion of two-phase olive mill solid residue

Aerobic biodegradability of the effluent derived from the anaerobic digestion of two-phase olive mill solid residue was assessed in laboratory-scale batch reactors of 1.5–l volume. Five experimental runs at influent substrate concentrations in the range of 660–13100 mg COD l−1 were carried out at a ratio of substrate-to-microorganisms concentration S0/X0 equal to unity. Chemical oxygen demand (COD), soluble chemical oxygen demand (SCOD), volatile suspended solids of the mixed liquor (MLVSS), pH, dissolved oxygen, saturation percentage (% sat.) and temperature were determined during the experiments. An increase in the influent substrate concentration caused a decrease in COD and SCOD removals at similar times of aeration θ. The non-biodegradable fractions of COD and SCOD were determined to be 43.1% and 53.1% of their initial values, respectively. It was found that a first-order kinetics adequately described the variation of biodegradable SCOD with θ. The values of the first-order reaction constants were found to be 0.074, 0.044, 0.040, 0.033, and 0.013 h−1 (1.776, 1.056, 0.960, 0.792, and 0.312 d−1) for Runs 1–5, respectively, with variance coefficients lower than 10% in all cases. During the experiments the growth of microorganisms appeared to be in the exponential phase. Hence, first-order reactions were assumed to describe the increase in MLVSS concentration with θ. The values of the specific growth rate μ were determined to be: 0.0064, 0.0075, 0.0084, 0.0091, and 0.0038 h−1 (0.154, 0.180, 0.202, 0.218, and 0.091 d−1) for initial substrate concentrations of 660 (Run 1), 1400 (Run 2), 2600 (Run 3), 6600 (Run 4), and 13100 (Run 5) mg COD l−1, respectively. Finally, the values of the maximum specific growth rate μM and the saturation constant KS of the Monod equation with their standard deviations (p<0.05) were determined to be 0.23±0.01 d−1 and 160±8 mg SCOD l−1, respectively.