Kinetics of forced aerated biodegradation of digested sewage sludge-reed mixtures at different temperatures

This work presents a kinetic analysis of the aerobic biodegradation of anaerobically digested sewage sludge and dried reed mixtures at different temperatures. Batch experiments were conducted in laboratory-scale reactors with temperature (T) control and forced aeration of the solid mixture. The biowaste mixture was treated at four different temperatures: 25, 40, 50 and 60 °C, with moisture controlled and samples taken weekly for carbon (C) and volatile solids (VS) measurements. The duration of experiments was either 90 d (at 25 °C) or 60 d (at 40, 50 and 60 °C). Two different kinetic models were used to fit the carbon mineralisation curves: the 2C model, which considers two organic fractions (biodegradable and non-biodegradable) and the 3C model, which considers three fractions (easily biodegradable, slowly biodegradable and non-biodegradable). In both cases, the kinetic rate constants were calculated by mathematical fitting and were compared with previously reported values. The temperature influence on the rate constants was studied for both models using a T-dependent equation. The calculated kinetic rate constants were in agreement with previously published values, and good fitting of the experimental data was obtained with both models. Similar rate constant values were obtained for mineralisation of the biodegradable fraction (2C model) and the easily biodegradable fraction (3C model). The rate constants for the slowly biodegradable fraction (3C model) were much lower. A good correlation between rate constants and T was observed. Different optimum temperature values were obtained for each rate constant depending on which carbon fraction was degraded. The T-dependent rate constant values obtained could be used for modelling the C mineralisation of real variable-temperature composting processes.