An evaluation of substrate degradation patterns in the composting process. Part 2: temperature-corrected profiles

In this paper, the patterns of 44 substrate degradation profiles obtained from the composting literature are examined following their correction to a constant temperature of 40 degrees C, using a new procedure presented in this work. The applicability of a single exponential model, a double exponential model and a non-logarithmic Gompertz model in describing their behaviour is then evaluated. Multi-phase profiles were most commonly seen, with convex shapes observed in only a relatively small proportion of the profiles. Convex shapes were also embedded within other profiles, either preceeded by a lag phase, or followed by non-convex behaviour. Sigmoidal patterns were relatively rare. Of the temperature-corrected data sets examined, 33 were found to be either not well modelled by, or inappropriate for, any of the above models. Two fits rated as good were obtained when using the single exponential model, and one fit rated as excellent, plus one fit rated as good, were obtained when using the double exponential model. A single fit rated as excellent was found when using the non-logarithmic Gompertz model. The lag phase, which was observed in many data sets, was successfully modelled using the non-logarithmic Gompertz function where excellent and good fits were obtained, but as expected this phase of the profile could not be modelled by either the single or double exponential functions. When the lag phase or post-convex curve data was removed from 20 data sets, use of the single exponential function resulted in three fits rated as excellent and two rated as good. When a double exponential model was applied to these data sets, three fits rated as good were obtained, whilst application of the modified Gompertz model gave one fit rated as good. The remainder of the fits were rated as moderate to fair. It is concluded that the evidence supporting the use of the single exponential model, the double exponential model or the non-logarithmic Gompertz model to describe full substrate degradation profiles in composting following their adjustment for temperature effects is limited. Further work is suggested in order to investigate the nature of those profiles which were not well modelled, to more precisely ascertain the cardinal temperatures for composting used in the function of Rosso et al. (1993) Rosso, L., Lobry, J.R., Flandrois, J.P., 1993. An unexpected correlation between cardinal temperatures of microbial growth highlighted by a new model. J. Theor. Biol 162, 447-463.], which was employed in the present temperature correction procedure, and to incorporate correction for varying moisture and oxygen concentrations.