An evaluation of substrate degradation patterns in the composting process. Part 1: profiles at constant temperature

This paper examines the patterns of 32 constant temperature substrate degradation profiles obtained from the composting literature, and evaluates the use of a single exponential model, a double exponential model and a non-logarithmic Gompertz model in describing their behaviour. Profiles were found to be predominantly either sigmoidal in shape, or to exhibit multi-phase behaviour, with a relatively small proportion of convex curves. Of the constant temperature profiles, 26 were either not well modelled by any of the above functions, or of such differing profiles that none of the above functions was applicable. Goodness of fit was measured using a normalised error function, and rated using a five-category descriptive scale, ranging from excellent to poor. No fits rated as excellent were observed. Fits rated as good were obtained for three data sets when using a single exponential function, for two data sets when using a double exponential function, and for one data set when using the non-logarithmic Gompertz function. The remainder of the fits were rated as moderate to poor. 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 substrate degradation profiles generated at constant temperature is limited. Further work is suggested in order to establish standard procedures and a standard simulated composting mixture for substrate degradation studies and to build a more comprehensive set of long-term substrate degradation profile data at constant temperature, and under non-limiting moisture and oxygen concentration conditions.