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Moisture is a key environmental factor that affects many aspects of the composting process. Biodegradation kinetics are affected by moisture through changes in oxygen diffusion, water potential and water activity, and microbial growth rates. These relationships are made more complex by the dynamic nature of the composting process, with changes in particle size and structure occurring over time. A deductive model of the effects of moisture on composting kinetics has defined these relationships based on fundamental physical properties and biological mechanisms. This study applies this model to experimental data from a manure and papermill sludge composting system. The results demonstrate that the optimum moisture content for biodegradation can vary widely for different compost mixtures and times in the composting process, ranging from near 50 to over 700 on a wet basis. While there is a significant reduction in biodegradation rate when operating outside the optimum range, the results also suggest opportunities to mitigate this effect through manipulation of substrate density and particle size. This framework for engineering analysis demonstrates the importance and challenges of maintaining optimum moisture content in dynamic composting systems, where biological drying, metabolic water production, and changes in compaction and porosity are all occurring over time.
Introduction
Composting process control has seen significant advances in the last three decades, with increasing understanding of the underlying scientific principles (Finstein et al. 1983, 1985; Miller 1991; Epstein 1997), sophisticated engineering analysis (Keener et al. 1992; Haug 1993; Hamelers 2001), and practical implementation (Rynk et al. 1992; Kuter et al. 1995; Leege 1996; Rynk and Richard 2001). Although the terminology varies, in each of these frameworks process control strategies can be seen as an attempt to manage the interactions between physical and biochemical material properties and microbial activity. This is generally accomplished through setting appropriate initial conditions and then maintaining key process variables in an optimum range. Moisture and air-filled porosity appear both as initial conditions (with C JN ratio), and as key process variables (along with temperature, oxygen concentration, and sometimes pH). Because moisture affects material and matrix properties as well as microbial activity, it has important implications for both the physical and the biological aspects of the composting process.
Moisture management requires a balance between two functions: encouraging microbial activity and permitting...