significant anaerobic core. As macro-scale aeration limits would certainly arise with this particle size if it were the sole component of the composting pile, then composting
particles will, in general, be larger than this size and have substantial anaerobic cores. Therefore, micro-environment formation can not be ignored in composting.
With new micro-environments forming at each interval, up until oxygen reaches the core of the particle, it follows that further insights into the phases of the composting time course noted in Figure 3-2 arise from considering the following:
Growth phase where there is a balance between biomass growth rate and the changing proportion of the particle which is aerobic.
Post growth phase but less than fully aerobic particle, where the observed composting rate is net of reduced composting rate in the old micro-environments and increased composting rate from a new micro-environment‟s contribution.
Fully aerobic particle, where the observed composting rate results from all micro- environments reducing their composting rate, albeit all in different states of degradation.
Micro-environments link the laws of diffusion, microbial dynamics and particle geometry effects within the composting dynamic. Therefore many aspects of compost
understanding would be enhanced by considering the micro-environment perspective. Some effects express primarily through the physical side of micro-environment analysis, such as:
Moisture content effect on the composting time course. For moisture contents below optimum, the particle will swell with increasing moisture content. A linear relationship between moisture content and particle diameter was used to determine wet particle size from the sieved oven dry sample size in the temperature trials in this work. At higher moisture contents, thesecondary particle formation of Hamelers & Richard (2001), in which the size of pores filled with water were determined from the matric potential which rises with increasing moisture content, meshes perfectly with micro-environment analysis.
Composting response to mixing. The matric potential and the size of pore filled with water could be used to determine the „size‟ of the contact point between adjoining particles. There is no reason why micro-environment analysis could not be used to determine the anaerobic area of this contact point. Summing the size of all these anaerobic contact points would indicate the proportion of the surface of the perfect sphere, the analytical boundary in this thesis, not available to oxygen. Over time, the anaerobic parts of these contact points would develop a different substrate concentration to their aerobic neighbours. When mixed, these points can be assumed to be exposed to oxygen and the effect on the composting process from this determinable physical effect predicted.
Other effects express through the microbial side, such as:
Micro-analysis of pH. The stoichiometry of the electron acceptor prevalent at the site (Harremoës, 1978)) and toxicity effects within composting can determine pH changes at the micro-scale. These are most likely to dominate in the anaerobic core and consequently will occur outside the micro-environment framework. However the „recovery‟ of a micro-environment from low pH may reveal insights into the nature of the pH changes in composting (Beck-Friis et al., 2003). An approach using stoichiometry, and hydrogen ion activity, was applied to the volatilization of ammonia from a composting pile by Liang et al. (2004). However, the same analysis framework could be done at the micro-environment scale.
Odorous chemical generation in the anaerobic core and the consumption of these breakdown products in the aerobic shell could be determined. The odour
production of the pile is net of these two processes and as each space is determined by its electron acceptor, they each have their own state-space parameters
determinable with micro-environment analysis. The changing balance between production and consumption of these odorous chemicals over the composting time course could be determined from these parameters. This effect was proposed by Tseng et al. (1995), and would imply that there is a critical particle size where anaerobic breakdown products are „scrubbed‟ in the aerobic zone.
While some effects express through a combination of all factors:
Optimisation of aeration need. The ability to detect the optimum aeration pumping frequency based on „seeing‟ the composting response consequent to an aeration event (Figure 5-3), could be subjected to a full thermodynamic analysis. This demonstrates the strength of the technique developed by the author. Any non-thermodynamic impact on the microbial response would express as a
thermodynamic imbalance. From this type of analysis, other aspects of aeration events e.g. pressure differential across the compost, non-steady state changes in the oxygen penetration depth and its effect on the aerobic biomass development, may be found to be important.
Determination of the diffusion coefficient appears to be possible from the composting time course. Although from the theory presented in this thesis, different diffusion coefficients could arise from the development of micro-porosity within the particle. Previous experiments using an early version of the reactor indicated that horse faeces had a higher fast fraction rate constant than pig faeces, and this may be due to greater micro-porosity from the higher roughage content in the horse faeces influencing the diffusion coefficient via micro-porosity.
With the increased understanding of composting that arises from application of the laws of diffusion via its analytical framework micro-environment analysis, the possibility that composting could be optimised from a microbial perspective is raised.