Section 7.7 discusses areas likely to yield useful insights from application of micro- environment analysis. However, many of these insights will only arise when there are:
statistical techniques to reliably extract rate constants and fraction proportions from experimental data;
no flaws in the experimental reactor.
In addition, all the physical effects that can be attributable to the application of diffusion laws to the major electron acceptor could be determined by use of a diffusible-substrate solution of diffusion laws in micro-environment analysis.
With a diffusible-substrate solution used in micro-environment analysis, the effect of diffusion laws on the distribution of oxygen would be adjusted for when determining the rate constants and substrate proportions. With adjustment for the physics of oxygen distribution, the observed variability from any experiment can be attributed to microbial complexity, and effects on/of its immediate environment; including alternative electron acceptors.
By way of example, the parts of the particle size trial data not explained by non-diffusible substrate solutions (as used here) may be explained by using a diffusible substrate solution. Any parts of the experimental time course not explained by the diffusible substrate solution would need further explanation, such as: other electron acceptors, pH, and/or microbial complexity effects.
Similarly, much of the inconclusiveness noted in the temperature trial data may be explainable by use of a diffusible substrate solution, although the structural limitations in the reactors (discussed in Chapter 5), may also be important. If the data remain
inconclusive after application of a diffusible substrate solution, repeating the trial with a single and presumably larger particle size for better experimental control, may improve the detection of the actual temperature effect on the rate constants and fraction proportions. These research needs can build on the theoretical framework presented in this thesis. Micro-environment analysis, being rooted in the laws of physics, provides a solid base on which other levels of complexity and analysis can rest.
Chapter 8
8 CONCLUSIONS
The aim of this research was to develop a method for extending instrumental resolution to sub-particle scales by using the laws of diffusion to predict substrate degradation due to aerobic composting at these scales.
This was successful in explaining large proportions of the composting time course. This success occurred despite the analysis being based on diffusion law solutions with non- diffusible substrates, and simple first-order microbial kinetics.
Micro-environment analysis has considerable potential as a composting research tool as it inherently adjusts for those parameters related to the physics of oxygen distribution in a composting particle. In particular, the adjustment occurs at the most fundamental level of its expression. For example, the range of fundamental impacts that temperature has on the observed composting time course (via k, D, C02), allows calculation of those aspects
arising from interactions between these fundamental impacts (via volume proportion Φm);
a full synecological approach. Because of this, the analysis framework is a robust analytical tool which contains a volume of compost at a known place in a composting particle whose energy density is known to high precision.
By enabling determination of parameters adjusted for the laws of physics, the analysis framework intensifies the experimental signal from any composting trial.
There is now a need to develop statistical tools to extract parameters, especially rate constants, from experimental data. Using statistical tools that utilise our considerable computing power, means these parameters can be accurately determined independently of the impacts of the laws of physics. The variation that remains in the experimental data can then be attributed to either microbial effects or secondary physical effects (such as substrate diffusion), or interactions between them e.g. pH effects.
Micro-environment analysis is a robust tool for analysis. Its robustness is such that the assumptions of micro-environment analysis, that is the occurrence and determination of spatial variability in substrate concentration at the sub-particle scale in a composting particle, are unaffected by introducing a diffusible substrate solution of diffusion laws. The computational complexity increases considerably, but the basic assumptions remain.
Similarly, other computational complexity, such as different microbial kinetic systems, toxicity effects consequent on microbial activity, Monod kinetics (substrate or oxygen based), mesh seamlessly with the current form of micro-environment analysis – they have an analysis space in which their (and their neighbours) parameters are known to high precision.
With better statistical tools and more experimentation using micro-environment analysis, the concerns of Schloss & Walker, (2001) re the statistical power of current composting experiments may be overcome.
It is suggested here that the fast fraction with a rate constant in the order of 1 d-1 is present in all composting materials and consists of cell contents, dead bacteria, and microbial breakdown products. However, this fraction may be present in such low proportions that its effect on the composting time course is not easily detectable, and consequently its effect is merged into current determinations of „fast‟ rate constants. It is further suggested (in agreement with Hamelers (2001)) that the hydrolysis rate constant largely determines the slow and humification fractions. The possibility that substrate types exist for which the rate constants fall within a range, such as suggested in this work for a faeces based
substrate, raises the possibility that fundamental differences between substrates exist only at the particle size and energy density aspects of these substrates. If this were the case, then considerable predictive power arises for a broad range of substrates.
The experimental evidence also points to the possibility that the oxygen diffusion coefficient can be determined from the composting time course. This could enable attribution of observed effects to their correct source e.g. an observed effect may be due to micro-porosity (influencing the diffusion coefficient) in a particle rather than substrate density.
The amount of the composting time course that can be explained by the physics of oxygen distribution, even with a simple microbial kinetics, strongly suggests that for composting, physics is a more important descriptor of the time course than biology.
What is less apparent, yet consequent from this work, is the possibility that with an analysis based on the laws of physics at a scale close to that needed for each microbe, composting systems could be optimised from the microbial perspective. Such an optimised composting system is likely to differ from current systems.
Acknowledgements
This research has been in large part a personal journey. Awareness of gaps in our scientific understanding of composting which discriminated against what I considered a desirable technology utilising composting, lead directly to the current thesis. People I have interacted with along this path have contributed in various ways, often indirectly. Warrick allowed me to use the workshop for building the reactors and helped with those little things that make experimentation a little easier, Kelvin for use of the coolroom, putting up with large polystyrene boxes occasionally half filling his lab and, when the refrigeration unit failed, exerted considerable effort, along with Don McKenzie to get a new unit fitted. Neil and Alistair for assistance with electrics and sensors.
My supervision team: Graeme Buchan, Keith Morrison and Mike Noonan provided the range of supervision needs. Keith for the big picture stuff and hours of non-thesis discussion which nevertheless filled parts of the framework, but will likely be most useful in the post thesis stage of life. Graeme for the detail, and uncovering some crucial mathematical errors and Mike for the microbial context.
Comments on drafts from Barbara, Shirley and Phil provided an extra-discipline view on what I had written.
To all my financial backers: Huntly and Pip; Phil and Shirley thanks for very reasonable accommodation. My tenants who tolerated deferred maintenance on the house so the cash book was a little better balanced. Mike for assistance with tuition fees.
As a mature student in my late 50‟s my motivations and experience were somewhat different from more traditional students, and in my case came with considerable non- scientific knowledge. This produced some tensions, for which the IP stance of Lincoln produced considerable grief, and almost caused the project to fail. It is however
symptomatic of wider changes in society and while it was not pleasant, it will remain one of those life experiences which will no doubt shape, in part the remainder of my life.
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