Explanations of the Rewarming Phase

The larger particle sizes in the dog sausage trials (discussed in Section 6.1) produced a rewarming part of the experimental time course that could not be explained by micro- environment analysis in its current form.

Two explanations for this rewarming need consideration:

 Diffusible substrate.

 Alternative electron acceptors.

6.1.7.1 Diffusion of Substrate

Alternative diffusion law solutions that may explain the rewarming section in Figure 6-2 include the possibility that diffusion of the substrate from the centre of the particle to the oxygenated surface is occurring.

For these trials, the energies released by the end of the 40 day composting period are comparable for all particle sizes, suggesting that substrate in the anaerobic core is being accessed (Table 6-4). If this were not the case, then the energy released would have closely matched the aerobic proportion.

Table 6-4 – Measured total energy released after a 40 day composting period; and modelled aerobic proportion.

0.8 cm 1 cm 1.5 cm 2 cm 2.5 cm

Dog sausage trial 1

(MJ L-1) 6.9 7.2 6.0 6.3 5.5

Dog sausage trial 2

(MJ L-1) 6.8 7.0 6.1 6.7 6.3

Aerobic proportion @

30 days 0.96 0.9 0.7 0.6 0.5

However, the data in Figure 6-2 and Figure 6-3 would seem to imply that diffusion of substrate would occur only after some days of composting (as solutions without that diffusion explain the early data so well). In this respect, Hamelers‟ solution would contain an implicit delay in the substrate diffusion effect, as the enzymes necessary for

solubilisation would need to be produced before sufficient solubilised substrate could diffuse to the aerobic zone and be detected experimentally.

Additional support for the diffusible substrate argument would be the observation made at the end of composting that the larger particles had „liquified‟, often fusing together

forming „secondary‟ particles (especially at the reactor ends where moisture contents were higher due to the temperature profile along the reactor). It was this linking of „liquified‟ particles that prompted the addition of sawdust to the second dog sausage trial to attempt to maintain particle integrity throughout the composting period.

However, with a diffusible substrate the essence of micro-environment analysis would still apply, that is limited, but gradually increasing, oxygen diffusion distance in conjunction with particle geometry effects. It is only the time course of E(t) which would differ with a diffusible substrate, as oxidised substrate would be replenished by diffusion from the core.

Micro-environment analysis is primarily an organisational framework based around diffusion laws and microbial kinetics. The composting space-time elements of micro- environment analysis have no particular affiliation to the steady-state, non-diffusible substrate diffusion law solutions used in this thesis. There is no reason why diffusion law solutions that allow for a diffusible substrate could not be used in the micro-environment organisational structure. So long as a solution contains an oxygen penetration depth, or a method of determining an oxygen concentration below which aerobic composting can be assumed to be zero, it would mesh with micro-environment derivations, as oxygen penetration depth is necessary to determine micro-environment thickness, z.

Micro-environment analysis however, would not explain alternative electron acceptors that do not need to diffuse (such as nitrates in the original substrate) – discussed further in Section 6.1.7.3.

6.1.7.2 Oxygen Flux Insights into the Diffusible Substrate Question

The surface flux of oxygen approach to determining composting rate (mentioned in

Chapter 3 and used by Hamelers, 2001) can give insights into the substrate diffusion effect. For a substrate with particles of a known size, for which the composting rate has been measured, then the stoichiometric relationship between heat output and oxygen

consumption means that the measured composting rate can be used to determine an oxygen consumption rate. Combining this with the known surface area: volume ratio, determines

the surface flux of oxygen, assuming all the heat released is from use of oxygen as an electron acceptor and that oxygen diffuses into the particle - Equation 6-6.

Equation 6-6 Particle Particle SA V Q Flux  0.0627 mg O2 cm-2 s-1

The flux can also be determined from knowledge of VOR, D, and CO2 ((Bouldin, 1968)):

Equation 6-7 VOR C D Flux  2  0 mg O2 cm -2 s-1 0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Day m g O 2 c m -2 s -1

Experimental (Equation 6-6) Substrate flux (Equation 6-7) Difference

Figure 6-8 - Dog sausage trial 2, the 2.5 cm cubical particles composting rate expressed as a surface oxygen flux, compared with the output of Equation 6-7, where VOR is determined by the MEA model.11

If the VOR used in Equation 6-7 were based on first-order kinetics then the time course of the calculated flux can be taken as that flux which would occur if there were no diffusion of substrate into the aerobic zone.

It follows that the difference between the time course data from experiment (using oxygen flux as determined by Equation 6-6and the measured Q), and the flux determined with

11 _{Note: the oxygen diffusion coefficient in Equation 6-7 needed to be changed to get the graph, using the}

micro-environment model parameters, to approximate the data curve in the first stage of composting (assuming substrate diffusion does not begin at this stage).

Equation 6-7 (using VOR based on first-order kinetics), could be taken as an indication of substrate which is diffusing into the aerobic zone (Figure 6-8).

While the above flux based analysis has a known flaw in that only a single VOR can be used in Equation 6-7, it does provide evidence of the diffusion of substrate from the core of a composting particle into the aerated layer, as proposed by Hamelers (2001). It also provides another package of tools giving insights into the dynamics occurring in a composting particle.

6.1.7.3 Other Electron Acceptors as Explanation of the Rewarming

That the additional heat is from electron acceptors other than oxygen must be considered as a possible (partial) explanation for the observed difference between micro-environment explanations and actual data. Bruno Dog Sausage contains sodium nitrite (and potassium sorbate) and this can be used as an electron acceptor by microbes. These electron

acceptors would release heat, and hence would be detected by the experimental apparatus. As these electron acceptors would be distributed throughout the sausage they would not be bound by diffusion laws, meaning that the whole particle will contribute to the observed composting rate. Micro-environment analysis would not apply to these effects.

The delay in expression of the observed effect can also be explained by an alternative electron acceptor, as microbes utilising this substrate would need to build up in the core of the particle before detectable heat is released from use of this electron acceptor.

Some explanation is required for the observations and either (or both) substrate diffusion from the core of the particle, or (and) nitrite oxidation could explain the disproportionate release of energy with the larger particle sizes. The experimental setup was not able to distinguish between these alternative explanations.