If a particle were fully aerobic there would be only one micro-environment and spatial variation would not arise. With oxygen penetration depths of only mm scale, most particles will contain an anaerobic core. However, particle geometry effects discussed in Chapter 4 mean that even though small particles may have an anaerobic core, the volume
proportion of this core is small and the micro-environment effect is not strongly expressed. The experimental strategy has three parts:
1) Particle size trial, where the effects of micro-environment volumes and their relationship to both their location within the particle and the particle radius can be determined with micro-environment analysis, and this prediction tested
experimentally.
2) Temperature trials, where the temperature effect on the impact of changing rate constants on VOR (and hence oxygen penetration depth) is tested experimentally. 3) Diffusioninto a composting pile, where the composting time course was monitored
at various depths in the pile.
In both the particle size and diffusion trials the same substrate, cubical cut dog sausage, was used. The temperature trials by contrast utilised pig faeces which necessitated some extra considerations:
When mixed with the bulking material a range of particle sizes resulted,
necessitating extension of micro-environment analysis to accommodate the range of particle sizes.
Consideration of the micro-scale distribution of the „substrate‟ within the „mix‟.
The effect of temperature on the growth phase as well as the rate constant.
5.4.1 Particle Size Trials
Dog sausage „Bruno - Beef, rice and vegetable‟ was used as substrate and was cut with a
sharp knife into cubical particles. Five particle sizes were cut from the same sausage (0.8 cm, 1 cm, 1.5 cm, 2 cm, 2.5 cm). The smaller sizes were initially cut into double sized slices and these were then halved at the appropriate stage (cutting a 1 cm slice off a sausage caused variation that was substantially reduced by cutting a 2 cm slice which was later cut in half). For the 0.8 cm size, a reactor load of 1 cm particles were „chopped‟, leading to a range of sizes but all < 1cm.
Old moist compost (particle size proportions noted in Table 5-3) served as both bulking material and inoculation. Two replicates were done using the same brand of dog sausage but likely to be different batches. Typically 700 grams of dog sausage were mixed with
170 grams of old compost, a mixing ratio (R) of 0.81. 500-600 grams of mixture (1.1 litres) was added to each reactor. Early trials had found the increased moisture content at the ends of the reactor (discussed above), combined with „liquefaction‟ of substrate
particles had caused some particles to merge (formation of secondary particles).
Consequently, the second particle size trial contained added moist sawdust but maintained the same ratio of sausage: old compost. The weight of dog sausage in each reactor was similar in both trials, but due to the added sawdust the weight of mixture added to the reactor was higher (750 g - 1.6 litres) and the proportion of this that was sausage (Rsausage)
was lowered to 0.6. The FAS also changed with the different particle sizes (Table 5-2). Trials were run for 40 days at a compost temperature of 16 °C. All reactors were started and finished at the same time. Particle sizes were rotated around the reactors in
subsequent trials to eliminate reactor differences. Measurements of MC and ash of the initial components and the mixed compost were done. From this, the volume of the dog sausage within the reactor was determined.
Micro-environment analysis was done on an equivalent particle radius being the radius of a spherical particle which would contain the same volume of compost as the cubical cut particle Table 5-2.
Table 5-2 – The equivalent spherical diameter and FAS of the cubical particles.
Cubical cut size
(cm) 0.8 1 1.5 2 2.5 Equivalent spherical diameter (cm) 1 1.2 1.9 2.5 3.1 FAS % 61.3 59.2 60.0 52.3 52.2
5.4.2 Temperature Trials
Pig faeces were collected over a period of several days from pigs living in a paddock but fed grain and food scraps.
All the faeces were mixed with old compost as bulking material. The mixture was divided into 5 equal sized parts, from which the amount required for each reactor was measured and added to the reactor. The residue (~650 ml) was oven dried (103 °C for 24 hours), sieved and ash determined at 550 °C. Free air space was 59.46 ± 0.29%
Ten particles (a range of sizes) where chosen for diameter measurements. These particles were held on small pieces of paper with double sided sellotape. A vernier scale was used to measure two diameters (typically maximum and minimum) at a point marked with white-out liquid paper and at each measurement the paper and particle were weighed. The sample then was air dried until the next measurement. The final measurement was done on an oven dried sample. The moisture content at each measurement could be determined from the weight difference from the oven dry sample. Regression analysis determined the relationship between wet particle diameter and dry particle diameter, where particle diameter was assumed to be the average of the two measurements. The
regression was then applied to the average sieve size (the size retaining the sample and the next larger size used) of the oven dried sample to determine the diameter of the wet
particle.
Sieve sizes were 16 mm, 8 mm, 4 mm, 2 mm, 1 mm and „dust‟.
The characteristics of the bulking material and mixture for pig trial 2 can be seen in Table 5-3.
Table - 5-3 The % of oven dry sample retained for each sieve size (bulking material and each trial temperature mixture) – pig faeces trial 2.
Sieve Size (mm) 16 8 4 2 1 Dust Bulking material 0 0 19 32 23 26 T6 30 22 27 16 5 2 T9 31 22 25 15 5 2 T12 29 24 26 15 5 2 T16 38 19 21 15 5 2 T20 28 29 21 14 5 2 Trial average 31 23 24 15 5 2
5.4.3 Diffusion into the Pile Trials
Three litres of the small particle size compost, composed of 0.8 cm dog sausage, old compost plus extras to give a good porosity (the FAS was 51.3 %), were loaded into a reactor with no mesh manifold. The reactor was stood on end and the restricted airflow was supplied to the upper part of the reactor. The reactor housing temperature was held at 16 °C and the compost temperature allowed to rise.
The temperature „bulge‟ that formed as oxygen penetrated further into the compost was monitored by an array of 5 thermocouples spaced evenly (each 70 mm apart) down the reactor (Section 5.2). The temperature difference between each array and the average of all arrays was used to determine the relative composting rates – rapidly composting parts appeared as strongly positive, while anaerobic and exhausted compost were strongly negative.