Physical Properties

Temperature

An increase in the temperature was clearly observed during the early phase of composting for all piles (see Figure 6-1). An internal temperature profile, characterised by an initial increase followed by a decrease before finally approaching ambient temperature, can be traced back to the typical rise and fall in temperature within the time expected in traditional windrow composting.

As Figure 6-1 shows, all piles exhibited typical composting temperatures, achieving thermophilic temperatures of more than 55°C and reaching approximately 67°C within two weeks, especially in pile P2 (thermophilic phase). Thereafter, the temperature declined slightly to around 60°C and remained above 50°C from week six to week eight

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(active phase), before dropping further during the second phase of composting (curing phase).

The addition of a bulking agent allowed for good aeration during composting, favouring microbial respiration and augmenting the exothermic activity of the decomposition process. As reported by Smith and Jasim (2009), the ideal thermophilic temperatures during a small-scale composting process are sometimes difficult to attain if only vegetable residues are composted.

Figure 6-1. Average temperature profiles during composting runs

As the process progressed, the temperature began to decrease gradually after five weeks and, with an ambient temperature, reached a constant level after twelve weeks. Notably, in P4 the short thermophilic development and continual temperature decrease indicated high initial C/N ratios (low nitrogen content) which indicates the carbonaceous degradation of raw compost material (Rynk et al., 1992;WSU, 2016).

In windrow composting, windrow size, turning frequency, initial C/N ratio, ambient temperature, moisture content and oxygen supply are among the variables that can affect the temperature (USEPA, 1985). According to USEPA guidelines for pathogen control, the thermophilic period for all piles was achieved during the active process (Tchobanoglous et al., 1993; Abbassi et al., 2015).

As the organic matter became more stable, the microbial activities and decomposition rate declined, and thus the temperature gradually decreased to the ambient level, marking the end of the active phase. The reduction of the pile temperature to that of the ambient temperature was evident in the last four weeks, indicating that the maturation of organic materials into biologically stabilised products was efficiently accomplished.

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Oxygen and carbon dioxide concentrations

Under aerobic conditions, emissions of carbon dioxide are a good indicator of the amount of degraded organic carbon. Therefore, the key factor in the composting process (i.e.

aerobic biological activity) can be monitored and optimised through the continuous measurement of oxygen levels. If the oxygen supply is limited, micro-organisms favour anaerobic conditions that have high odour potential (Haug, 1993; Abbassi et al., 2015).

The concentration of O2 and CO2 was used as an indicator for turning the piles, regardless of the turning schedule previously discussed in the research methodology. If the O2

concentration was found to be close to zero in any sampling location, turning was immediately applied to provide the microorganisms in the pile with the required oxygen.

As Figure 6-2 clearly shows, the initial concentration of oxygen within the body of the piles was very low due to the high rate of biological activity. This was also evident from the results of CO2 concentrations in the initial phases, which were very high. As the composting process progressed, the oxygen concentration increased and CO2

concentrations decreased. This is attributed to the decreasing rate of biological degradation (Haug, 1986; Abbassi et al., 2015).

Figure 6-2. Average O2 and CO2 concentrations for all piles during composting runs As shown in Figure 6-2, CO2 concentrations inside the entire pile immediately increased to around 20% (v/v) during the first week of composting. O2 concentrations exhibited the opposite trend, declining rapidly to the lowest level (0%) from the start of composting.

Thereafter, O2 concentrations gradually increased as the composting process progressed, reaching their highest levels during the curing phase. The change in trends regarding O2

and CO2 concentrations is attributable to vigorous microbial activity during aerobic composting. Therefore, concentrations of O2 and CO2 in an aerobic process serve to monitor the provision of sufficient oxygen to the piles via windrow turning practices.

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Bulky materials, such as woodchips, are often used to maintain structure and porosity because they decompose at a much slower rate than other carbon sources, such as sawdust. Furthermore, the structure of the plant residues used in P2, P3 and P4 were better than those of fruit and vegetables (P1). This was critical in providing porosity and, hence, aeration. Well-aerated mixtures result in low turning frequencies and high-quality products (Willson, 1993). The concentration of CO2 in the final curing phase of composting (approximately 3%) clearly demonstrates that existing biological degradation proceeds at a very low rate.

Moisture content

Moisture content (MC) is a critical parameter in the composting process. It influences the oxygen uptake rate, free air space, microbial activity and temperature of the process (Petric et al., 2012). According to Bernal et al. (2009), the optimal MC for effective composting depends on the waste type or form. They opined that the feedstock MC should be at 50–60%. To this end, moisture content levels inside the composting piles remained close to 50% to ensure high organic matter degradation with sufficient porosity and proper aeration and, consequently, aerobic degradation and composting throughout the entire experiment. Water was added to maintain the moisture levels at around 50% for the first eight weeks. This ensured optimum microbial activity and a sufficient oxygen supply. In general, the moisture content decreased gradually during composting, causing slow decomposition and low temperatures. The moisture content values ranged from 33 to 38%

for different types of compost (see Figure 6-3). The lowest value of moisture content (33%) was found for fruit/vegetables with plant residues (1:0) compost while the highest value of moisture content (38%) was obtained for fruit/vegetables with plant residues (1:3) compost.

Figure 6-3. Average moisture content during composting runs

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Large variations in the moisture content during the active composting phase, especially between week two and week eight, were attributed to the thermophilic temperatures caused by vigorous microbial activity, as well as the excessive frequency of turning which reduced the existing moisture content inside the piles. Moreover, higher porosity and aeration led to greater sensitivity in the variation of moisture (Abbassi et al., 2015). When the moisture of the compost decreased, limiting the process, the temperature decreased, which was especially evident in the treatment with the high bulking agent ratio (P3 & P4) where the highest decrease in temperature was observed; similarly, the temperature in P3 and P4 increased quickly when enough moisture was provided by watering once again.

When the curing phase began at the end of week eight, the moisture content in all piles gradually decreased to around 30%, especially in P3. By the end of composting, the highest water content, around 38%, was found in P4. This was attributed to the high initial C/N ratios (high carbon content) within the raw materials used, which reduced the rate of decomposition (Lynch and Cherry, 1996).

Bulk density, volume of piles and water consumption

As a result of the biological activities, organics in the composting material (substrate) were mineralised and transferred into stable materials and carbon dioxide. The consequence was a reduction in the piles’ volume (Figure 6-4). These results were found to be in agreement with the findings of Yue et al. (2008) and Abbassi et al. (2015). The pile volumes decreased in the range of 22–30% by the end of the composting processes (after twelve weeks), which was attributed to vigorous microbial activity within the pile (see Figure 6-4). This is crucial in further justifying the use of composting, as it means a 30% reduction in transportation requirements for composted material compared to non-composted and unstable organic material.

The results indicate that the bulk density value ranged from 340 to 603 kg m^-3 for different types of compost. The highest bulk density (603 kg m^-3) was found for P1, which was formed from one portion of fruit and vegetables and zero portions of plant residues (1F&V: 0P). The lowest value of bulk density (340 kg m^-3) was found for P4, which was formed from one portion of fruit and vegetables and three portions of plant residues (1F&V: 3P). Hurerta-Pujol et al. (2010) found that bulk density values ranged between 350 and 502 kg m^-3 for different compost types, results supported by those of Raviv et al.

(1987), Larney et al. (2000), Mohee and Mudhoo (2005) and Romeela et al. (2008).

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Figure 6-4. Pile volume and bulk density during composting runs

This reduction in the volume of material was offset by an increase in bulk density, where an approximate 15% increase was observed (Figure 6-4). This can be attributed to the fact that the bulk density of compost increases as total organic matter decreases (Figure 6-5).

Conversely, the bulk density of compost decreases as the total organic matter increases.

For instance, bulk density increases from 340 to 603 kg m^-3 when the total organic matter decreases from 39.7 to 24.3%.

Figure 6-5. The relationship between bulk density and total organic matter

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The relationship between the bulk density of compost and the total amount of organic matter within the compost is shown in the following equation:

BD = -17.694 TOM + 1039 R² = 0.90 (4) Where:

BD is the bulk density (kg m^-3) TOM is the total organic matter (%)

The internal heat generated due to microbial activity and the ambient environment affected the provision of optimum moisture levels inside the composting piles. A summary of the initial volume of organic material, final composting volume and water added volume is shown in Figure 6-6. This shows that the amount of added water (in volume) was more or less equal to the amount of organic fertiliser produced (in volume), therefore a 1:1 ratio can be assumed. This means that the ratio of water added in relation to the amount of raw composting material was about 60% for the four piles, which is in line with Abbassi et al.’s (2015) findings.

Figure 6-6. Pile water consumption during composting runs

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