2.2. Composting process
Two different composting mixtures were prepared with the following composition (dry weight basis):
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PM: poultry manure (78%) and barley straw (22%) as bulking agent.
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PM + B: the same PM mixture with 3% of added biochar.
To ensure the homogeneous composition of the piles, an initial mixture of poultry manure with barley straw (12,000 kg on a fresh weight basis) was prepared. After that, one half was amended with the biochar. Finally, each treatment (PM and PM + B) was divided into two piles (3,000 kg each per pile; fresh weight).
The mixtures were composted, in trapezoidal piles of 1.5 m high with a 2 × 3 m base in a pilot scale plant, by using the turned pile (windrow) system. The piles were turned weekly using a front-end loader tractor during the active phase of the process (approximately 90 days, Fig. 1) in order to homogenise and aerate the mass, reactivate the process and avoid compaction. Moisture level was maintained during the process between 40% and 60% by adding water periodically. The temperature was measured by inserting a thermocouple into four different points of the pile every three days during the bio-oxidative phase and fortnightly afterwards. The total duration of the composting process, including the maturation phase, was 135 days.