Composting is a feasible, environmentally-friendly management tool for the treatment
of animal manures based on nutrient recovery. However, the liquid character of the pig
slurry, with low dry matter, requires previous solid-liquid separation and the mixture of
the solid fraction with an adequate bulking agent in the correct ratio. This work studies
the influence of the bulking agent proportion on the composting of the solid fraction of
pig slurry, focussing on the development of the process and on the greenhouse gas
emissions. For this, two mixtures of the solid fraction of pig slurry and cotton gin waste,
in different proportions, were prepared (4:3 and 3:4 solid fraction of pig slurry:cotton
gin waste, v:v) and composted by the Rutgers static pile system in a pilot plant (2000 kg
each pile). The temperature profiles of the composting piles were similar, but the pile
with the greater proportion of bulking agent showed higher temperatures and a longer
thermophilic phase, with greater aeration demand. The specific heat capacity of cotton
is lower than those of other bulking agents used frequently for animal manure
composting, which means that this material requires less energy from microbial activity
for its temperature to increase. However, the easily degradable organic matter present in
the pig slurry explained the faster organic matter mineralisation occurred in the pile
with the higher proportion of solid fraction of pig slurry; in pile with higher proportion
of bulking agent, the mineralisation process was slower due to the lignocellulosic
character of the cotton gin waste. The organic matter mineralisation was closely related
to the gaseous emissions (mainly as carbon dioxide): significant methane emissions
were detected when the highest organic matter degradation occurred in the pile with the
greater proportion of solid fraction of pig slurry, together with the highest nitrogen
oxide emissions - indicating the existence of anaerobic pockets within the composting
mass. So, it can be concluded that the thermal properties of the bulking agent were
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responsible for the temperature development and aeration demand, while the gaseous
emissions were related to the organic matter degradation process. The composts
produced were stable, with a good degree of maturation; the compost with the higher
proportion of solid fraction of pig slurry had greater organic matter humification and
higher nutrient concentrations