Gaseous emissions and process development during composting of pig slurry: the influence of the proportion of cotton gin waste

Abstract

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 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.