The effects and consequences of anaerobic digestion on organic matter and nitrogen in pig slurry

Abstract

Pig slurry is an abundant resource in terms of mass (around 18 million tonnes produced per year in France). A field experiment carried out on the site of the Guernevez pig station (Chamber of Agriculture of Brittany) made it possible to compare the fate of the main elements of the slurry (carbon, nitrogen and phosphorus) and of two indicator germs of treatment, depending on whether it follows a traditional management method or undergoes a passage through a mesophilic digester. Spontaneous anaerobic digestion in slurry pits causes methane emissions into the atmosphere over the course of months. Passing through a digester accelerates this phenomenon while recovering the biogas emitted. For slurry, only 22% of organic matter is degraded. In terms of energy, the benefit is modest since the pig slurry is a diluted effluent (92 to 96% water) with a limited energy production potential. The addition of 3% by mass of a cookie factory residue has doubled the production of biogas per tonne of input. In this case, more than 90% of the organic matter of the co-substrate has been degraded so that the composition of the digestate differs little from that of the slurry digestate. Regarding nitrogen, there is a slight increase in the ammoniacal fraction (72% for the digestate against 65% for the slurry), but above all a net increase (doubling) of losses by volatilization, both during storage and spreading, correlated with an increase in pH from 7.2 for slurry to 8.2 for digestate. On phosphorus, the effects are insignificant. The dissolved forms are barely more abundant in the digestate and the mineral fractions precipitated in the form of fine particles remain dominant (80%). From an agronomic point of view, in this particular case, the slurry digestates alone or mixed with the by-product of the agro-food industry have characteristics similar to those of raw slurry after storage, apart from the increased risks of ammonia volatilization mentioned above. Their use for fertilizing maize did not give rise to significant differences in yields or in nitrous oxide emissions. Some differences in the kinetics of organic nitrogen and carbon mineralization were however observed.