Influence of Treatment Techniques for Pig Slurry on Methane Emissions during Subsequent Storage

Methane (CH4) is an important greenhouse gas and recent inventories have suggested that livestock manure management systems greatly influence the global CH4 emissions. The present study was conducted to evaluate this assumption using pig slurry as the representative livestock manure. CH4 emissions rates were determined through a series of four laboratory experiments comparing, during a 50 day storage monitoring period, the evolution of a range of raw, separated, dilute, aerated or chemically amended slurries. The experiments were performed using 60 l storage reactors equipped to allow a continuous measurement of the biogas (CH4 and CO2) emitted. The laboratory procedure based on a discontinuous replenishment of the air above the stored slurry, enabled the production curve for carbon emissions to be established. A range of pig slurries were studied including four raw slurries differing in their dry matter content, two dilute slurries (slurry to water ratios of 1:1, 1:2), two differently separated slurries (screened or centrifuged), two aerated slurries, and three slurries chemically amended by the use of commercial additives. From raw slurries, the average daily rate of CH4 emission comprised between 9 and 77 g [CH4 C] m−3 [slurry] day−1, cumulating in a total CH4 emission of up to 3·6 kg [CH4 C] m−3 [slurry] for the 50 day monitoring period. The percentage of carbon emissions (CH4 C+CO2 C) varied from 5 to 30% of the total initial carbon content. Biogas emissions from separated slurry were rather similar to that of the raw slurries. The dilution of the slurry tended to decrease the production and the emission of CH4. The two aerated slurries had very low CH4 emissions, showing a 70% to near 100% reduction of emissions compared to the untreated slurry. The use of the three additives lowered the CH4 emissions from 47 to 64%. For all experiments, the physico-chemical changes of the slurries during storage is reported including total solids, total carbon, chemical oxygen demand, biochemical oxygen demand and the volatile fatty acids.