The effect of incubation conditions on the laboratory measurement of the methane producing capacity of livestock wastes

The effects of incubations conditions (dilution, mixing, incubation time and inoculum amount and origin) on the determination of the maximum methane producing capacity (B0) from various livestock slurries were evaluated. For this purpose, the methane yields of different livestock slurries were determined using batch anaerobic incubations performed at 30 °C as regard these different conditions. The B0 and the methane (CH4) generation as a function of time were used to study the processes and to determine the best incubation conditions. Methanogenesis was identified as the major rate-limiting step during the anaerobic degradation of slurries, probably due to inhibition by volatile fatty acids. In some cases, high free NH3 concentrations were suspected to inhibit the hydrolysis process. The addition of inoculum and/or the dilution of the substrate reduced the inhibition and allowed to reach the B0 more rapidly. However, the addition of inoculum must be minimized to reduce the possible errors made by considering a similar production by the inoculum with and without the substrate. All experiments performed during this study allowed to define the incubation conditions required for the determination of the B0 from livestock slurries. Applying these conditions, the B0 values determined for swine slurries varied from 244 to 343 L CH4kg Vadded-1, from 204 to 296 L CH4kg Vadded-1 for dairy cattle slurries and equalled 386 and 319 L CH4kg VSadded-1 respectively for calves and duck slurries.