Effect of prehydrolysis of milk fat on its conversion to biogas.

Milk fat is considered to be the main limiting component of the kinetics of dairy wastewater anaerobic digestion. The objective of this work was to give a better understanding of the nonelucidated anaerobic degradation steps of milk fat. For that purpose, the kinetics of fat degradation was quantified in comparison with other milk components (lactose, proteins), regarding the milk fat polluting load and structure [globular (native state), triglycerides]. This work confirms that milk fat is degraded after a lag phase of several days, with a maximal degradation rate 2 to 5 times less than the degradation rate of the other milk components. It was shown that (1) the structure of the fat does not influence the limits of its anaerobic degradation; (2) the lag phase before biogas production is mainly due to unsaturated free fatty acids (FFA); and (3) conversion to biogas occurs at a lower rate for saturated than for unsaturated FFA. Therefore, the prehydrolysis of fat, which increases the instantaneous concentration of unsaturated FFA, sharply increases the length of the lag phase with no significant change in the maximal biogas production rate. To reduce the delay imposed in the biogas production, it is necessary to reduce the concentration of unsaturated FFA.