Digestion anaérobie des déchets de bovins mélangés au macrophyte "Pistia stratiotes"

Résumé

Rural populations (12.7 million in Morocco) use either forest wood and straw or fossil fuels as energy sources for cooking and household needs. It is essential to develop alternative energy sources, as fossel fuels are becoming more expensive and natural forests have to be protected from further degradation. One alternative that is used throughout the world involves the production of biogas via anaerobic digestion of cattle manure. We studied how to improve the performance of the cattle manure fermentation process through the addition of ground plant material, especially water lettuce (Pistia stratiotes). This aquatic plant can be grown on effluent ponds, with the following advantages: (1) purification of effluents through active absoption of organic matter and minerals; (2) use as animal feed; and (3) enhancement of biogas production by grinding this material and adding it to the fermentation substrate. In these experiments, a series of continuous fermentors were used with mixtures of cattle manure containing 12.5%, 16.6%, 25% or 50% of Pistia stratiotes. The best biogas yields were achieved with a proportion of 50% of water lettuce in the mixture, giving a biogas yield of 0.82 m3.m3.d, with a methane content of 76.8% and a 15-day hydraulic retention time, at a constant temperature of 35° C. The kinetic study showed that the process is well represented by the Monod model (Table 2). The performances were much better than those obtained with anaerobic digestion of cattle manure alone (Table 1). The improved performance can be explained by the mineral and protein enrichment of the substrate provided by the water lettuce, thus enhancing the buffering potential of the digestors and increasing the CH4 content in the biogas. This is due to a bacterial consortium adapted to the substrate, which balances the pH and efficiently degrades the volatile fatty acids (VFA) (Figures 2 and 3). Indeed, the best biogas production corresponded to the lower VFA concentration obtained with 50% ground water lettuce.