Leaves of Mangifera Indica (MI, mango leaves) and Manihot Utilissima (MU, cassava leaves) are available in tropical regions and are the most accessible vegetal wastes of Kinshasa, capital of Democratic Republic of Congo. These wastes are not suitably managed and are not rationally valorized. They are abandoned in full air, on the soil and in the rivers. They thus pollute environment. By contrast, they can be recuperated and treated in order to produce methane (energy source), organic fertilizer and clean up the environment simultaneously. The main objective of this study was to investigate methane production from MI and MU leaves by BMP tests at 30°C. The yields achieved from the anaerobic digestion of up to 61.3 g raw matter in 1 l medium were 0.001 l/g and 0.100 l CH4/g volatile solids of MI and MU leaves, respectively. The yield of MU leaves was in the range mentioned in the literature for other leaves because of a poor presence of bioactive substrates, and low C/N ratio. This methane yield corresponded to 7% of calorific power of wood. By contrast, the methane yield from MI leaves was almost nil suggesting some metabolism inhibition because of their rich composition in carbon and bioactive substrates. Whereas classical acidogenesis and acetogenesis were recorded.Therefore, methane production from the sole MI leaves seems unfavorable by comparison to MU leaves at the ambient temperature in tropical regions. Their solid and liquid residues obtained after anaerobic digestion would be efficient fertilizers. However, the methane productivity of both leaves could be improved by anaerobic co-digestion.

Vegetal waste and some wastewater of agro-food industries contain plant secondary metabolites (PSMs). It was showed in nutritional researches that these substances such as saponins and tannins reduced the methane production in the rumen. To our knowledge no study was done in the waste treatment domain to evaluate the inhibitory effect of the principal glycosidic metabolites from the wastewater or vegetal waste on their own methaneproducing anaerobic digestion. Therefore in this paper BMP tests were carried out at 30°C with four commercial PSMs (CPSMs) in mixture with glucose monohydrate (Gl) used as control sample. These CPSMs were saponin from Quilaja Saponaria Molina Pract (Sap), tannic acid (Tan), salicin (Sal) and aloin from Curacao Aloe (Alo) representing respectively saponins, tannins, alcoholic glycosides and anthraquinones sources. Acidogenesis and acetogenesis were recorded for all the mixtures of Gl and CPSMs; however their conversion rates decreased with the increase of the concentrations of CPSMs. By contrast, the methanogenesis was inhibited at concentrations of CPSMs above 0.3 g/l. The inhibition degree for aromatic compounds on the anaerobic biodegradation of Gl seemed directly to depend on the numbers of benzene rings in the medium and the synergism. Thus, the highest inhibition of the biogas production from Gl was recorded for Alo, followed by Sap, Tan and Sal. However, the highest inhibition of the methane production from Gl was recorded with Sap, Alo, Tan and Sal. It was supposed that the toxicity potentials of these PSMs on the own biomethanization would be in following decreasing order: Sap or Alo, Tan and Sal.Therefore, the concentration of PSMs alone or in mixture in a digester should be below 0.3 g/l. for a better methanization.