Energetic Balance of Biogas and Fertilizing Potential of Digestate from Anaerobic Digestion of Manihot Utilissima

Abstract

Optimal scale production of biogas from the anaerobic digestion (AD) of organic wastes (OWs) as substrates has become one of stimulating environment-friendly procedures to foster the replacement of fossil energies by renewable ones. AD of OWs generated from households is desirable as an effective method to fight against environmental pollution effects of the latter in developing countries. Notably in Africa where each year there are more than 600,000 premature deaths following the use of solid biomass energy (especially charcoal). Yet, if assessing the potential of biogas production, that is, biochemical methane potential (BMP) has played a prominent role in the choice of substrates, in the other hand the evaluation of the optimal ratio between the quantity of variety OWs and the amount of energy to be produced has not received several attentions. The latter is valuable not only for energetic productivity but also for profitability. Thus, in this report, an energetic balance between the amount of leaves and stems of Manihot Utilissima (MU) annually produced as well as the energetic potential of their biogas were investigated. Cow manure (CM) was employed as inoculum, under mesophilic conditions of the collection sites: Ngaba and Ndjili in Kinshasa City Province (KCP), Congo DR, where the leaves and stems of MU are among the most generated wastes. Furthermore, we evaluated the fertilizing potential of digestates from the AD of the leaves and stems by their Carbon/Nitrogen (C/N) ratios.
 The annual energetic potentials of biogas produced were estimated to be 1.362 ± 0.028 109 kWh for the leaves and 0.337 ± 0.006 109 kWh for the stems. These were associated to the energy needs for the KCP households corresponding to the use of charcoal. The latter was evaluated to be 166 103 tons for leaves and 41 103 tons for the stems of MU, respectively.
 The fertilizing potential of digestates from the AD of the leaves and stems of MU assessed by their C/N ratios were determined to be 5 and 10, respectively; indicating that they are favorable for the cultivation of vegetables and fruit trees in KCP soils (C/N~5) but also optimal for the organisms, soil conditioning and could improve the soils hydraulic conductivity (C/N~10).