Water for small-scale biogas digesters in sub-Saharan Africa

Biogas could provide a more sustainable energy source than wood fuels for rural households in sub-Saharan African. However, functioning of biogas digesters can be limited in areas of low water availability. The water required is approximately 50dm(3)day(-1) for each cow and 10dm(3)day(-1) for each pig providing manure to the digester, or 25 (+/- 6)dm(3)day(-1) for each person in the household, using a digester volume of 1.3 (+/- 0.3)m(3) capita(-1). Here, we consider the potential of domestic water recycling, rainwater harvesting, and aquaculture to supply the water needed for digestion in different countries of sub-Saharan Africa. Domestic water recycling was found to be important in every country but was usually insufficient to meet the requirements of the digester, with households in 72% of countries need to collect additional water. Rooftop rainwater harvesting also has an important role, iron roofs being more effective than thatched roofs at collecting water. However, even with an iron roof, the size of roof commonly found in sub-Saharan Africa (15 to 40m(2)) is too small to collect sufficient water, requiring an extra area (in m(2)) for each person of (R/100) (where R is the rainfall in mm). If there is a local market for fish, stocking a pond with tilapia, fed on plankton growing on bioslurry from the digester, could provide an important source of additional income and hold the water required by the digester. In areas where rainfall is low and seasonal, the fishpond might be stocked only in the rainy season, allowing the pond to be covered during the dry period to reduce evaporation. If evaporative losses (E in mm) exceed rainfall, an extra catchment area is needed to maintain the water level in the pond, equivalent to approximately (1.5x((E-R)/R))m(2) for each person in the household.