Constructed wetlands for resource recovery in developing countries

Constructed wetlands (CW) are effective in treating wastewater, particularly in settings that require low technology and low maintenance as operational constraints. Biomass harvested from CW can be used as a renewable energy source and treated effluent can provide irrigation for agricultural uses. Biomass yields for four selected wetland plants in CW, namely Phragmites spp., Typha spp., A. donax, and C. papyrus, ranged from an average of about 1500 g of dry mass per square meter (g/m2) for Typha spp., up to 6000 g/m2 for A. donax. The energy yield for direct combustion of these plants occupied a narrow range, averaging about 18 megajoules per kilogram of dry mass (MJ/kg) for all plant types, a comparable amount to Acacia spp. Methane yields varied from about 170–360 L of methane (normalised to standard conditions) per kilogram of dry mass (LN/kg). 1 m2 of CW planted with A. donax can produce on average 110 MJ through direct combustion or 1660 L of methane from biogas production. In a village of 200 people the biomass from a CW planted with Typha spp. can reduce cooking fuel needs by 4–55% and therefore save up to 12 ha of forest per year. The water footprint of these plants was measured as the percent loss in water in the CW from evapotranspiration (ET). Under a fixed set of assumptions on climate and operation, the water used through ET, the CW could deliver from 64% to 76% of the influent water for subsequent use. In summary, CW have the potential to offset energy and irrigation needs at scales ranging from small communities to peri-urban areas. Constructed wetlands used to treat wastewater have the potential to provide a sustainable bioenergy source without placing burdens on water resources or displacing other food or energy crops.