Constructed wetlands as biofuel production systems

Clean biofuel production is an effective way to mitigate global climate change and energy crisis(1). Progress has been made in reducing greenhouse-gas (GHG) emissions and nitrogen fertilizer consumption through biofuel production(2-4). Here we advocate an alternative approach that efficiently produces cellulosic biofuel and greatly reduces GHG emissions using waste nitrogen through wastewater treatment with constructed wetlands in China. Our combined experimental and literature data demonstrate that the net life-cycle energy output of constructed wetlands is higher than that of corn, soybean, switchgrass, low-input high-diversity grassland and algae systems. Energy output from existing constructed wetlands is similar to 237% of the input for biofuel production and can be enhanced through optimizing the nitrogen supply, hydrologic flow patterns and plant species selection. Assuming that all waste nitrogen in China(5) could be used by constructed wetlands, biofuel production can account for 6.7% of national gasoline consumption. We also find that constructed wetlands have a greater GHG reduction than the existing biofuel production systems in a full life-cycle analysis. This alternative approach is worth pursuing because of its great potential for straightforward operation, its economic competitiveness and many ecological benefits.