Closing the loop - Recovery of nutrients and energy from wetland biomass

Abstract The aim of our research was to estimate the amount of energy, C, N and P that potentially could be extracted from the plant biomass harvested annually in the Narew National Park (NE Poland). We found that the conversion of biomass through anaerobic digestion (AD) delivers not only power and heat but also ensures the recovery of nutrients and organic matter for their reuse in agriculture. The most productive community with the highest biomass yield was reed bed of Phragmites australis (9.78 ± 1.66 t d.w. ha−1). It was followed by Phalaridetum and tall sedge communities with Carex elata and C. gracilis (6–7 t d.w. ha−1), rushes of Glyceria maxima (3.61 ± 0.63 t d.w. ha−1), and sedge moss communities with C. lasiocarpa (~1 t d.w. ha−1). Annual vegetation management would result in ~33·103 t d.w. of biomass. The primary net energy output was related to the productivity of the vegetation and ranged from ~17 GJ ha−1 for the sedge moss communities to 160 GJ ha−1 for Phragmitetum australis. The methane potential of the studied species was low and varied between 89 Nm3 t−1 d.w. for Phragmites and 188 NL kg−1 d.w. for Glyceria maxima. Consequently, the unit related net gain of energy in CH4 ranged between 23 and 30 GJ ha−1 for the perennial grasses and tall-sedge communities, and 3.5 GJ ha−1 for the low-productivity sedge-moss communities. As a result of low energy inputs for harvesting, the wetland biomass exhibited a favorable energy balance. The annual potential of CH4 for the whole studied valley section was 3286 ± 439·103 m3 resulting in net electricity potential of ~9.1 GWh and waste heat –32.3 TJ. An “average” hectare of wetland could power 1.2 household and provide 1010.4 ± 169.4 kg of C, 117.8 ± 19.6 kg of N, and 11.9 ± 2.0 of P applied do crop soils along with a digestate.