Concerns about energy supplies and negative environmental consequences of fossil fuels have spurred the search for renewable biofuels. Biofuels may offer a promising alternative to fossil fuels, but serious concerns arise about the adverse greenhouse gas (GHG) consequences from using nitrogen (N) fertilizer. Attempts and progresses worldwide have been made to reduce N use, such as switchgrass and low-input high-diversity (LIHD) grasslands or even to reuse waste nitrogen through microalgae. However, such biofuel generation systems are not as ‘clean’ as expected, still be N-dependent. Hence, new ideas that produce biofuels using from the municipal waste via constructed wetlands (CW), can provide energy without N requirement (or N-neutral). In a CW, biomass is produced together with the pollutants (especially waste N) removal, and thus can be harvested and used as biofuel. In this paper, we summarized the potential aboveground biomass yield from CW plant evaluated as cellulosic ethanol bioenergy production, and combined the life-cycle analysis with mass balance approach to estimate the energetic, environmental and economic performance of CWs, compared with other biofuel production systems.
Results/Conclusions
Our combined experimental and literature data results showed that annual biomass yields of CWs averaged 37813 kg ha-1yr-1 as the by-product of treating waste N, about one order of magnitude larger than traditional biofuel production systems (corn, soybean, switchgrass, LIHD grassland). CWs have 4-17 folds higher energy yields than other biofuel production systems supported by plenty N in wastewater supply. Biomass yield in CW produce 500% more renewable energy than energy consumed during production and have life cycle environment benefits of 8.8 Mg CO2 equivalent ha−1 yr−1GHG emission reduction. Moreover, constructing CW is more economical feasible than wastewater treatment plants (WTP), since the cost of building a CW (13.6 to 70 yuan RMB per kg N) is only one-third to one-half the cost of a WTP (37.5 to 135 yuan RMB per kg N) and the operation cost of CW is about one-tenth to the cost of a WTP (approximately 0.5 yuan to 5 yuan RMB per kg N of wastewater, compared to 13.75 to 37.5 yuan RMB per kg N for WTP).
This study demonstrate that plants grown and managed in CW as biomass energy crop produce more renewable energy than energy consumed in its production and has significant environment benefits, reducing agrichemical use especially N fertilizer compared with traditional biofuels, suggesting itself the potential for another source of biofuel feedstocks.