Environmental and energy assessment of the substitution of chemical fertilizers for industrial wastes of ethanol production in sugarcane cultivation in Brazil

Purpose

Vinasse and filter cake are residues of bioethanol processing that are used to be recycled as fertilizers in sugarcane plantation. Studies related to the environmental dimension on this practice are concerned only with the effects on water and soil. The present study examines the systemic effects of replacing chemical fertilizers with vinasse and filter cake on the environmental performance of ethanol, via life cycle assessment (LCA).

Methods

The analysis was carried out by comparing various scenarios structured from two control variables: crop management techniques (manual and mechanized harvesting) and source of nutrients (for supplying the nutritional needs of sugarcane crops): chemical fertilizers, chemical fertilizers + vinasse, and chemical fertilizer + vinasse + filter cake. Impact assessment was carried out in terms of primary energy demand, climate change, terrestrial acidification, freshwater eutrophication, human toxicity, and terrestrial ecotoxicity. LCA has been applied according to both attributional and consequential perspectives. Moreover, a sensitivity analysis was performed in order to verify the effects of the varying amounts of nitrogen (N), phosphorus (P), and potassium (K) in the composition of vinasse on the results obtained for the impact profile.

Results and discussion

From the attributional LCA perspective, the most expressive contributions regarding primary energy demand occurred in terms of depletion of non-renewable fossils. Replacing chemical fertilizers with vinasse and filter cake was beneficial for the environmental performance of ethanol as it reduces climate change, terrestrial acidification, and human toxicity impacts and sustains freshwater eutrophication and terrestrial ecotoxicity unaltered in relation to scenarios using only fertilizers. In terms of consequential LCA, ethanol’s environmental performance is influenced by the inclusion of the production of calcium fluorite to compensate the hexafluorosilicic acid deficit occurring in conjunction to the decrease of phosphate fertilizer and is compensated by the benefits provided by the general reduced consumption of chemical fertilizers for most of the impact categories. The exception occurred for primary energy demand.

Conclusions

The reuse of residues from bioethanol production—vinasse and filter cake—as primary nutrient suppliers for the cultivation of sugarcane instead of chemical fertilizers is a valid practice that improves the environmental performance of ethanol, even if it is analyzed under a consequential LCA perspective. The transport of these inputs to the field must be managed, however, in order to minimize primary energy demand and climate change impacts.