Vinasse treatment using hybrid tannin-based Coagulation-Microfiltration-Nanofiltration processes: Potential energy recovery, technical and economic feasibility assessment

The sugarcane industry has a relevant contribution in the Brazilian economy. However, the inadequate disposal of vinasse in the fertigation practices has led to environmental issues. Therefore, the current research investigated the applicability of microfiltration (MF) and nanofiltration (NF), with and without the use of tannin as an organic coagulant, to treat vinasse. Among the different coagulant concentrations used, 8.0 mg/L was selected as optimum dosage due to greater removal efficiencies. Removal efficiencies for COD, color and ions by MF-NF and C-MF-NF routes were similar and higher than 90.6%. As floc sizes increased with the coagulant addition, the cake layer became more porous and compressible, reducing fouling resistance in MF process. In that sense, back transport velocity of particles became an important mechanism affecting the membrane fouling. Overall, the NF permeate quality allows its various uses, such as washing water and cooling water. The combined concentrate from MF and NF was considered for energy production and fertigation, presenting higher potential for methane production compared to raw vinasse. MF-NF route was chosen after an economic analysis due to the high chemical expenditures related to tannin acquisition in the C-MF-NF process. Considering the prices currently practiced for industrial water acquisition (2.8 US$.m(-3)), a 3 year would be required for the investment to be recouped. Moreover, an internal rate of return of 52.3% was obtained, which was higher than the market attractiveness considered. Hence, the economic analysis reassures the MF-NF rout feasibility for vinasse treatment.