Improving the Treatment of Sugarcane Vinasse in a Microalgae-Based System

Ethanol production generates vinasse as waste, a liquid effluent with a high percentage of organic matter and nutrients and with a high rate of acidification. The use of vinasse as a source of nutrients for microalgae has shown to be viable. In this research, the raw vinasse was treated with a microalgae-bacteria consortium in two High-Rate Algal Pond reactors (11.8 L each) operating in parallel. The first HRAP was composed of one reactor and the second was divided into two series reactors of 5.7 L each. The experiment was conducted for 132 days and was divided into 5 stages, with applied organic load varying from 7 to 11 g COD m2 d−1, hydraulic retention time of 7 days, continuous feeding, and added concentration of bicarbonate and urea varied, from 0 to 1000 mg NaHCO₃ L−1 and 0 to 50 mg (NH₂)₂CO L−1. The pH of both reactors in the 5 stages remained above 8.1 throughout the experimental period, the COD removal in the reactors always remained higher than 78%, reaching 95% in the experimental condition with 200 mg NaHCO₃ L−1 and 100 mg (NH₂)₂CO L−1 (C/N = 5.9). The NH4+ removal recorded in the reactors was ≈ 78% in all conditions except in condition IV, where no bicarbonate was added, but urea (330 mg (NH₂)₂ COL−1) was added (C/N = 2.2). The biomass productivity reached a rate of 0.67 g m−2 d−1 in stages I, II e II, in both reactors, and in stages IV and V a growth of biomass adhered to the walls of the reactors was observed. The microalgae-bacteria system is a viable method for the treatment of raw vinasse, requiring dilution and buffer addition. The growth in suspension and adhered to the reactors formed naturally as a response to the stress that the system was exposed to, without compromising the efficiency of removal and recovery of the reactors, therefore, being a robust system. Based on the results obtained in this research, pilot photobioreactors will be designed for the treatment of vinasse in sugar and alcohol plants, making it possible to produce energy and by-products of commercial interest using microalgal biomass.