The main objective of this study was to evaluate the anaerobic conversion of vinasse into biomethane with gradual increase in organic loading rate (OLR) in two upflow anaerobic sludge blanket (UASB) reactors, R1 and R2, with volumes of 40.5 and 21.5 L in the mesophilic temperature range. The UASB reactors were operated for 230 days with a hydraulic detection time (HDT) of 2.8 d (R1) and 2.8–1.8 d (R2). The OLR values applied in the reactors were 0.2–7.5 g totalCOD (L d)−1 in R1 and 0.2–11.5 g totalCOD (L d)−1 in R2. The average total chemical oxygen demand (totalCOD) removal efficiencies ranged from 49% to 82% and the average conversion efficiencies of the removed totalCOD into methane were 48–58% in R1 and 39–65% in R2. The effluent recirculation was used for an OLR above 6 g totalCOD (L d)−1 in R1 and 8 g totalCOD (L d)−1 in R2 and was able to maintain the pH of the influent in R1 and R2 in the range from 6.5 to 6.8. However, this caused a decrease for 53–39% in the conversion efficiency of the removed totalCOD into methane in R2 because of the increase in the recalcitrant COD in the influent. The largest methane yield values were 0.181 and 0.185 (L) CH4 (g totalCOD removed)−1 in R1 and R2, respectively. These values were attained after 140 days of operation with an OLR of 5.0–7.5 g totalCOD (L d)−1 and totalCOD removal efficiencies around 70 and 80%.

The efficiency of individual and integrated processes applied to organic matter reduction and biodegradability improvement of a biodigested sugarcane vinasse wastewater was assessed. Strategies considered were Fenton's oxidation (Strategy 1), coagulation/flocculation (Strategy 2) and the combination of both processes (coagulation/flocculation followed by Fenton's reaction) - Strategy 3. It was found that Fenton's oxidation per se allowed reducing the organic matter, increasing the wastewater biodegradability and a non-toxic effluent was generated; however the cost of treatment was very high (86.6 R$/m(3) - 21.2 €/m(3)). Under optimized conditions, coagulation/flocculation provided a slight increase in effluent's biodegradability, toxicity towards Vibrio fischeri was also eliminated and moderate removals of total organic carbon - TOC - (30.5%), biological oxygen demand - BOD5 - (27.9%) and chemical oxygen demand - COD - (43.6%) were achieved; however, the operating costs are much smaller. The use of dissolved iron resulting from coagulation/flocculation (270 mg/L) as catalyst in the second stage - Fenton's oxidation - was shown to be an innovative and economically attractive strategy. Under optimal conditions overall removals of 51.6% for TOC, 45.7% for BOD5 and 69.2% for COD were achieved, and a biodegradable (BOD5:COD ratio = 0.54) and non-toxic effluent was obtained. In order to increase the efficiency of the process but using less hydrogen peroxide, the Fenton's oxidation was performed by gradually adding the oxidant. This procedure allowed to obtain the highest organic matter removal efficiency (as compared with the addition of all hydrogen peroxide at the beginning of the reaction). This way it was possible to minimize the reagent consumption and, consequently, reduce the treatment cost.

The post-treatment of biodigested sugarcane vinasse was envisaged in the present study following three different approaches: i) coagulation/flocculation e Approach 1; ii) combination of coagulation/flocculation with photo-Fenton reaction e Approach 2; iii) coagulation/flocculation followed by biological degradation and photo-Fenton reaction e Approach 3. The coagulation/flocculation per se provided a slight increase in effluent's biodegradability, a decrease in toxicity and turbidity, and provided moderate removals of total organic carbon (TOC), biochemical oxygen demand (BOD 5) and chemical oxygen demand (COD). The operating costs associated with chemicals consumption are low (5.7 R$/m 3 z 1.4 V/m 3). The second and third treatment approaches allowed to reach a nontoxic effluent, improved the biodegradability and led to similar high global organics removal efficiencies. The combined treatment consisting of coagulation/flocculation plus photo-Fenton oxidation (Approach 2) revealed to be a promising solution for treating this wastewater, in order to be recirculated into the high rate anaerobic reactor. Additionally, this solution provides smaller costs (84.9 R$/m 3 z 20.2 V/m 3) associated with chemicals and energy consumption, which are however very high. Nonetheless, this cost can be reduced by about 40% if solar radiation is used in the photo-Fenton process.

Biogas production from sugarcane vinasse has enormous economic, energy, and environmental management potential. However, methane production stability and biodigested vinasse quality remain key issues, requiring better nutrient and alkalinity availability, operational strategies, and knowledge of reactor microbiota. This study demonstrates increased methane production from vinasse through the use of sugarcane filter cake and improved effluent recirculation, with elevated organic loading rates (OLR) and good reactor stability. We used UASB reactors in a two-stage configuration, with OLRs up to 45gCODLd, and obtained methane production as high as 3LLd. Quantitative PCR indicated balanced amounts of bacteria and archaea in the sludge (10-10copiesgVS), and of the predominant archaea orders, Methanobacteriales and Methanosarcinales (10-10copiesgVS). 16S rDNA sequencing also indicated the thermophilic Thermotogae as the most abundant class of bacteria in the sludge.