Biomethane Production from Sugarcane Vinasse in a Circular Economy: Developments and Innovations

Abstract

Sugarcane ethanol production generates about 360 billion liters of vinasse, a liquid effluent with an average chemical oxygen demand of 46,000 mg/L. Vinasse still contains about 11% of the original energy from sugarcane juice, but this chemical energy is diluted. This residue, usually discarded or applied in fertigation, is a suitable substrate for anaerobic digestion (AD). Although the technology is not yet widespread—only 3% of bioethanol plants used it in Brazil in the past, most discontinuing the process—the research continues. With a biomethane potential ranging from 215 to 324 L of methane produced by kilogram of organic matter in vinasse, AD could improve the energy output of sugarcane biorefineries. At the same time, the residual digestate could still be used as an agricultural amendment or for microalgal production for further stream valorization. This review presents the current technology for ethanol production from sugarcane and describes the state of the art in vinasse AD, including technological trends, through a recent patent evaluation. It also appraises the integration of vinasse AD in an ideal sugarcane biorefinery approach. It finally discusses bottlenecks and presents possible directions for technology development and widespread adoption of this simple yet powerful approach for bioresource recovery.

1. Introduction

The Importance of Biohydrogen Production

The idea of using anaerobic digestion (AD) to reclaim energy while treating residues from sugarcane ethanol production was described as early as 1983 [1]. Although the technology was recognized as one of the most feasible ways to treat vinasse [2,3], many industries were still looking at vinasse solely as a fertilizer (or a nuisance) until recently. The biorefinery and circular economy wave brought a new impulse to integrate vinasse processing into the existing autonomous distilleries (those that convert all sugarcane into ethanol) or annexed distilleries (those that use residual molasses from sugar production for ethanol production). This trend also aligns with many of the United Nations SDGs (Sustainable Development Goals), most directly with SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action), with the potential to increase the recovery of green energy stored in sugarcane, and to reduce pollution [4,5].

Sugarcane is one of the major world energy crops, traditionally used as feedstock for bioethanol, sugar, and bioelectricity production. The high demand for biofuels has boosted production, favored by increasing fossil fuel prices, environmental impacts, global warming, concerns with greenhouse gases (GHG) emissions, and increasing governmental incentives [6,7]. In 2018, sugarcane ethanol production appeared in second place with 30 billion liters—the primary feedstock for ethanol has been coarse grains (i.e., corn) with 61 billion liters. Together, these two feedstocks comprise 84% of the world’s ethanol production [8]. Differently from sugarcane vinasse, the residue from grain fermentation has significant nutritional content and is generally concentrated and dried into DDGS (distillers dried grains with solubles) [9,10]. In contrast, vinasse from sugarcane ethanol has limited direct use [11].

Sugarcane production worldwide reached about 1.86 billion tons in 2021, with 75% of this production concentrated in Brazil, India, China, Pakistan, and Thailand (respectively, 38.5, 21.8, 5.8, 4.8, and 3.6% of the worldwide production). Brazil produced 75% more sugarcane than India, but similar amounts of refined sugar (38.1 million tons in Brazil and 35.8 million tons in India, in the same period [12]). These figures show the importance of sugarcane bioethanol in Brazil—where more than half of the sugarcane, about 55%, is converted into biofuel—and also the potential for other countries to add a clean fraction of biofuel to the automotive fleet, most remarkably India, which has an estimated production capacity of 30 billion liters of ethanol annually [13].

Brazilian production areas are located mainly to the South of the Amazon Rainforest; therefore, there is no competition with native forest areas. South-Central Brazil is the heart of the country’s sugarcane production and industry. In addition to the technological potential, Brazil has much land available for production, using about 100,000 square kilometers for plantation, less than 1.2% of the national territory [14,15]. Various by-products/wastes are generated in sugarcane mills, such as sugarcane bagasse, straw, leaves and grasses, molasses, vinasse, and CO₂ [16]. Developing integrated processes using these residual fractions could amplify sugarcane processing units’ viability and sustainability through biorefinery approaches under a circular bioeconomy [17]. Vinasse is a necessary coproduct of distillation, generated in large amounts (10–15 L per liter of ethanol) [18]. It is rich in organic and mineral residues, leading to a tremendous environmental problem [19,20,21], and also to many processing opportunities. Increasing circularity towards zero-waste biorefineries makes different vinasse applications ever more attractive, such as a fertilizer, animal feed, or energy source through AD. The latter is increasingly important, with active research and recent large-scale implementation. The anaerobic digestion of vinasse can increase process circularity and energy efficiency while still allowing further use of the digested vinasse. Although the simple digestion technology is not new, it is not optimized for the composition and scale of the sugarcane processing industry, and there is space for technology improvement. Aspects such as pretreatments, nutrient amendments, microbiota control and succession, biogas and biomethane yields, and digestate destination are still under intensive research and present many challenges and opportunities, as discussed in the following sections.

2. Sugarcane Vinasse

Vinasse is a residue of complex composition, with an acidic pH. The effluent has a high organic load: on average, it has 46 g O₂/L of COD (chemical oxygen demand), a dark brown color caused by melanoidins, and an unpleasant, strong odor [22,23]. Its typical composition is presented in Table 1. The effluent can be a source of salts, micronutrients, and macronutrients, especially phosphorus, potassium, calcium, and nitrogen. These are essential for microbial growth [24]. Vinasse also contains smaller amounts of other ions, such as SO₄²⁻, Mg²⁺, Cl⁻, and Fe²⁺ [25], and trace amounts of heavy metals, such as Cr, Ni, Cu, Zn, and others [25,26]. The effluent’s physicochemical composition is somewhat variable because of the variability of the raw material, alcohol fermentation conditions, the proportion of molasses and juice, conditions used in distillation, cane plant variety, and maturity of the crops [27,28]

Environmentally speaking, the effluent is considered highly polluting because it can impact the different environmental matrices (soil, water, and air). Excessive vinasse application harms soil composition and its microbial population [18,29], reducing up to 90% of the dissolved oxygen in the soil [11].

Vinasse can also highly pollute water bodies and marine ecosystems because its high nitrogen and phosphorus contents cause eutrophication. With its high COD and acidity, the residue causes depletion of the dissolved oxygen in the water and changes its pH [11,23]. If improperly applied to soils or stored in lagoons, vinasse can also be a source of greenhouse gases from organic matter decomposition, mainly CH₄, N₂O, and CO₂ [18,30,31,32]. The nitrogen content in vinasse is high and usually expressed as total Kjeldahl nitrogen, and is probably primarily organic because of prior consumption of inorganic sources by yeasts; in one analysis by [33], NO₃⁻-N represented ca. 27% of the total nitrogen, and in another, 17% [34]. The repeated application can increase the content of heavy metals in soils, with potential toxicity to plants, animals, and humans: excessive amounts of Cd, Cr, and Ni are carcinogenic, while Pb, Cu, and Zn can affect the central nervous system, as well as the gastrointestinal system [35]. However, proper use of vinasse (75 t.ha⁻¹ per growing season for up to 18 years) showed a negligible biomagnification effect [36].

Table 1. Physicochemical composition of sugar cane vinasse before utilization.

Reference/ Components	[37]	[38]	[25]	[39]			[40]	[26]	[41]	[42]	Average (1st Generation)
pH	5.4	4.2	nd	4.4	4.3	4.6	4.6	4.4	5.25	4.4	4.6
COD (g O2/L)	103	28.5	nd	36.0	39	49.0	31.7	25.2	33	67.3	45.8
BOD (g O2/L)	57.4	16.5	nd	nd	nd	nd	13.4	7.9	15	21.0	21.87
Ca (mg/L)	719	515.2	3160	741	1502	1304	828	671	1180	nd	1180
Cl (mg/L)	nd	1218.9	59.4	nd	nd	nd	nd	nd	2161	nd	1146
P (mg/L)	190	120.8	560	111	35	64	5518	207	135	nd	852.5
Mg (mg/L)	237	244.7	162.4	354	428	543	321.3	264	523.5	nd	319.3
N (mg/L)	1190	356.6	nd	1603	570	762	234	171	329.5	1100	698.09
K (mg/L)	2056	1750.9	1620	3147	2334	2827	3276	3401	2557.9	nd	2551.5
SO4 (mg/L)	710	1537.6	1680	2300	2700	2900	340.3	2993	2264	nd	1936.1

nd: no data—the component may be present in vinasse, but its concentration was not reported in the referenced text.

Table 1. Physicochemical composition of sugar cane vinasse before utilization.

Reference/ Components	[37]	[38]	[25]	[39]			[40]	[26]	[41]	[42]	Average (1st Generation)
pH	5.4	4.2	nd	4.4	4.3	4.6	4.6	4.4	5.25	4.4	4.6
COD (g O2/L)	103	28.5	nd	36.0	39	49.0	31.7	25.2	33	67.3	45.8
BOD (g O2/L)	57.4	16.5	nd	nd	nd	nd	13.4	7.9	15	21.0	21.87
Ca (mg/L)	719	515.2	3160	741	1502	1304	828	671	1180	nd	1180
Cl (mg/L)	nd	1218.9	59.4	nd	nd	nd	nd	nd	2161	nd	1146
P (mg/L)	190	120.8	560	111	35	64	5518	207	135	nd	852.5
Mg (mg/L)	237	244.7	162.4	354	428	543	321.3	264	523.5	nd	319.3
N (mg/L)	1190	356.6	nd	1603	570	762	234	171	329.5	1100	698.09
K (mg/L)	2056	1750.9	1620	3147	2334	2827	3276	3401	2557.9	nd	2551.5
SO4 (mg/L)	710	1537.6	1680	2300	2700	2900	340.3	2993	2264	nd	1936.1

nd: no data—the component may be present in vinasse, but its concentration was not reported in the referenced text.

2.1. Current Pretreatments, Treatments, Disposal, and Usage

Dealing with vast volumes of vinasse is one of the biggest challenges in the sugar-alcohol industry [43] due to the potential environmental risk, especially to water bodies, if deposited without proper treatment. In this way, several alternatives for in situ valorization of vinasse have been evaluated, with variable success.

The main application of vinasse today is as an irrigation fertilizer (in “fertigation”), mainly for sugarcane [23]. Returning vinasse to fields can supply up to 50% of the required nitrogen, phosphorus, and potassium [44,45]. However, repeated use may increase soil salinity, favor lixiviation to water sources [18], increase the content of organic matter and nitrogen in the soil, release unpleasant odors, increase greenhouse gas emissions, and foster insect proliferation [30,46,47]. To mitigate these problems, vinasse is usually diluted before being applied as fertilizer [48].

Another classical method to remove organic matter from effluents, aerobic treatment, has been reported for vinasse—mainly for removing color and reducing COD. The large amount of biomass formed in aerobic processes has also been studied as a protein source. However, the high concentration of organic matter limits oxygen transfer [27,49], and the required intensive aeration increases operational costs.

Regarding effluent treatments, anaerobic digestion is more common because of its low operational costs and the possibility of producing by-products of commercial interest, such as biogas and agricultural fertilizer [27]. This process is mediated by different microorganisms that work in symbiosis, converting complex organic compounds into different biomolecules, such as methane and carbon dioxide. Anaerobically digested vinasse has also been studied to produce food and feed fungal biomass [50].

Other uses for vinasse are in development, such as in producing hydrogen or volatile fatty acids [51], or energy production through concentration and incineration [43,52]. However, one of the most significant interests today is the production of biomethane while still recovering mineral nutrients in the secondary effluent and the sludge [53] and removing up to 80% of the COD [54].

The high content of organic matter and nutrients make the effluent a tempting substrate for fermentation. However, finding microorganisms that can quickly convert residual carbon from a previous fermentation is not trivial. There are reports of the production of biosurfactants [55], lipids [47], solutions for hydroponic culture [25], yeasts [18], and as a culture medium for several fungal species [56,57].

Reducing the large volumes of vinasse may be required for some applications, which can be done using evaporation or ultrafiltration systems. While these pretreatments increase energy costs, they can reduce disposal costs [43,58]. Other methods are investigated to treat vinasse, such as trickling filters, coagulation-flocculation, electrocoagulation, ultrasonication, photo-Fenton reactions, and ozone pre-processing. These methods can decrease vinasse’s phenolic compounds, color, and COD content, but usually require much energy [59,60,61].

Vinasse pH amendment—The natural pH of vinasse is low, around 4.6 (Table 1). This is far from the optimal range required for the development of a methanogenic microbiota of pH 6.5 to 8 [62]. Although many reports describe the direct biodigestion of vinasse, it is good practice to increase its pH before processing, as is disclosed in patents [63,64,65]. This can be done by adding alkalis, such as NaOH or Ca(OH)₂ [18,66,67], residual filter cake (also known as press mud) from the sugarcane juice liming process, or even boiler ash [68,69]. However, the pH quickly drops in batch anaerobic digesters [67] because of the low buffering capacity of the vinasse and the formation of volatile fatty acids (VFA). With the subsequent consumption of VFAs, the pH rises again in batch reactors. Therefore, a strategy for pH maintenance is partitioning the system into two reactors, one optimized for the acidogenic and another for the methanogenic phase. It is possible to recycle effluent from the methanogenic reactor as a buffer for the acidogenic reactor [70,71]. Vinasse with an average pH of 4.6 (Table 1) would require at least 1 kg³ of NaOH per cubic meter of vinasse to raise the pH to 7.5, at the cost of about USD 0.31–0.53 per kg of NaOH [72,73], not considering buffering effects.

2.2. Energetic Potential in Vinasse

Vinasse may be efficiently used as a substrate for bioenergy generation, even with its variable [19,27,74,75,76,77] composition. The anaerobic digestion process can reclaim part of the energy content in organic molecules, currently lost to decomposition in fields, while still maintaining the mineral nutrients necessary for fertigation. Santos et al. [74] found similar bioenergy yields and electrical energy potential for different types of sugarcane vinasse. Brazil’s sugarcane harvest in the season 2020/2021 was 665 million tons, with an ethanol production of 29.8 billion liters [78]. The generation of vinasse in the same period was about 360 billion liters, considering the average production of 12 L of residue per liter of ethanol [20,29]. With the above values and based on the methodology used by Bernal et al. [76], the energy production potential from the vinasse of all annexed and autonomous plants of ethanol in Brazil could reach 3.28 TWh per season—or 2.3% of the domestic electricity consumption in Brazil in 2019 [78]. This is equivalent to 1.9 million barrels of oil.

3. Vinasse Anaerobic Digestion

Anaerobic digestion (AD) is a natural and ubiquitous process where a microbial community works synergically, converting the available organic carbon into methane and carbon dioxide. This process can be explained in four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [79,80]. The gaseous mixture produced is commonly termed biogas, containing 60–70% methane and 30–40% carbon dioxide. Trace gases, such as hydrogen and hydrogen sulfide [81], can also be found in small amounts. The purified fraction, after CO₂ absorption and, therefore, with a higher methane concentration, is termed biomethane. This fraction can be pure enough to be fed to natural gas pipelines.

Refs. [82,83,84,85,86] show that vinasse anaerobic digestion depends on detailed information about its chemical composition, the inherent microbial community, and the biochemical steps to which the organic matter is subjected [84,85,86]. The scientific literature is rich in studies showing relations between process parameters and the optimal efficiency of biodigestion systems [82,83]. However, it is frequent that these conditions are not reproducible in other environments. This is a consequence of the microbial community’s variable composition, which depends on nutrient availability.

3.1. Microbial Community Diversity in Vinasse AD

Microbial activity is central to AD. All four stages, from the breakdown of complex organic matter to the final generation of methane, are mediated by different microorganisms [87]. Besides the obvious exchange of metabolites, the signaling and quorum sensing between populations [88,89] and the extracellular transport of electrons [90] are hot topics for research that can further improve processes. The more efficient and specialized microorganisms are present, the more efficient the chemical conversion. Ideally, attaining a productive microbial community would be done by inoculation and using selected nutrients and culture conditions. However, understanding and manipulating these complex microbial communities is not a simple task: recent metagenomic analyses seldom identified the microorganisms at the species level [91], but show a complex and variable ecosystem.

An analysis of 134 studies of the microbial community diversity in the anaerobic digestion of vinasse identified 1635 MAGs (metagenome-assembled genomes); however, only 25 MAGs were identified frequently in the samples, which clearly shows that the microbial community is vastly variable. In this study, >96% of the identified MAGs belonged to bacteria [92]. Bacteria are responsible for the hydrolysis of complex organic matter (Clostridium, Cellulomonas, Bacillus, Thermomonospora, Ruminococcus, Bacteroides, Acetovibrio, and Microbispora genera) [27], bioconversion of the readily fermentable substrates into volatile fatty acids (Lactobacillus, Streptococcus, Bacillus, and Escherichia) [93], and production of acetic acid and biohydrogen (Acetobacterium, Syntrophomonas, Clostridium, Sporomusa, Syntrophospora, Thermosyntropha, and Eubacterium) [27]. Among the 61 MAGs of Archea identified by Campanaro et al. [92], 53 referred to Euryarchaeota: Methanococcus, Methanosarcina, and Methanolobus are considered the main archeobacteria in methane production. The acetoclastic biosynthesis (from acetate) contributes about 70% of the methane in anaerobic digestion, while the remainder, 30%, comes from methanogenic hydrogenotrophs (e.g., Methanosarcinia sp.). However, a high degree of inter cooperation and even interspecies hydrogen transfer occurs in the microbial community [94,95].

This overview is compatible with the composition of a mesophilic biogas production from sugarcane vinasse where the prevalent phyla identified were Bacteroidetes (58.5%), Firmicutes (14.1%), and Proteobacteria (13.1%), while the methanogens were mainly composed by Crenarchaeota (2.39%) and Euryarchaeota (Methanosaeta 1.97%, Methanomassiliicoccaceae 1.15%, and Methanobacterium 0.7%) [96]. In a thermophilic biogas production from the co-digestion of vinasse with filter cake, Methanothermobacter (order Methanobacteriales), Thermotogae, and Thermodesulfovibrio (Nitrospirae) were identified as the most common thermophilic microorganisms [53]. The intricate relationship between the anaerobic microbiota and its action on organic matter for biogas production is shown in Figure 1.

3.2. Vinasse Methanogenic Potential and Digestion

Although vinasse is produced from a widely known and established process, its composition varies with the season, the quality of the processed sugarcane, the raw material (sugarcane juice or molasses), and fermentation efficiency. There is a variation in composition within the same ethanol industry and, consequently, in the potential for biomethane production from vinasse. For example, Leite et al. [98] observed more than 20% in the methane yield using vinasse from the beginning and end of the operation season.

The biochemical methane potential (BMP) can be expressed in liters of methane per kilogram of chemical oxygen demand or volatile solids; since gas volume varies with temperature, the volume is expressed as “normal” liters of gas at 25 °C, leading to units such as NL_CH4.kg_COD⁻¹. The BMP of vinasse was estimated as 231 NL_CH4.kg_vs⁻¹ [99,100], 215 NL_CH4.kg _COD⁻¹ [101,102], and 324 NL_CH4.kg _COD⁻¹ (recalculated from [103]). The latter is closer to the theoretical maximal BMP of 350 NL_CH4.kg _COD⁻¹, considering a substrate such as a carbohydrate, lactic, or acetic acid, with the minimal formula CH₂O.

Among the bioreactors used for vinasse biodigestion, UASB (upflow anaerobic sludge bioreactor) is the most common due to its high biomass retention capacity, which allows for a low hydraulic retention time. Moreover, UASB systems are less expensive than traditional CSTR systems (continuous stirred-tank reactors) [104,105]. The efforts to increase biomass concentration through sludge retention are evident in the studies that describe the use of an AFBR (anaerobic fluidized bed reactor) with immobilized biomass [39] and membranes in AnSBBR (anaerobic sequencing batch biofilm reactor) [106]. The higher the biomass retention, the fewer total suspended solids are lost, increasing the effluent quality: 90% COD removal can be reached [107] by favoring the maintenance of slow-growing microorganisms, such as methanogens. Despite the advantages of AnSBBR over conventional reactors, membrane fouling is common and brings additional operational costs [108] that were not overcome to date. UASB and CSTR continue to be the most frequently used bioreactors.

3.3. Vinasse Composition Effects on Methanogenesis

As presented in Table 1, sugarcane vinasse is rich in organic matter (carbohydrates and organic acids), nitrogen, potassium, and phosphorus. Despite the high potassium content (1.6 to 2.4 g/L) [19,25], no study indicates adverse effects of this ion on the anaerobic community. The C:N ratio of vinasse from sugarcane (12:1) or sugarcane + vinasse (16:1) is below the recommended for anaerobic digestion (20–40:1) [99]. This can result in the accumulation of ammonia due to the excess nitrogen that inhibits the whole process. A possible way to increase the C:N ratio in N-rich vinasses is the addition of carbon sources, e.g., small amounts of sugarcane juice or bagasse.

In descending order, vinasse’s most abundant micronutrients are sulfur, calcium, magnesium, iron, manganese, and copper [19]. The sulfur content must be quantified in the vinasse from sugarcane and molasses mixtures. Unlike potassium, excess sulfur is known to disturb AD, especially at concentrations above 200 mg/L, when sulfate-reducing bacteria are favored instead of methanogens and can inhibit methane production [109].

The most common toxic chemical species for methanogens are oxygen, nitrates, and nitrites, higher fatty acids, heavy metals (Ni > Cu > Cd > Cr > Pb), aromatic compounds, chlorinated hydrocarbons, sulfides [110], and antibiotics [111], the last two largely used in ethanol fermentation. Another aspect to consider is the low pH of vinasse, which is due to the presence of volatile fatty acids [19] and low alkalinity [112]: the acidogenesis in AD results in further acidification, which is incompatible with the optimal pH for methanogenic bacteria (6.5–8.0). The pH must be regulated, but the chemicals used must consider the sustainability of the process [29].

Methanogenic bacteria are more sensitive to unbalanced nutrients and process parameters, such as pH reduction and high organic loading rates, which require careful system operation. Concerning the toxic compounds of vinasse, Methanosarcina is less susceptible than Methanothrix [27]. Both genera can produce methane from acetate and are responsible for >70% of the methane generated. However, the first has a higher growth rate, lower affinity to acetate [113], and sensibility to the presence of propionate [96]. It is directly related to the medium’s cobalt, manganese, molybdenum, nickel, and tungsten content [114]. In the case of UASB reactors, the presence of Methanotrix in granular sludge is crucial for stabilizing the system [115]. Finally, the main enzyme involved in acetate degradation in methanogens is carbon monoxide dehydrogenase, which contains iron, sulfur, and nickel in its composition [113]. Iron and sulfur content in vinasse is generally sufficient, but nickel is not [99] and may have to be added to the system.

3.4. Improving Vinasse Biodigestion with Co-Substrates

The choice of co-substrates to correct the above-mentioned low C:N ratio should also consider the need to supplement desired nutrients and dilute toxic compounds. Moreover, the low pH of vinasse, the presence of toxic compounds, and the low biodegradable fraction content should also be considered in the strategy of co-digestion [116]. Some examples of co-digestion are presented in

Table 2. Biochemical methane potential, BMP, in vinasse co-digestion.

Co-Substrate	Proportion (Vinasse: Co-Substrate)	Solids Content	BMP, as Reported	BMP, Recalculated, NL CH4 kg−1 COD	HRT or Batch Duration, Days	Reference
Press mud (filter cake)	75:25, VS% basis	7.1%	365 L CH4 kg−1 VS	268.1 NL CH4 kg−1 COD	24.1	[117]
Hemicellulose hydrolysate	75:25 Plus 1 g/L yeast extract and 15 g/L ash	5%	279 CH4 kg−1 COD	279 NL CH4 kg−1 COD	34	[34]
Glycerin	50:50, COD-basis	0.5%	15.25 mol CH4 kg COD applied	341.6 NL CH4 kg−1 COD	15	[118]
Tofu wastewater	20:80 by volume (74:26 by COD)	2.1%	159 NL CH4 kg−1 COD	159 NL CH4 kg−1 COD	20	[119]
Cheese whey	75:25	2.4%	15.76 mmol CH4 gCOD−1	353 NL CH4 kg−1 COD	20	[116]

.

Sugarcane processing into bioethanol gives by-products such as filter cake, straw, and bagasse that should be prioritized as co-substrates—especially considering logistics and a circular economy context. Press mud, for instance, is generated at about 2–3% of the sugarcane crushed [120]; it is a nutrient-rich residue with 25–30% solids [121], used as fertilizer and with incipient use in anaerobic digestion [122]. On the other hand, using solid substrates in biodigestion has some technical limitations, such as the total solids content, which explains the clear preference for studying liquid co-substrates. Furthermore, attention must be given to lignocellulosic substrates with lower biodegradation rates [99], which increase the hydraulic retention time and biodigester size, impacting process economics.

Using molasses as a co-substrate improves the efficiency of vinasse anaerobic digestion compared to sucrose and sugarcane juice [19]. Co-digestion of vinasse with sugarcane filter cake (press mud) may improve the process stability and methane production (by 21 and 64%, based on reports by Volpi et al. [103] and López González et al. [117]). Hemicellulosic hydrolysates, yeast extract, and sugarcane bagasse ash have also been recently studied [34] and showed a positive impact on methane production when added to vinasse.

3.5. Early Implementations of Industrial Processes

Although biodigestion technologies are widespread in India and China [123] and are expanding in other sugarcane-producing countries, such as Thailand and Pakistan, vinasse is still primarily used for fertigation, eventually after anaerobic treatment in lagoons [124]. However, the technology is being picked up by industry now, with examples of economic and process evaluation in Mauritius [125], Cuba [126], Colombia [127,128], Thailand [129], and Indonesia [130]. Most reports of industrial processing come from Brazil, where 10 out of 355 distilleries have developed vinasse biogas facilities in the past, most no longer in operation [131]; however, there are large projects recently launched, further described in Section 4.2.

The first experience of biodigestion of vinasse in Brazil took place in Rio de Janeiro in 1981, with a modified Indian model reactor of 330 m³. In 1986, a mesophilic UASB was installed in São Paulo’s state with a total capacity of 1500 m³. Since 1987, the “Usina São Martinho” in São Paulo has carried out a thermophilic process (55–57 °C) for the digestion of vinasse, with a pilot reactor type UASB with 75 m³ of capacity and gas production two times higher than that reported for mesophilic processes, and at significative lower hydraulic retention time (10.8 h versus 8.6–15 days). Although the system showed promising results, it had a high operation cost and did not reach an industrial scale [132]. The number of pilot and real-scale studies (0.5 to 15,000 m³) is much smaller than those carried out at the laboratory scale [117,133,134,135,136,137]. At large scales, the methane concentrations in the biogas (60–68%) [134,136] are lower than those obtained at the lab scale (60–84%) [108,138], with methane production rates 5–34% lower [133,137,139].

The anaerobic degradation of vinasse on an industrial scale is a challenge and still needs scientific and technological development [140], primarily due to the difficulties of maintaining process stability and a high rate of vinasse degradation [137]. Moreover, sugarcane seasonality must be considered. Government incentives are essential for installing biodigestion plants in the sugarcane-based ethanol industry [141]. Considering the increase in ethanol production for the coming years, the energy produced from vinasse biomethane could add 29.4% more than ethanol alone to the Brazilian energy matrix [142]. Moreover, vinasse biomethane is a biofuel produced locally with little connection to fossil fuel prices and variations in exchange rates.

4. Current Trends and Technologies

Although anaerobic digestion, in general, is a mature technology, applying it to vinasse processing in sugarcane biorefineries is not trivial. It implies working with the enormous volumes required for economic bioethanol production and dealing with specific problems, such as the C:N ratio, its high ionic content, and the lack of vinasse out of the harvesting season. A look at the patent literature and market developments can inform how research impacts the development of vinasse AD and give a glimpse of the necessary directions to be investigated.

4.1. Patents in Vinasse Biodigestion

To assess the state of the art in biomethane production from sugarcane vinasse, a patent search was carried out in the Derwent Innovations Index database, considering the period between 2005 and 2020. The search retrieved 132 documents, and 51 relevant documents were selected. Data were exported to the software Excel^® and analyzed to exclude patents or utility models unrelated to biogas production.

The first patent deposit for biomethane production from vinasse dates back to 1981 [143], but patent filings’ most pronounced evolution occurred in the last 15 years. In the evaluated period (2005–2020), 51 patent documents were found, peaking in 2009 (8 filed documents). This can be attributed to the increase in the energy crisis and green movements due to the eagerness to find alternative energy sources [144]. In the subsequent years, around four patents were filed per year, except for the years 2007 and 2008, where only one patent document was found. In the case of the years 2019 and 2020, only two and three patent filings were found, respectively (Figure 2A), but it should be considered that there is a latency period of approximately 18 months between the filing date of the patent and the date of publication [145].

Among the countries protecting technologies to produce biomethane from sugarcane vinasse, Brazil, China, and the United States dominate, with a share of 20%, 18%, and 18%, respectively. Other countries such as India, Canada, France, Germany, Mexico, Spain, Japan, and the United Kingdom have less participation. In general, it is observed that the countries filing patents in biomethane production from vinasse are the largest producers of sugarcane. As the primary producer of sugarcane ethanol and, consequently, vinasse, it makes sense that Brazil has filed most patents on technological advances to use vinasse in biomethane production. This is because the large volume of effluent generated is a problem for the sugar and alcohol industry and calls for a sustainable solution [146]. The production of energy as biomethane is a promising alternative since the energy recovery from the effluent improves environmental indicators and positively impacts local and regional socio-economic parameters [45].

The top two most recurrent International Patent Classification (IPC) codes [147] were C12P (fermentation or enzyme-using processes to synthesize a desired chemical compound or composition or to separate optical isomers from a racemic mixture) with 51% of patent documents, and C02F (treatment of water, wastewater, sewage, or sludge), found in 23% of patent documents. Other patent classification codes were C10L (fuels not otherwise provided for; natural gas; synthetic natural gas obtained by processes not covered by subclasses) with 16%, C12M (apparatus for enzymology or microbiology) with 14%, and C12N (microorganisms or enzymes; compositions thereof; propagating, preserving, or maintaining microorganisms; mutation or genetic engineering; culture media) with 12%, among others (Figure 2B).

While anaerobic digestion itself cannot be patented, improvements are described as processes or utility models. For example, WO2008/040358Al describes a sequential process for recovering proteins from the liquid phase of vegetable materials (sugarcane) and producing fermented bioproducts from fermentable sugars, such as alcohols, fertilizers, and bioenergy in the form of biomethane from vinasse. The process starts with treating the raw material (mechanically or enzymatically), disrupting the cells, and making proteins available for recovery and carbohydrates for fermentation. Proteins are recovered by precipitation (acid or salt) or by chromatography. Sugars are fermented to alcohol using yeast or bacteria, and the resulting effluent is used to produce biomethane using bacterial methanogens [148]. In the patent document WO2009/010959A2, Yogev and Gamzon [138] describe methods for producing fuels from simpler molecules, such as CO₂, H₂O, or CH₄, the latter obtained from the anaerobic fermentation of wastewater as, for example, stillage.

The patent BRPI0915815B1 presented a process to produce biogas using effluents from the sugar and alcohol industry, mainly vinasse and biogenic material (washing waters and other organic wastes from the process) with up to 85% of conversion [63]. The effluents are mixed and transferred to a bioreactor, the pH is adjusted, and the fermentation is carried out by adding or immobilizing bacteria. The process can operate at temperatures between 37 and 50 °C and a pH between 6 and 8. The gas produced is collected in a gas reservoir. A recirculation pipe takes the biomethane to the bottom of the bioreactors to support the CH₄ reaction and expel the reaction-inhibiting CO₂. The discharge effluent, as well as the sludge, can be used for irrigation or fertilization purposes. The patent WO2012/153189A2 shows a process for biogas production, which could operate in a semi-continuous or continuous mode using a mixture of solid and liquid residues: lignocellulosic biomass from sugarcane, and vinasse [64]. Unlike the patent BRPI0915815B1, in this case, the lignocellulosic biomass was pretreated through steam explosion. In a second bioreactor, anaerobic digestion is carried out with 10–20% v/v of anaerobic bacteria (anaerobic sludge from treatment stations), enzymes (cellulases and xylanases), and up to 20% v/v vinasse. Digestion can be done under mesophilic or thermophilic conditions (20–70 °C) and at a pH between 5 and 8.

Finally, Huandong et al. [65] presented a process for producing biogas rich in biomethane from cane vinasse in the patent document CN105039422B. The process is conducted in batch mode, using a 75% bioreactor working volume and alternating agitation at 120 rpm (3 to 15 min each hour). The substrate fermentation uses a 16% v/v inoculum, pH 7.5–8.5, and a temperature of 55 °C for 44 days.

A patent search was also run in the Brazilian patent database, considering that the country is the primary producer of vinasse and the main depositor of patents in this field and that the national patent database is not yet fully integrated with the Derwent worldwide database. The results of relevant patent documents are presented in

Table 3. Relevant patent documents retrieved from the Brazilian patent database (National Institute of Industrial Property, INPI).

Document Number and Year	Assignee	Translated Title	IPC	Technology	Status	Reference
BR 11 2019 009768 6 A2/WO 2018/091004 (2017)	Christine Apelt (Germany)	Process for material and energy recovery of residues from sugar cane processing and arrangement for performing the process	C12P 7/06; C12P 5/02; C02F 1/24; C02F 1/40; C02F 11/04; C02F 3/28; C02F 9/00; C05F 11/00; C05F 17/00; C05F 5/00	Liquid and solid (ground) residues are hydrolyzed and fermented in continuous multi-stage cultures	Filed, active	[149]
BR 10 2016 023277 5 A2 (2016)	Federal University of Sao Carlos (Brazil)	Valorization process of effluents from the sucroenergetic sector	C02F 11/04; C02F 103/32; C05F 5/00	The vinasse is pretreated to separate solid and liquid fractions; the liquid is used for microalgae cultivation, and the solid is used as a fertilizer, animal feed, or as a substrate in composting and biodigestion processes	Filed, active	[150]
BR 10 2015 031011 0 A8 (2015)	Fermentec—Tecnologias em Açúcar e Álcool Ltda. (Brazil)	Integrated process for the production of oil and biogas from vinasse	C10L 1/08; C10L 9/08; C12P 7/62	Oleaginous yeasts are cultivated in the vinasse, and the residual broth is used for biogas production in a UASB reactor	Filed, active	[151]
BR 13 2014 025044 0 E2 and BR 10 2014 009156 4 A2 (2014)	Jorge Vinicius da Silva Neto (Brazil)	Process for the production of bioenergy and biofertilizers through anaerobic digestion and algae cultivation using agro-industrial by-products	C02F 9/14; C02F 3/28; C02F 3/32; C05F 5/00; C02F 103/20; C02F 103/32; C02F 11/04	Vinasse is used for biogas production in an anaerobic reactor, and the digested broth is used for microalgae cultivation; CO2 from biogas combustion feeds the algal culture, and algal biomass feeds the biodigester	Abandoned	[152]
BR 10 2014 024757 2 A2 (2014)	Geo Energética Participações S.A. (Brazil)	Equipment and process for anaerobic vinasse biodigestion and biogas production	C02F 11/04; C12M 1/107	Bioreactor of high vertical dimension with temperature control and continuous biodigestion process with biomass recirculation and O2 injection for biological consumption of H2S	Filed, active	[153]
BR 10 2013 021902 9 A2 (2013)	Veolia Water Solutions & Technologies Support (USA)	Method of ethanol and biogas production, and ethanol facility for the production of ethanol and biogas	C12P 7/06; C12P 7/14; C02F 9/02; C12M 1/107	Biogas is produced from “distilled beer” vinasse in a membrane anaerobic bioreactor	Abandoned	[154]
BR 10 2012 020335 9 A2 (2012)	Geo Energética Participações S.A. (Brazil)	Biological purification production plant for recycling vegetable waste from sugar and alcohol production	C02F 103/32; C02F 9/14; C02F 9/08	Two tank bioreactors and one lagoon to process solid and liquid wastes from distilleries producing biogas and an organic fertilizer	Filed, active	[155]
PI 1100736-2 B1 (2011)	Arka Ambiental Ltd.a. (Brazil)	Vinasse treatment system with power generation, reuse water generation, and concentrated organic fertilization	C12F 3/00; F03G 7/00; C05F 5/00	Anaerobic reactor of internal circulation; system of biogas washing and drying for use in electric energy generation; system of ultrafiltration and reverse osmosis to separate reuse water and fertilizer	Granted, active	[156]
PI 0903984-8 A2 (2009)	Algae Biotecnologia Ltd.a. (Brazil)	Method of sequential treatment of wastes from the sugar and alcohol sector with production of microalgal biomass and production of renewable fuels	C12S 3/10; C12S 3/02; C12R 1/865; C12R 1/89	Cultivation of microalgae in clarified vinasse, while the removed solids are destined for biogas and biofertilizer production; the residual liquid can alternatively be used for microalgae cultivation, and microalgal biomass can be alternatively biodigested; microalgae can biologically purify biogas	Abandoned	[157]
PI 0915815-4 B1/WO 2010/003397 (2009)	Johann Rietzler (Germany)	Process for the production of methane from process waters and biogenic material	C12P 5/02; C02F 3/28; C02F 3/30; C12M 1/113	Process water containing biogenic material is converted to methane by immobilized or free bacteria, with biogas recirculation inside the bioreactor	Granted, active	[63]
PI 0704885-8 A2 (2007)	Bruce Wilson Giannetti (Brazil)	Incentivized methanization of the concentrated organic matter of the effluent from ethyl alcohol distillation	C07C 9/04; C02F 11/04	The process comprises a reactor containing a membrane of expanded clay spheres serving as support for methanogenic bacteria; the effluent is pumped through sprayers inside the membrane; part of the biogas is reinjected inside the membrane to force the release of gas bubbles	Abandoned	[158]
PI 0002731-6 A2 (2000)	Ricardo Audi (Brazil)	Processes for obtaining CO + H2O through the reform of CH4 from biogas in an aluminum/nickel and water vapor mixed-bed gasifier, as well as CO2/hydrocarbon reform through the integral biogas passage into the fixed-bed reformer to obtain CO	C01B 3/40; C01B 3/44; C01B 32/40	A process that involves biogas production from vinasse anaerobic digestion, separation of CO2 by monoethanolamine absorption, and CH4 reform in a fluidized bed gasifier to produce CO and H2; also, the biogas passes through a fixed-bed tubular reactor with catalyzers and is converted to CO	Denied	[159]
PI 0002730-8 A2 PI 9905239-3 A2 PI 9905240-7 A2 PI 9706185-9 A2 PI 9704382-6 A2 (1997-2000)	Ricardo Audi (Brazil)	Various	Various	Technologies involving the catalytic reform of CH4 obtained from vinasse anaerobic digestion, resulting in syngas for the synthesis of organic molecules	Abandoned or denied	[160]

Search terms (in the abstract): (CANA* OR SUCRO* OR AÇÚCAR* OR ETANOL OR ÁLCOOL OR VINHAÇA) AND (VINHAÇA OR VINHOTO OR EFLUENTE* OR ÁGUA* RESIDU* OR RESÍDUO* LÍQUIDO* OR SUBPRODUTO* LÍQUIDO* OR ÁGUA* DE PROCESSO) AND (METANO OR BIOMETANO OR BIOGÁS OR CH4 OR BIODIGESTÃO OR DIGESTÃO ANAERÓBICA OR ENERG*). Date of search: 2 January 2021. Because the INPI database does not provide rewritten titles and abstracts, the search strategy included additional keywords, as does the Derwent database. The search retrieved 73 documents, analyzed based on the abstract and claims, and 18 relevant documents were selected. The database does not cover patents filed before 1997.

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4.2. Recent and Announced Projects

The enormous volumes of sugarcane vinasse produced in Brazil have stimulated the development of projects to convert this liquid residue into biogas for energy generation, first at the academic/research level and, today, at the industrial level. Since 2011, the São Carlos School of Engineering (University of São Paulo) has been developing compact and robust bioreactors that allow the optimized conversion of vinasse to biogas using bacteria and archaea that remain immobilized in biofilms, in a project with the cost of USD 428,000. An important bioethanol research center in Brazil, CTBE, has launched a project (USD 927,000) of a virtual “first-generation biorefinery”, which aims to develop mathematical models to optimize sugarcane production plants in all their stages, including the anaerobic digestion of vinasse to generate bioenergy, employing the software Environment for Modeling, Simulation and Optimization (EMSO) [161].

Despite the considerable potential for energy generation, biogas production from sugarcane vinasse is not yet a widespread technology. In 2017, in the state of São Paulo, the major Brazilian bioethanol producer, only 8 of 165 distilleries applied alternative treatments for vinasse, including anaerobic digestion. In all other cases, vinasse was used directly in fertigation [76]. However, there is an estimation that the production of biogas in the sugar and alcohol segment will increase by 80% in the next seven years, considering that the use of sugarcane vinasse and filter cake as raw materials for biogas production boosts the energy recovery from sugarcane by 31% [162].

In 2018, a vinasse biogas production project was scaled up in the Ivinhema production plant in Minas Gerais. This project was a partnership between the company Methanum and the Cooperative Adecoagro, initiated in 2013 and funded (BRL 4.87 million, or approximately USD 2,250,000 at the time) by the Brazilian Innovation Agency FINEP (Financiadora de Estudos e Projetos), in the frame of the PAISS Program (Plano BNDES-Finep de Apoio à Inovação dos Setores Sucroenergético e Sucroquímico), an initiative dedicated to supporting innovation in the sugar and alcohol segment. Today, the produced biogas is used to heat the water used in the industrial processes, and the digestate is employed as an organomineral fertilizer in sugarcane cultivation. The vinasse is concentrated before the biodigestion process [163]. In the São Martinho production plant, located in São Paulo, biogas from vinasse is produced to provide heat for the drying process of yeast cells [164].

Recently, biogas production technology from the anaerobic digestion of sugarcane vinasse for electricity generation has reached an industrial scale in Brazil. In July 2020, large-scale electricity production in biogas moto-generators started in the Bonfim production plant in Guariba (São Paulo). This joint venture between the companies Raízen (a joint venture between Shell and Cosan) and Geo Energética, announced in 2018, had an investment of BRL 153 million (approximately USD 30 million in December 2020) and represents the first project in the world that uses vinasse and filter cake from ethanol production to generate electric energy from biomethane at a large scale. The production plant operates with three turbines that generate 8955 kW. The estimated annual production is 138,000 MWh. Another project of BRL 160 million (approximately USD 31 million in December 2020) applying the same technology shall be implemented in the Narandiba production plant, which belongs to the Cocal group and is in Presidente Prudente (São Paulo). This plant will produce around 114,000 Nm³ of biogas per day during the sugarcane harvest. Part of the biogas will be used for electric energy generation (up to 33,300 MWh/year). Around 24,000 Nm³/day of purified biomethane will be introduced in the distribution pipeline of the company Gas Brasiliano. The production plant will be started in April 2021 [165,166,167]. The company Jalles Machado, located in the state of Goiás, also plans to start biogas production from vinasse in 2021 [164].

Mixing two different substrates allows for regular biogas production throughout the year. The vinasse has an irregular organic load and energetic density, and its storage is not economical due to the high water content and huge volumes. The filter cake, in contrast, has a more stable composition and high solids content, and thus can be used to compensate for the variations of vinasse composition. Before being scaled up to the Bonfim commercial plant, this technology, developed by Geo Energética, was successfully applied in the Regional Cooperative of Sugarcane Producers (Coopcana) in the state of Paraná, in a demonstration plant with the capacity to produce 4 MW [167].

5. Vinasse Treatment and the Circular Economy

5.1. Current Practices of Circular Economy in the Sugarcane Industry

The circular economy concept involves all material streams and flows and their utilization routes in an industrial process, from raw materials, other inputs, and resources to products, by-products, and residues, including gas emissions. Unlike the traditional linear economy, which considers only resource consumption and residue disposal, the circular approach adopts reuse and recycling to close the material flow cycle to the maximum. Thus, a well-established circular economy reduces the extraction of natural resources.

Analysis tools, such as modeling, simulation, and optimization, can aid in increasing the process’s yield and productivity, reduce capital and operational costs, and improve the management and conservation of energy resources to reduce greenhouse gas emissions. Life cycle analysis (LCA) is a technique used to minimize environmental problems, evaluating the impacts from raw material production to product consumption based on the input and output of materials and energy [168]. These analyses are fundamental to determining an ideal biorefinery configuration, such as sugar and ethanol plants that convert the feedstock into sugar, ethanol, steam, and electricity [16].

Since the 1970s, in Brazil, approaches have been adopted to increase circularity in sugarcane ethanol production plants. From this decade until the mid-1990s, the transformation of sugarcane bagasse into electric energy grew linearly and, from then on, exponentially (Figure 3). This co-generation strategy is the most significant measure towards a circular economy that allows production plants to become self-sufficient in energy. Other approaches highlighted by Raízen, one of the leading Brazilian ethanol-producing companies, were: strategies for water reuse, e.g., efficient recovery of condensates; the use of ashes from burning bagasse as fertilizer in the field, together with vinasse and filter cake; and the conversion of sugarcane bagasse and straw to second-generation ethanol, which reduces the carbon footprint by 35% [169].

5.2. The Next Challenge: Valorization of Liquid Residues

A critical aspect for the ethanol industry, both for the first-generation (from sugarcane juice or molasses) and the second-generation (from lignocellulosic biomass, such as straw and bagasse), is the environmental impact of the liquid residues generated in the process, mainly vinasse [171]. Effluent biodigestion can improve the energy balance in ethanol processing and the environmental suitability of waste disposal [172]. One of the advantages of biogas energy is the prospect of storing biogas and production in periods of low energy generation in hydroelectric plants, which can make the price more competitive [173]. Therefore, the inclusion of a unit of anaerobic digestion of vinasse using process integration techniques in sugarcane biorefineries is the reason for studies by several researchers [117,172,173,174]. A summary of climate change impacts analyses of biogas production from sugarcane vinasse is shown in

Table 4. Climate change impacts per unit of ethanol energy produced in different configurations and scenarios of biorefineries for ethanol production. “X” indicates if a specific step is present. Generally, scenarios including 2^nd generation ethanol production and vinasse biodigestion have a lower impact.

Biorefinery Products					Climate Change Impacts (gCO2eq/ MJethanol)	Reference
First-Generation Ethanol	Second-Generation Ethanol	Sugar	Vinasse	Energy
X	-	-	-	X	17.2	[172]
X	-	-	X	X	16.8
X	X	-	-	X	15.5
X	X	-	X	X	15.2
X	X a	-	X b	X	15.9
X	X a	-	X c	X	15.6
X	-	-	-	-	23.7	[175]
X	-	-	X	X	13.5
X	X	-	X	X	10.9
-	-	-	X	X	11.3 e	[76]
X	-	X	-	X	23.0 f	[174]
X	-	X	X	X	22.6 f
X		X	X d	X	22.8 f

^a Fermentation of hydrolyzed cellulosic fraction; ^b biodigestion of hydrolyzed hemicellulosic fraction; ^c biodigestion of vinasse and hydrolyzed hemicellulosic fraction; ^d biogas-H₂ purification for sale; ^e calculated from article data (100,000 ha); ^f calculated from article data, considering the specific energy of ethanol 25 MJ/kg.

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The energy potential of biogas production vinasse AD was evaluated both as a product and in electricity generation [76,105]. The surplus electricity, not used in the factory’s energy demand, can be sent to the local grid companies. Using vinasse biogas could have generated about USD 85 million in carbon credits in revenues for Brazil in 2009 [173]. Electricity production is one of the potential uses of biogas that can maximize energy recovery based mainly on the availability of well-established efficient conversion technologies, such as engines and turbines [176]. Moraes et al. [173] estimated that about 12% of the energy produced by bagasse burning could be supplied by vinasse biogas. Barrera et al. [177] showed that anaerobic digestion plants improve the vinasse lagoon’s environmental profile. LCA analyses indicated a reduction of up to 77% of the total score, and the exergy recovery from the natural environment was up to 46% [177]. Pereira et al. [105] evaluated the economic viability using net present value and electricity cost parameters for biogas production in São Paulo state, Brazil (one of the world’s largest ethanol-producing regions). Total annual electricity generation was determined to be 659 GWh, with one-third of the municipalities studied considered economically viable. The vinasse could supply electricity to almost 295,000 inhabitants, representing 0.45% of the state’s energy demand [105].

On alternative anaerobic digestion methods, Fuess et al. [141] analyzed two-phase biodigestion to increase vinasse energy production compared to single-phase schemes. The two-phase system increased biogas production by 20 to 30% without harming the profitability of the biorefinery. Using an optimized alkalinization strategy can improve the environmental performance of the ethanol production chain. López-González et al. [126] investigated the feasibility of anaerobic digestion of press sludge previously pretreated by liquid hot water and alkaline heat. Economic, energy, and environmental analyses showed that co-digestion of the vinasse from the pressed sludge without pretreatment was the most viable for a sugar and ethanol plant.

Biomethane can be injected into the natural gas network or used as a substitute for fossil diesel oil in the sugarcane harvesting and transportation operations at the plant [131]. In the ethanol life cycle, fossil energy (diesel oil) is used in the mechanical harvesting and transport of sugarcane and emits greenhouse gases [178]. Diesel consumption depends on the conditions of mechanical harvesting, such as crop density, distance traveled, and truck capacity [131,179]. Transportation and sugarcane harvesting is responsible for consuming about 0.98 and 0.9 L of diesel per ton of sugarcane, and operations can reach 3.5 L/ton [180].

Longati et al. [181] analyzed different configurations to produce biogas from vinasse as complementary fuel in the boiler and as a substitute for diesel in agricultural operations. There was an insignificant reduction in environmental impacts using biogas as a complementary fuel in a boiler. Biogas used as a substitute for diesel obtained significant improvements in all categories analyzed, such as abiotic depletion, global warming, depletion of the ozone layer, human toxicity, and aquatic marine ecotoxicity. Thus, the application of biogas is fundamental for an efficient biorefinery. Studies by da Silva Neto et al. [131] show that using vinasse can improve energy balance indicators by 3.5% in electricity generation, 7.9% in the injection of biomethane in the gas network, and 27% in the substitution of diesel in plant operations. Total greenhouse gas emissions have declined by about 7% in all three applications.

5.3. The Ideal Sugarcane Biorefinery

A biorefinery that uses all waste and by-products to generate energy or product tends to minimize environmental impacts regardless of its configuration. For example, the ecological advantages of producing second-generation ethanol or biogas from the hemicellulosic biomass fraction are similar. Thus, the application of the generated product also needs to be evaluated. According to Junqueira et al. [175], second-generation ethanol can boost bioethanol production by 27% in autonomous distilleries, reaching 108.4 L of first- and second-generation ethanol per ton of sugarcane. However, Longati et al. [172] estimated an increase of about 9% in electricity production if a first-generation ethanol plant used vinasse biodigestion. Therefore, by replacing part of the biomass used in the boiler, biogas would increase by almost 8% the biomass diverted to second-generation ethanol. The studies showed that the best scenario for most of the parameters studied was the production of first- and second-generation ethanol with the fermentation of the cellulosic and hemicellulosic fraction hydrolyzed and biodigestion of vinasse. This scenario produced 113.64 L of ethanol per ton of cane and 9.2 m³ of biogas per m³ of vinasse. However, the increase in ethanol production would generate even more vinasse, requiring extra capital investment. The biodigestion of vinasse has reduced the environmental impacts in all simulations analyzed.

Different configurations of sugarcane biorefineries have economic, energy, and environmental potential, but the most promising ones, which can already integrate sugar and alcohol plants, are those with biodigestion of vinasse. Figure 4 shows the main biogas production steps according to the studies presented in Table 4.

6. Perspectives and Challenges in Vinasse Anaerobic Digestion

The unique combination of large volumes, high conductivity, high COD, and nutrient content in vinasse makes its efficient processing challenging yet full of opportunities—and industrial implementations of vinasse AD, described in Section 4, prove that.

The potential for many countries to add a clean fraction of bioethanol to the automotive fleet, most remarkably Brazil, India, and China, may push industries towards increasing circularity and a complete water recycling process. With the surplus energy provided by biomethane, more sophisticated uses for sugarcane bagasse can be implemented, starting with second-generation ethanol (and second-generation vinasse, in a virtuous cycle) and evolving to other fermentation products. Further ahead, membrane technologies may altogether change the composition of press mud and vinasse, calling for processes supporting higher solids content. In the meantime, the surplus energy can continue to be sold to feed power grids—a welcome green component to the world energy matrix.

For now, seven aspects and trends are especially promising routes to improve the anaerobic digestion of vinasse, increasing process sustainability: (i) developing and modulating microbial consortia; (ii) improving vinasse pretreatment for enhanced AD; (iii) developing microalgal processes coupled to biorefineries; (iv) improving the AD of second-generation vinasse; (v) developing biohydrogen or biohythane production from vinasse; (vi) recovering organic acids from the digestate; and (vii) developing bioelectrochemical systems. These trends are detailed below.

Microbial consortia are critical to effective biodigestion. The makeup of the microbial communities in anaerobic reactors, and its modification through space (bioreactor zones) and time (crop season), must be better understood. Modern molecular biology tools can tell us a lot about these microbial communities. Microbiota composition and dynamics can lead to insights on strategies for inoculation boosts, nutrient amendment, and process modifications, ultimately leading to higher energy recovery and a lower residual COD [96].

Residue pretreatment and composition amendment strategies can intensify CH₄ production: chemical or thermal treatment can be used to better control the outcome of AD [182,183], especially in systems with co-substrates. Furthermore, additives such as iron nanoparticles [184] can aid in enhancing methane yield.

Microalgae culture can be done in the digested vinasse, further improving the use of the residue and expanding the possibilities in sugarcane biorefineries, and increasing process circularity [185]. Microalgal cultures can grow quickly in wastewater media [186,187], although nutrient correction may be necessary [185]. The microalgal biomass can be fed directly to biodigesters in a first-generation biorefinery, enhancing biomethane generation. In a second-generation biorefinery, algal biomass can be further processed into value-added products such as lipids, protein meal, and carotenoids [188]. This requires robust strain development that can grow on the clarified but acidic and still COD-rich vinasse or strategies for residue pretreatment, e.g., filtration to reduce self-shadowing [189]. Anaerobic digestates tend to have high contents of acetic, propionic, butyric, and valeric acids, ref. [81] and vinasse digestate is no exception.

Second-generation ethanol will probably increase its participation in sugarcane biorefineries over time. That implies adapting the processes developed for first-generation vinasse to mixed residues also containing second-generation vinasse. Is that vinasse richer in organic compounds? Will typical by-products of hydrolysis (furfural, hydroxymethyl furfural, and acetic acid) make their way to the anaerobic digester? And to what effect? The literature on the composition of second-generation vinasse is still scarce (an example is Silverio et al. [190]), with most of the research effort being put into hydrolysis and fermentation. This is because second-generation processes are yet to reach a large scale, allowing the potential of residues to be unleashed before it is produced on a large scale, thus developing processes with circularity by design.

Biohydrogen can also be produced anaerobically from vinasse or as an upgrade from biomethane. The technology is in development, and it is already clear that to be economical, biohydrogen production must be done on a large scale [191]. Similarly to classical biodigestion, microbial consortia and nutrient amendment are critical to the process, with the extra concern of avoiding the domination of a methanogenic microbial population—a challenge for continuous systems. Biohydrogen produced via methane reforming can be used to produce ammonia. There is a recent agreement between the Dutch–Brazilian company Raízen to provide biomethane to the Norwegian company Yara [192] for this purpose.

Organic acids can be recovered from vinasse or its digestate. Their origin may be the sugarcane juice [193], molasses or fermentation metabolites in the initial vinasse, or the residues from the acidogenic phase. These acids may become especially relevant in biorefineries integrating lignocellulosic bioethanol. Their relatively low concentration would make profiting tricky: recovery processes, such as adsorption, extraction, or even electrodialysis, could be used but require technology development and techno-economic analysis. Still, it is a promising line of investigation when the large volumes involved are considered, and organic acid removal may be unavoidable in 2G ethanol production.

Bioelectrochemical systems have high capital cost, but are elegant processes that use microbiota capable of direct electron transfer to electrodes, generating electrical energy during effluent digestion, without combustion. This technology is compatible with high-conductivity effluents such as vinasse [194]. With the development of new materials and cost reduction, these systems could reclaim part of the vinasse energy.

7. Conclusions

Both ethanol fermentation and anaerobic digestion (AD) are long-known technologies. However, the mapping of patents and business news shows that large-scale processes are still uncommon, and most of the vinasse still proceeds to field application (fertigation) with minimal treatment. However, good results have already been obtained in small-scale methanogenic fermentation: the experimental BMP of vinasse ranges from 215 to 324 NL_CH4.kg _COD⁻¹.

Since the basics of vinasse anaerobic digestion are established, further developing the technology is paramount in order to lead to stable and profitable processes. The opportunities for improvement abound, from developing stable consortia and co-digestion to better understanding bioreactor dynamics and developing microalgal bioproducts. The techno-economical assessment of large-scale biorefinery strategies is also essential in deciding on biorefinery routes and optimizing capital and operational costs, ultimately leading to a return on investment and increased circularity.