Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel

Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel Bioenergy systems for the future 5 anaerobic biodigestion for enhanced bioenergy generation in ethanol biorefineries understanding the potentials of vinasse as a biofuel

Anaerobic biodigestion for

enhanced bioenergy generation

in ethanol biorefineries:

Understanding the potentials

of vinasse as a biofuel

L.T Fuess*, M.L Garcia†

*University of Sa˜o Paulo, Sa˜o Carlos, Brazil,†Sa˜o Paulo State University,

Rio Claro, Brazil

CODsewage COD of sewage

CODvinasse COD of vinasse

DDGS distillers dried grains with solubles

EPbagasse energetic potential of bagasse

Bioenergy Systems for the Future http://dx.doi.org/10.1016/B978-0-08-101031-0.00005-3

© 2017 Elsevier Ltd All rights reserved.

energetic potential of vinasse

LHVCH 4 lower heating value of methane

LHVEtOH lower heating value of ethanol

PCCwater per capita water consumption

Biofuel industries, such as ethanol and biodiesel plants, may be considered the mostrepresentative examples of biorefineries, based on the application of different (bio)processes to efficiently obtain value-added products and bioenergy from a wide range

of feedstocks (Poggi-Varaldo et al., 2014; Moraes et al., 2015) Focusing on ethanolproduction, important advantages must be considered, such as the worldwide techno-logical consolidation of the process and the suitability for applying a series of crops,grown under different climatic conditions, as raw materials (Willington and Marten,1982; Hill et al., 2006; Cavalett et al., 2012; Fuess and Garcia, 2014) In addition,depending on the type of feedstock, ethanol production may be coupled to sugar refin-ing and electricity generation, which enables a more flexible use of the raw material.Such interesting scenarios are particularly observed in sugarcane-based plants, so thatthe amount of total reducing sugars (TRS) directed to ethanol or sugar productiondepends intimately on their market price, whereas the residual lignocellulosic

material, that is, bagasse, is burned in cogeneration systems for both steam and tricity generation (BNDES and CGEE, 2008; Dias et al., 2013; Moraes et al., 2015).The efficient use of the raw material in ethanol plants is also strictly related to theproper management of the wastewater streams generated within the processes, so thatspecial attention must be direct to vinasse Vinasse or stillage constitutes a high-strength dark-brown wastewater resulting specifically from the step of distillation,being characterized by some common compositional aspects regardless of the feedstock,including high levels of organic matter, solids, and acidic compounds (Espan˜a-Gamboa

elec-et al., 2011; Fuess and Garcia, 2014) Based on the highly polluting character of vinasse,Willington and Marten (1982)indicated that small-scale sugarcane-based distilleries(250 m3 of ethanol—EtOH—per day) may generate polluting loads equivalent tothe ones observed for populations as high as 2 million inhabitants Although vinassegeneration rates depend directly on the type of feedstock and technological level ofthe processes, an average proportion of 10–15 L of vinasse per liter of produced ethanol

is usually observed in ethanol plants (Willington and Marten, 1982; Cavallet et al., 2012;Moraes et al., 2015) For relatively large-scale sugarcane-based Brazilian distilleries(1000 m3EtOHday
1,Dias et al., 2011), the annual volume of vinasse generated mayreach up to 3.5 million cubic meters, based on an average sugarcane harvesting period

of 232 days (CONAB, 2011) Such enormous value is of great interest in terms ofenvironmental concern and energetic potential, as further discussed

The reference literature includes a wide variety of approaches for the management

of vinasse in ethanol plants Although prior studies presented direct discharge intowater bodies and marine outfall as available options (Sheehan and Greenfield,1980; Willington and Marten, 1982), the recent literature focus on the application

of different (bio)technological approaches to vinasse, usually in an effort to reachenvironmental adequacy Studied processes include anaerobic (Mohana et al.,2009; Ferraz et al., 2016) and aerobic (de Bazu´a et al., 1991; Ferreira et al., 2011)digestion and conventional physical-chemical (Zayas et al., 2007; Rodrigues et al.,

2014) and advanced oxidative processes (Sangave et al., 2007; Asaithambi et al,

2012) Alternatives applied to vinasse management also include incineration to ashes(Willington and Marten, 1982; Akram et al., 2015) and evaporation and concentrationfor producing animal feed and reducing costs with transportation (Wilkie et al., 2000;Pimentel et al., 2007; Murphy and Power, 2008); however, both cases demand expres-sive amounts of energy, hindering the global energy balance of the process.Particularly, vinasse streams resulting from sugarcane processing are characterized

by appreciable levels of nutrients, mainly potassium (Fuess and Garcia, 2014), whichfavors their direct land disposal through fertirrigation Fertirrigation constitutes virtu-ally the sole management approach applied to vinasse in the Brazilian sugarcane-to-ethanol industry (BNDES and CGEE, 2008; Fuess and Garcia, 2014), leading toimportant reductions in the consumption of freshwater and mineral fertilizers(BNDES and CGEE, 2008; Sivaloganathan et al., 2013) However, the continuous soilapplication of vinasse may potentially generate a broad range of negative environmen-tal impacts in the long term, such as soil salinization and structural destabilization,water bodies acidification, leaching of toxic compounds to groundwater, emissions

of greenhouse gases (GHG), and release of unpleasant odors (Mohana et al., 2009;Anaerobic biodigestion of vinasse for enhanced bioenergy generation 151

Oliveira et al., 2013; Fuess and Garcia, 2014; Moraes et al., 2015) GHG emissionsand odors releasing result directly from the conversion of the biodegradable organicfraction found in vinasse by soil microbial populations, characterizing a second majordrawback from fertirrigation, since the uncontrolled organic matter conversion asso-ciated to this practice leads to expressive bioenergy losses.

According to Palacio et al (2012), the energetic potential of 1 ton of cleanedsugarcane (TC) is approximately 1718 Mcal, which is distributed as sugars (153 kgand 608 Mcal), bagasse with 50% moisture (216 kg and 598 Mcal), and straw with15% moisture (165 kg and 512 Mcal) Given that the ethanol yield in autonomous dis-tilleries, that is, TRS directed only for ethanol production, reaches usually82.9 LEtOHTC
1 (Moraes et al., 2014), a fraction of about 421 Mcal is recoveredthrough biofuel production, so that roughly 30% of the energetic content of the sugars(or 10% of the total sugarcane energetic potential) remains unconverted in vinasse Inturn, the remaining energy fraction from ethanol production in annexed plants, that is,sugar and ethanol production, accounts for slightly more than 20% of the energy fromsugars, based on average sugar and ethanol yields of 50.8 kgsugarTC
1 and53.4 LEtOHTC
1 (Moraes et al., 2014) Such estimates do not consider the amount

of energy directed to cell growth in fermentative processes that may lead to slightlyoverestimated values Nevertheless, the values presented herein indicate that vinasseshould be considered a highly energetic raw material from ethanol plants rather than aresidual stream, with great potential toward the application of anaerobic processes.Anaerobic digestion (AD) constitutes an effective alternative to the management ofvinasse and other high-strength wastewaters (HSW), in order to concentrate importantadvantages compared with other treatment approaches The application of AD as thecore technology in treatment plants enables associating reductions in the pollutingload of effluents with the generation of bioenergy through biogas, a methane-rich gas-eous blend resulting from the activity of several microbial populations under syntrophicassociations The use of biogas as energy source in industrial plants should lead to

a minimum-to-average energy recovery potential (ERP) of about 50%–60%(Borzacconi et al., 1995) In fact, a few studies on the energetic potential of vinasses fromthe corn-to-ethanol industry indicated reductions in the consumption of fossil fuels in therange of 43%–65% (Stover et al., 1984; Khanal, 2008; Agler et al., 2008; Cassidy et al.,2008; Schaefer and Sung, 2008) Therefore, considering the concept of biorefineries, theimplementation of anaerobic processes provides a more efficient exploitation of the baseraw material, with emphasis on the use of residues (by-products) as energy sources.Numerous studies on the application of anaerobic processes to vinasses from differ-ent feedstocks are presented in the literature, such as sugarcane (Craveiro et al., 1986;Costa et al., 1986; Souza et al., 1992; Ferraz et al., 2016), beet (Braun and Huss, 1982;Boopathy and Tilche, 1991; Vlissidis and Zouboulis, 1993), corn (Stover et al., 1984;Agler et al., 2008; Lee et al., 2011; Andalib et al., 2012), cassava (Luo et al., 2009, 2010and 2011), and cellulosic materials (Good et al., 1982; Callander et al., 1986 and 1987;Kaparaju et al., 2010) However, most reports are based on the use of bench-scale sys-tems, so that some studies tend to superficially discuss the ERP of the biogas Param-eters related to methane production are often characterized as complementary data forperformance assessment, which is usually focused on organic matter removal In thiscontext, wider energetic analyses are required to fully comprehend the potentials of

vinasse for bioenergy generation in the ethanol industry Thus, in this chapter, the tial of AD for bioenergy recovery in the ethanol industry is carefully considered, based

poten-on different approaches The energetic potential of vinasses from different feedstockswas calculated, in order to assess the ERP of methane (biogas) compared with theenergy consumption in different steps from ethanol production The contribution frombiogas energetic content to the energy balance of ethanol production was also consid-ered Special attention was direct to the sugarcane-to-ethanol industry, based on theenergetic self-sustaining character of such plants In this case, a detailed economicassessment was applied to assess the feasibility of implementing AD systems to thetreatment of vinasse in large-scale distilleries

generation

Ethanol production is based mainly on two basic processes regardless of the feedstock,which include the fermentation of a sugar-rich substrate followed by the distillation of thefermented solution (Fig 5.1) Feedstock processing steps may differ according to theready availability of reducing sugars, that is, the broth extracted from sugar-rich crops,such as sugarcane, beet, and sweet sorghum, may be directed employed in fermentation,whereas starch-rich (e.g., corn, cassava, and barley) and cellulosic materials require pre-treatment steps (Fig 5.1) Ethanol production from sugar-rich feedstocks may be con-ducted using directly the sugars extracted from sugarcane (juice)—autonomousdistilleries—or blends between the juice and molasses, a sucrose-rich concentrated resid-ual solution from sugar production (Fig 5.1) In this case, distilleries are named annexedplants, which constitute most of ethanol plants in Brazil (70%,Cavalett et al., 2012).Molasses may also be employed directly in ethanol production, such as in the Indiansugarcane-to-ethanol industry, where juice is used solely in sugar refining (Soam

et al., 2015) Nevertheless, regardless of the feedstock and process characteristics, vinasse

is generated specifically within the distillation step (Fig 5.1), presenting some interestingcompositional aspects in terms of biodegradable organic matter

Compositional characteristics for vinasses from different feedstocks are compiled

inTable 5.1, usually indicating high values (>0.4) for the ratio between the ical (BOD) and chemical (COD) oxygen demands, that is, BOD/COD ratio, regardless

biochem-of the feedstock This relation roughly describes the suitability biochem-of a given wastewater

to biological process treatments, so that for values higher than 0.3–0.4, the able fraction may be considered high (von Sperling, 2007) Biodegradable organiccompounds in vinasses result mainly from three sources: (i) residual fractions ofnonconverted sugars, (ii) residual concentrations of nonrecovered ethanol, and(iii) fermentation of by-products, that is, metabolites excreted by yeasts and contam-inant microorganisms, such as glycerol and organic acids (Wilkie et al., 2000; Dias

biodegrad-et al., 2012) According toWilkie et al (2000), every 1% of residual reducing sugarsand ethanol may lead, respectively, to increments of about 16 and 20 gL
1in the COD

of vinasse Since such compounds are easily assimilated in anaerobic environments,the application of AD to vinasse theoretically enables obtaining high biogas produc-tion rates as a direct result from an efficient conversion of the organic matter

Anaerobic biodigestion of vinasse for enhanced bioenergy generation 153

5.3 Bioenergy generation from vinasse: Input data

and estimates

In short, EP constitutes the energy potentially recovered as biogas from the organiccontent found in vinasse In this study, the energetic analysis was initially applied

to vinasses from various feedstocks: sugarcane (juice—SGJ—and blends of juiceand molasses—SJM), beet molasses (BML), corn (CRN), cassava (CSV), and ligno-cellulosic materials (LCL).Table 5.2 compiles the raw data used in the estimates,

Sugar-rich solution (broth)

Fermentation

Distillation Ethanol

Sugar

Vinasse

Starch-rich crops

(corn, cassava, cereals)

Lignocellulosic materials

Fig 5.1 Technological routes for ethanol production from different feedstocks Note:aIncludesthe application of different physical-chemical and/or biological processes to disrupt vegetalfibers found in lignocellulosic materials, converting polysaccharides into fermentablesugars (Moraes et al., 2015)

Table 5.1 Compositional characterization for vinasses from various feedstocks

Feedstock BOD (g L1)

COD(g L1) BOD/COD

TKN(mg L1)

TP(mg L1) K (mg L1)

SO4 2

(mg L1) pH ReferencesSugarcane

Table 5.1 Continued

Feedstock BOD (g L1)

COD(g L1) BOD/COD

TKN(mg L1)

TP(mg L1) K (mg L1)

SO4 2

(mg L1) pH ReferencesBeet

(molasses)

27.5–44.9 45.0–115.8 0.49–0.63 56–4700 56–175 5500–10,030 1040–3716 4.6–6.7 Braun and Huss

(1982),Boopathyand Tilche (1991),

Vlissidis andZouboulis (1993),

Wilkie et al

(2000),

Lutoslawski et al.(2011)

Corn 26.9–68.6 60.0–129.3 0.42–0.56 755–2000 1170–4140 1100–5560 nd 3.3–4.6 Stover et al

(1984),Wilkie(2008),Agler

et al (2008),

Alkan-Ozkaynakand Karthikeyan(2011),Nasr et al.(2011),Andalib

et al (2012)

Cassava 24.0–35.0 40.0–70.4 0.50–0.60 304–1440 83–400 nd nd 3.9–4.2 Luo et al (2009),

(2010), and(2011);Wang

Table 5.2 Reference data on vinasse characterization and typical performance of AD systems applied

to vinasses from various feedstocks

Reference Wilkie et al (2000) Costa et al (1986)

Reference Costa et al (1986),Prada et al (1998) Souza et al (1992)

Reference Wilkie et al (2000) Good et al (1982)

Parameters: COD, chemical oxygen demand; OLR, organic loading rate; ER COD , COD removal efficiency; MY, methane yield.

a Values corrected to standard temperature and pressure conditions (STP) if not indicated in the reference study

b Temperature not indicated

including the compositional characteristics of vinasse (COD) and typical performancedata on COD removal and methane production from AD systems reported elsewhere.

EP calculations were based on Eq.(5.1), so that an average vinasse production rate(VPR) of 13 m3m
3EtOHwas considered (BNDES and CGEE, 2008):

In Eq.(5.1), the terms EP, VPR, COD, ERCOD, MY, and LHVCH4are, respectively, theenergetic potential of vinasse (MJ m
3EtOH), the average generation rate of vinasse(13 m3m
3EtOH), the chemical oxygen demand of vinasse (kgCODm
3), the CODremoval efficiency (dimensionless), the methane yield (N m3kg
1CODremoved), and thelower heating value of methane (50 MJ kg
1or 35.72 MJ N m
3,Heywood, 1988).Performance data compiled inTable 5.2refer to the application of thermophilic tem-perature conditions (50–55°C) to the reactors, which may be considered a feasiblealternative for vinasses, based on two main advantages: (i) Vinasse is released fromdistillation columns at approximately 85–95°C (Dias et al., 2012; Moraes et al., 2015),naturally reaching the adequate temperature for the treatment without requiringenergy inputs, and (ii) thermophilic systems may handle higher OLRs compared withmesophilic ones, which enables the use of more compact reactors and consequentlyimproves the economics of full-scale anaerobic processes (Wilkie et al., 2000)

balance (EB) estimates

Estimates on the ERP from biogas were based on an energy balance for the production

of ethanol, in which the fossil energy used in feedstock processing and conversion intoethanol comprised system inputs, whereas the outputs corresponded to the EP previ-ously calculated (Section 5.3.1) Eq.(5.2)describes the calculations for the ERP (%),

so that the terms EC and EP are, respectively, the amount of consumed fossil energy inethanol production (MJ m
3EtOH) and the energetic potential of biogas (MJ m
3EtOH):

Table 5.3 Range of values for energy consumption raw data used in ERP and EB estimates

Transportoperationsa

Globalconsumption

(Agricultural

phase)

(Ethanolproduction)Sugarcaneb 1590–5620 260–820 330–470 2340–6630 Khan and Fox (1982),Macedo (1992),Macedo

(1998) Oliveira et al (2005),Boddey et al (2008),

Macedo et al (2008),Turdera (2013)

Corn 4110–26,060 11,800–31,530 100–2730 19,700–48,680 Worley (1992),Shapouri et al (1995),Graboski

(2002),Pimentel (2003),Shapouri et al (2003),Kimand Dale (2005),Oliveira et al (2005) Pimentel andPatzek (2005),Lavigne and Powers (2007),Pimentel

et al (2007),Yang et al (2009),Salla et al (2010a),

Yang and Chen (2012)

Cassava 1430–8910 6690–21,720 340–3290 12,060–25,730 Dai et al (2006),Nguyen et al (2007a),Nguyen

et al (2007b),Leng et al (2008),Soares et al

(2008),Silalertruksa and Gheewala (2009),Salla

et al (2010b),Papong and Malakul (2010),Le et al.(2013),Liu et al (2013)

a Include feedstock transportation to distilleries and/or ethanol distribution.

b Values regarding the Brazilian sugarcane-to-ethanol industry, in which most of energy requirements in distilleries (electricity and steam) is supplied by bagasse burning in cogeneration systems.

sugarcane, corn, and cassava Ethanol production chains based on corn (the UnitedStates) and sugarcane (Brazil) processing account for over 80% of the world ethanolproduction (DOE, 2016) In fact, almost 60% of the world ethanol production in 2014resulted from the US corn-to-ethanol industry, whereas the Brazilian sugarcane-basedproduction reached slightly more than 25% Regarding cassava, the interest on its use

as feedstock for ethanol production is due to important favorable characteristics, such

as the cultivation suitability to a wide variety of soils and climatic conditions andminimal nutritional requirements (Nguyen et al., 2007a; Soares et al., 2008) Suchcharacteristics enable the continuous operation of distilleries, which constitutes aremarkable advantage over sugarcane-based processes

EB calculations considered both the usual production layout (Eq.5.3), that is, out the application of anaerobic processes to vinasse and scenarios including the EP ofvinasse (Eq.5.4) Particularly, for sugarcane-based production chains, the energeticpotential resulting from bagasse combustion was also inserted (Eq 5.5) InEqs (5.3)–(5.5), the terms EB, LHVEtOH, EC, EPvinasse, and EPbagasse are, respec-tively, the energy balance for ethanol production (dimensionless), the lower heatingvalue of ethanol (21,220 MJ m
3EtOH), the energy consumption in ethanol production(MJ m
3EtOH), and the energetic potentials of both vinasse (MJ m
3EtOH) and bagasse(MJ m
3EtOH) EPbagasse considers only electricity generation (46.2 kWh TC
1,CONAB, 2011) In short, the energy balance relates the amount of bioenergy producedand the energy inputs required for the biofuel production (Hill et al., 2006):

A detailed technological assessment was conducted specifically for large-scalesugarcane-based distilleries, including technical, environmental, and economic aspects.Analyses were based on three scenarios, considering ethanol production in autonomousand annexed distilleries and the total Brazilian ethanol production in the 2014–15harvesting period.Table 5.4compiles the reference data considered for each scenario,including characteristics of the ethanol plant, AD system, and power plant Performancedata regarding the application of anaerobic processes to vinasse were based onCosta

et al (1986) and Souza et al (1992), whereas bioenergy (electricity) generationfrom biogas was based on the application of internal combustion engines (ICEs)(Table 5.4), taking into consideration commercially available technical data

The environmental assessment included estimates on the equivalent population thatallows representing the pollutant load of a given wastewater in terms of a populationwith the same polluting capacity (Moraes et al., 2014) Calculations were performed

Table 5.4 Input data for the technological assessment

Scenario

Autonomousdistillery

Annexeddistillery

2014/2015harvesting

(GE-J620 GS-F12(GE-Jenbacher)

J620 GS-F12(GE-Jenbacher)Electric conversion

f Costa et al (1986) and Prada et al (1998)

g Calculations for the 2014/2015 harvesting period were based on compositional characteristics of vinasses from sugarcane juice.

h Souza et al (1992)

i Heywood (1988)

j Procknor (2009)

k Corrected values for Dec 2014.

l Operating cost includes both AD system and power plant.

m EPE (2014)

Anaerobic biodigestion of vinasse for enhanced bioenergy generation 161

in terms of organic matter levels (COD), according to Eq.(5.6), so that the terms EQP,VPR, CODvinasse, CODsewage, PCCwater, and CR are, respectively, the equivalent pop-ulation (number of inhabitants), the production rate of vinasse (L day
1), the COD

of vinasse (g L
1), the COD of sewage (0.6 g L
1,von Sperling, 2007), the per capitawater consumption (225 L inhab
1day
1, von Sperling, 2007), and the sewage-to-water return coefficient (0.8,von Sperling, 2007):

EQP¼ VPR CODvinasse

The economic feasibility of implementing AD treatment plants in large-scalesugarcane-based autonomous and annexed distilleries was assessed by calculatingthe internal rate of return (IRR), net present value (NPV), and discounted paybackperiod of the project, with an acceptable minimum rate of attractiveness (MRA) of8.76% per year (Nogueira et al., 2015) Project lifetime, depreciation, and constructionwere set as 25, 10, and 2 years, whereas tax rates were fixed at 34% Investment andoperating costs and selling prices were quoted in Dec 2014, using a conversion rate of0.43 USD per unit of Brazilian real Reference data (capital and operating costs) forthe AD system and power plant were obtained fromProcknor (2009) (Table 5.4),based on the installation of internal circulation reactors coupled to hydrogen sulfideremoval systems and internal combustion engines without recovering thermal energy,that is, biogas was applied solely for the generation of electricity

In this section, potentials for bioenergy recovery from vinasse AD are detailedaccording to three main themes Firstly, the EP of vinasses from different feedstocks

is presented, followed by estimates on the ERP and EB for specific production chains,that is, sugarcane-, corn-, and cassava-based plants Lastly, environmental andeconomic aspects regarding the implementation of AD systems in sugarcane-baseddistilleries are addressed, also considering a wider energetic analysis

EP values obtained for vinasses from various sources are depicted inFig 5.2A, withvalues ranging from 3294 to 7592 MJ m
3EtOH Similar EP values referring to vinassesfrom molasses (6762 MJ m
3EtOH) and starch-based feedstocks (5789–6700 MJ m
3

EtOH)were previously reported elsewhere (Prakash et al., 1998; Nguyen et al., 2007b; Khanal,

2008), which confirm the potentials of vinasse as a biofuel The high values observed forvinasses from starch-rich and lignocellulosic materials (6342–7592 MJ m
3

EtOH,Fig 5.2A) probably result from the pretreatment methods applied to the raw materials.The aggressive conditions usually employed in such step, which may include theprovision of high temperatures and acidic conditions, enhance the conversion of com-plex substrates into more easily assimilable organic compounds, so that the fractions notuptaken by yeasts remain as residual organic compounds in vinasse Vinasses from

3294 –2163

0.06 0.08

0.26 0.21

Fig 5.2 Estimates on energy production and consumption and sludge production in anaerobicand aerobic treatment processes applied to vinasse: (A) energetic potential of vinasse (positivevalues) and energy consumption with aeration (negative values), (B) ratio between theenergy consumption in aerobic processes (ECaerobic) and the energetic potential of vinasse(EPvinasse), and (C) sludge production Abbreviations:SGJ, sugarcane juice; SJM, sugarcanejuice + molasses;BML, beet molasses; CRN, corn; CSV, cassava; LCL, lignocellulosics.Note that “EC aerobic main” and “EC aerobic post” represent the energy consumption withaeration in treatment systems considering the aerobic process as the main and posttreatmentsteps, respectively Sludge production values were calculated from biomass coefficient yields

of 0.40 and 0.06 g VSS g
1COD for aerobic and anaerobic microorganisms, respectively(Metcalf and Eddy, 2003)

Anaerobic biodigestion of vinasse for enhanced bioenergy generation 163

cereal-feedstocks conversion, such as corn, are characterized by high amounts of proteincompounds, which constitute highly energetic substrates (Wilkie, 2008; Westerholm

et al., 2012) On the contrary, the lower EP levels associated to sugarcane juice vinasses(3294 MJ m
3EtOH,Fig 5.2A) result from a more efficient substrate uptake during fer-mentation, as the organic fraction of the broth is characterized almost uniquely by sugarsthat are promptly utilized by yeasts The addition of molasses to the fermentable brothand the direct use of molasses provide considerable gains in the EP of sugar-based feed-stocks (4681 and 7501 MJ m
3EtOH, respectively, for SJM and BML,Fig 5.2A), alsoresulting from higher inputs of organic matters prior to fermentation

Despite the lower EP values observed for sugarcane vinasses herein, it should behighlighted the high MY levels (0.307–0.329 N m3

kg
1CODremoved,Table 5.2) obtainedeven for the application of high OLR values (18.3–26.5 kgCODm
3d
1,Table 5.2).Similar MY values reported for vinasses from molasses and lignocellulosics(Table 5.2) were obtained at organic loadings approximately two- to threefold lower

In practical terms, such results enables achieving high relations between bioenergygeneration and treatment system compactness in sugarcane-based plants, whichmay exert beneficial effects over the economic feasibility of the plants Yet focusing

on sugarcane-based chains, the implementation of second-generation (2G) processes

in biorefineries, that is, the use of sugarcane straw and bagasse for enhanced ethanolproduction tends to considerably increase the bioenergy production from vinasse,based on the coupling between the streams from both sugarcane and lignocellulosicmaterials processing Global EP values could reach levels 1.6- to 2.3-fold higher com-pared with the ones calculated for first generation plants (SGJ and SJM,Fig 5.2A).However, fundamental studies on the characterization of vinasses from the processing

of lignocellulosic materials and on their suitability to AD must be further considered,

as the type of pretreatment applied to disrupt vegetable fibers may generate a widerange of interfering and/or toxic compounds to biological systems, such as hydro-xymethylfurfural, phenolic compounds, and sulfates (Moraes et al., 2015)

At this point, the role of sulfate as an interfering compound should be considered,since negative effects over methane production may be observed in AD systems sub-mitted to high sulfate levels Sulfate sources in sugarcane vinasses include the use ofsulfuric acid to prevent microbial contamination and cell flocculation during the fer-mentative step (BNDES and CGEE, 2008; Oliva-Neto et al., 2013; Barth et al., 2014;Moraes et al., 2015) Particularly, the higher sulfate concentrations observed in molas-ses vinasses (Table 5.1) also result from the use of sulfuric acid in specific steps fromsugar processing (BNDES and CGEE, 2008) The application of acidic pretreatmentmethods to lignocellulosic materials may also lead to high sulfate concentrations invinasse (Table 5.1), as sulfuric acid constitute one of the main chemicals employed

in the process (Moraes et al., 2015)

Sulfate is converted into reduced sulfur compounds, mainly sulfide, by reducing bacteria (SRB) in anaerobic environments in a process known as sul-fidogenesis SRB constitute a versatile microbial group that grows on different types

sulfate-of substrates (Lens et al., 1998; Chen et al., 2008), competing mainly with methanogenicmicrobial populations by common substrates (Chen et al., 2008) This process hindersthe conversion of organic substrates into methane due the diversion of the electron flow

to sulfidogenesis, consequently impacting bioenergy recovery Toxic effects associated

to sulfide also impair the methanogenic activity in AD systems submitted to sulfate-richwastewaters (Chen et al., 2008; Camiloti et al., 2014) MY values (Table 5.2) used in EPestimates for vinasses from sugarcane, beet, and lignocellulosics are approximately90%–95% similar to the theoretical value (0.35 N m3kg
1COD), suggesting reducedeffects of interfering compounds over the studied processes Nevertheless, it is impor-tant highlighting the demand for critical studies on the influence of sulfate on the AD

of vinasse, in an effort to prevent overestimated studies on the bioenergy recovery fromthis HSW, mainly for molasses vinasses

EP values may also be used to emphasize the advantages of selecting AD as thecore treatment technology in ethanol biorefineries compared with other approaches,such as aerobic processes Assuming an energy consumption with aeration of about

2 kWh kg
1COD, that is, 7.2 MJ kg
1COD(Cheng et al., 2012), approximately 60%–80%

of the achievable EP with AD would be required in the operation of aerobic digesters

as the main treatment step to reach the same COD removal levels than AD (Fig 5.2Aand B) Most aerators demand about 1 kWh kg
1of oxygen supplied to the reactionsystem (Khanal, 2008) In terms of sludge generation, over 20 kg of solids per cubic meter

of treated vinasse could be produced in such systems depending on the vinasse source(Fig 5.2C) On the other hand, the application of sequential anaerobic-aerobic treatmentschemes to vinasse could associate several beneficial characteristics, as expected

In short, no more than a quarter of the EP from vinasse would be demanded in theaeration of aerobic reactors (Fig 5.2A and B), enabling the achievement of both globalpositive bioenergy balances (considering only the energy recovered from vinasse) andenvironmental adequacy in terms of reductions in the organic polluting load of vinasse.Moreover, sludge generation levels in the aerobic phase would be reduced to similarpatterns compared with AD (Fig 5.2C), considerably reducing problems with themanagement of excess sludge Such values highlight the importance of applying ADsystems as the first conversion step in treatment systems to efficiently handle highorganic inputs

One final remark should be taken into consideration regarding the EP from vinasse,based on the effective amounts of energy potentially recoverable from biogas, that is,assuming intrinsic losses from available conversion technologies in different applica-tions, such as electricity and thermal energy production Electric efficiency values, that

is, the proportion of the total EP converted into electricity, in current large-size enginesand gas turbines (>3 MW) may reach over 40% (EIA, 2015) in systems operated withgaseous fuels The application of the combined cycle (CC), which considers the cou-pling between gas and steam turbines, may increase the electric efficiency to levelsabove 50% In turn, combined heat and power (CHP) systems enable achieving globalconversion efficiencies as high as 80%–85% almost equally divided into electric andthermal conversions, depending on the prime mover used (EIA, 2015) CHP systemsare based on the recovery of the heat fractions dissipated within the cooling systems(engines) and exhaustion gas streams (engines and gas turbines) to produce hot water

or steam, which may be directly used in industrial processes, such as heating and dryingsteps and mechanical driving of machinery (EIA, 2015; Moraes et al., 2015) Therefore,over 80% of the EP values presented inFig 5.2Amay be recovered as electric and ther-mal energy in ethanol plants, which represents, in absolute values, available energyamounts of at least 2635–6074 MJ m
3

EtOH, depending on the feedstock

Anaerobic biodigestion of vinasse for enhanced bioenergy generation 165