Anaerobic Digestion-Based Biorefinery for Bioenergy and Biobased Products

Abstract

Population growth and increasing affluence worldwide have resulted in a significant increase in energy and material consumption as well as waste generation. Currently, the main source of energy and materials is petroleum, which has serious implications for energy security and the environment (e.g., climate change). The efficient conversion of abundant renewable bioresources into bioenergy and biobased products has significant potential to contribute to meeting the ever-increasing demand for energy and products. Anaerobic digestion (AD)-based biorefineries have great potential to serve as a technology for efficient conversion of a variety of low-value feedstocks, ranging from municipal and industrial organic wastes, to agricultural and forest residues, and energy crops, into high-value biofuels and biobased products with concurrent waste valorization. A more comprehensive study supported by research and development, however, is crucial for developing an AD-based biorefinery system analogous with today's petroleum refineries. This review focuses on an AD-based biorefinery approach. It offers a critical analysis of recent advances in AD-based biorefineries for producing bioenergy and biobased products.

Introduction

Industrialized society's reliance on fossil-based fuels and products has serious implications for energy security and the environment. Energy and products derived from renewable bioresources are regarded as potential alternatives to petroleum-based fuels and materials, among other products. Recently, the focus on bioenergy production has shifted toward the use of non-food/feed feedstocks such as lignocellulosic biomass (e.g., agricultural and forest residues and energy crops) to address the food-versus-fuel debate associated with the use of feedstocks such as corn, sugarcane, and soybean.

Lignocellulosic biomass, however, is composed primarily of cellulose, hemicellulose, and lignin, and the interactions of these components create a highly recalcitrant biomass structure. Several studies have focused on enhancing the deconstruction of lignocellulosic biomass into simple sugars—such as C5 and C6 sugars—through physical, chemical, biological, and hybrid pretreatments for the production of biofuels (primarily ethanol) via biochemical pathways.^1,2 Pretreatment is not only costly, but also often generates a significant amount of liquid and solid wastes that require further treatment before disposal into the environment. The high cost of pretreatments and the biomass loss in the waste stream are major challenges for achieving the economic viability and environmental sustainability of lignocellulose-based biofuels.^3

–8

A biorefinery concept for lignocellulosic biomass conversion is one avenue for achieving economic viability and environmental sustainability in bioenergy production. A biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass.⁹ The major objective of any biorefinery is to optimize the resource-use efficiency and minimize waste, thereby increasing economic benefits and environmental sustainability. In summary, the biorefinery concept is analogous to petroleum refineries that generate multiple fuels, power, and chemical products from crude petroleum.¹⁰ In a conventional biomass-to-bioenergy conversion process, variation in the biomass composition—which varies with biomass species, geographical locations, and crop growing conditions, among other factors—has been treated as a challenge because such differences create variation in the consistency and yield of the end-products. However, in a biorefinery approach, such variation is treated as an opportunity for producing diverse biobased products.

Anaerobic digestion (AD), which is already widely used in the wastewater-treatment field, has now been extensively adopted for producing bioenergy (i.e., methane [CH₄]-rich biogas) using renewable bioresources. AD can efficiently convert organic waste (liquid and solid) into biogas for use as a source of renewable energy, with remediation of wastes. More importantly, AD can also serve as an effective biological pretreatment of lignocellulosic feedstocks, facilitating the subsequent breakdown of such biomass into its constituent sugars (i.e., glucose, galactose, xylose, arabinose, and mannose) and/or short chain fatty acids (namely, acetic, propionic, and butyric acids), which can be further converted to many valuable chemicals and biofuels.^11
–13

AD-Based Biorefinery

Recently, AD has emerged as a promising bioconversion technology for producing renewable bioenergy (i.e., biogas/CH₄) from agriwastes such as animal manure and crop residues, and dedicated energy crops including maize silage, Napier grass, and energy cane. There are already more than 13,000 commercial AD plants for bioenergy production in Europe, and Germany alone has more than 8,000 such plants in operation.¹⁴ However, the conversion of dedicated energy crops into CH₄ alone may not optimize both substrate-use efficiency and the economic return of a commercial biogas facility.^15,16 AD can serve as a centerpiece biorefinery approach with diverse functions, such as providing a technology for waste remediation, for biological pretreatment of lignocellulosic biomass, and for producing bioenergy and biobased products.¹⁵ Fig. 1 presents a schematic of an AD-based biorefinery concept.

Fig. 1.

Schematic of an AD-based biorefinery for producing biofuels and biobased products.

As a technology for the treatment and recovery of resources from organic wastes (industrial, municipal, and agricultural), AD not only generates bioenergy in the form of biogas and biobased products, such as organic acids and biopolymers, but also simultaneously remediates the wastes, reducing the environmental footprint of such industries. When operating conditions such as solids retention time (SRT), temperature, and pH are maintained within the optimal range, the consortium of anaerobic microorganisms may selectively convert the plant solubles and hemicellulose into biogas, while effectively exposing lignin and cellulosic fibers in the digested fiber. In the downstream processes, commercial enzymes (cellulases) can be added to saccharify digested cellulosic fibers into soluble glucose. The monomeric sugars can then be used as precursors in the production of diverse products ranging from bioenergy to organic acids (e.g., succinic acid) and biopolymers (e.g., bioplastic).^1,16

–19 After enzymatic hydrolysis, the insoluble solid residue, mainly consisting of lignin and residual fiber, can either be combusted for heat and electricity generation or be further processed into different biobased products such as lignosulfonates. The digester effluent (i.e., the liquid stream after separation of solid residue from the digestate) is generally rich in nutrients (primarily nitrogen and phosphorus), and the soluble organic matter may require further pretreatment before disposal.^20,21 Though AD plant effluent is currently being used in fertigation (fertilization through irrigation), the high concentration of elements such as copper and zinc—in the case of a digester fed with animal manure, where such elements originate from the micro- and macronutrients supplemented in animal feed—or ammonia in the effluent is phytotoxic when applied directly.^20,22

A significant opportunity does exist to utilize AD effluent for macro- and microalgae production. Such effluent-based algae cultivation offers both nutrient removal from the effluent, which can be recycled back as process water into the AD plant, as well as algal biomass that can be further processed into biofuels and biobased products. The integration of AD in the biorefinery concept as a technology for organic waste remediation, as well as the utilization of digestate and effluent from AD for biofuels and biobased products generation, are discussed in greater detail in the following sections.

Resource Recovery from Wastes Using AD Technology

AD for volatile fatty acids (VFAs) production from organic wastes

Rapid population growth, urbanization, and industrialization have made management of solid and liquid wastes a major challenge, both in developed and developing countries. Among different waste streams, organic wastes (e.g., food waste and the organic fraction of municipal solid wastes, agriprocessing wastes, and animal manures) management is difficult because of the high moisture content and rapid decay under ambient conditions.²³ Most waste management practices currently focus on either disposal (e.g., landfill) or treatment/stabilization to meet environmental regulations.²⁴ Such practices are not only costly, but also have an adverse impact on the environment, contributing to surface and ground water pollution and greenhouse gas emissions. Waste management practices should be directed toward resource recovery. The bioconversion of organic wastes into useful chemicals such as VFAs through the AD process is one such avenue. For example, waste containing glycerol from biodiesel production, food waste, olive mill wastewater, starch-rich potato-processing wastewater, and waste activated sludge have been used as suitable substrates for VFAs production through AD.^{23,25

–30} Moreover, AD has been reported to be a cost-effective and environmentally friendly technology for VFAs production from waste streams.^25,31 Table 1 summarizes the production of the VFAs from selected waste streams.^{32

–39}

Table 1.

Volatile Fatty Acids (VFAs) Production from Selected Organic Wastes^a

WASTE TYPE	ORGANIC CONTENT (mg COD/L)	REACTOR TYPE AND OPERATING CONDITIONS	VFAs CONCENTRATION (mg/L)
Kitchen waste³²	166,180	Batch reactor, pH 7.0, 35°C, 4 days	36,000
Organic fraction of municipal solid waste^33,34	196,700	Plug flow reactor, pH 5.7–6.1, 37°C, HRT=SRT 6 days, OLR 38.5 g VS/L/d	23,110
	150,600^b	Plug flow reactor, pH 6.6, 55°C, HRT 6 days, OLR 22.5 g VS/L/d	19,581
Palm oil mill effluent³⁵	88,000	Semi-continuous reactor, pH 6.5, 30°C, HRT 4 days	15,300
Dairy wastewater^36,37	4,420	Continuous flow—completely mixed reactor, pH 6.8–7.2, 35°C, HRT 0.5 day	3,100
	4,000	Upflow anaerobic sludge blanket reactor, pH 5.5, 55°C, OLR 6 g COD/L/d	1,032
Gelatin-rich proteinaceous wastewater³⁸	4,000	Upflow anaerobic sludge blanket reactor, pH 6.5, 37°C, HRT 0.5 day	1,573
Food waste+sludge³⁹	29,050	Continuous upflow reactor, pH 5.5–5.9, 18°C, HRT 1 day, 25% food waste+75% primary sludge (on weight basis)	3,610

COD, chemical oxygen demand; HRT, hydraulic retention time; SRT, solids retention time; OLR, organic loading rate; VS, volatile solids.

mg COD/kg.

VFAs produced may be further processed into biogas or converted biologically/chemically into alcohol-based fuels such as ethanol and butanol or other value-added products such as polyhydroxyalkanoates, or they can be used directly to generate electricity in microbial fuel cells.^40,41 Moreover, VFAs can be used as a carbon source in biological nutrient removal in wastewater treatment and lipid production by oleaginous microorganisms for subsequent biodiesel production.^42
–44 Elefsiniotis and Wareham demonstrated the removal of nitrate by adding VFAs produced during anaerobic treatment of potato processing effluent and municipal primary sludge.²⁸ The denitrifiers used carbon derived from the VFAs, with the highest nitrate consumption rates found when acetic acid was used as a carbon source.²⁸

Oleaginous microorganisms such as microalgae, yeasts, and molds accumulate lipids up to 70% of their biomass under nutrient-limiting conditions.⁴⁴ These microbial-derived lipids serve as a renewable source for biodiesel production through transesterification. However, the high cost of the carbon source—about 80% of the total medium cost when glucose was used—for cultivation of oleaginous microorganisms makes the whole process economically unfeasible.⁴³ VFAs obtained from agriprocessing wastes and a variety of biodegradable organic wastes can be used as an alternative carbon source for lipid production by using oleaginous microorganisms.⁴⁵ Various studies have shown the ability of oleaginous microorganisms, such as Cryptococcus albidus, Cryptococcus curvatus, and Yarrowia lipolytica to use VFAs for growth and lipid accumulation.^42
–44 However, a higher concentration of VFAs (≥5,000 mg/L) can inhibit microbial growth.⁴³ Use of a two-step process can avoid toxicity of VFAs at high concentrations.⁴⁴ Y. lipolytica was initially grown on glucose and glycerol separately for biomass production, followed by the addition of VFAs under nitrogen-limited conditions to induce lipid accumulation. High lipid content (∼40%) was obtained when glucose and an acetic acid/VFAs mixture was used.⁴⁴ Under pH-controlled conditions, higher concentration of VFAs can still support the growth of C. curvatus.⁴⁶ Chi et al. demonstrated a strong correlation between the pH and concentration of VFAs used, where higher concentration of VFAs (potassium acetate) required higher pH to support cell growth.⁴⁶

The yield and composition of VFAs produced from different organic wastes depend on the composition of the wastes as well as the digester operating conditions, such as temperature, pH, organic loading rates (OLR), hydraulic retention time (HRT), and SRT, among others. Various studies have shown the significant effect digester pH has on the production of VFAs. For example, alkaline conditions (pH=8.0–11.0) enhanced hydrolysis (and subsequently VFAs production) from waste activated sludge, whereas slightly acidic conditions (pH=5.3–6.0) were favorable for VFAs production from food waste and industrial wastewater such as dairy whey effluent and pulp and paper mill effluents.^24,47
–49 Moreover, alkaline environments (pH=8.0–11.0) prevent consumption of the produced VFAs by methanogens and thus favor VFAs production by acidogens. Chen et al. found that the optimal operating conditions for maximum VFAs production from the codigestion of food waste and waste activated sludge were a pH of 8.0, carbon-to-nitrogen ratio (C:N) of 22, temperature of 37°C, and a fermentation time of 6 days.⁴¹ However, the chemical composition of the waste that was used to produce the VFAs was not taken into consideration.

More interestingly, the optimal pH for VFAs production varies by substrate type.²⁴ In the case of dairy wastewater, production of propionate was higher at lower pH (4.0–4.5) while the production of acetate and butyrate was favored at higher pH (6.0–6.5).⁵⁰ In the case of cheese whey, the amount of propionate produced increased when pH was increased from 5.3 to 6.0, while the amount of acetate and butyrate produced decreased.⁴⁸

VFAs production is feasible at low temperatures, but it decreases with decreasing temperature.^30,51 A significant improvement in both yield and production rate of VFAs were reported when the digester temperature was increased from psychrophilic to mesophilic conditions. The result, however, was inconsistent in terms of improvement in VFA production when operating temperature was increased from mesophilic to thermophilic conditions.^24,52,53 Moreover, the effect of temperature on the composition of VFAs was insignificant.²⁴

A longer HRT improves VFAs production to some extent, as microbes get more time to act on the substrates.^24,48,53,54 Since the acidogens are fast growing compared to the methanogens, at a longer HRT methanogens are likely to metabolize VFAs into CH₄, resulting in lower VFAs yield. Alkaya and Demirer showed that there was higher total VFAs production (2,159–3,635 mg/L as acetic acid) at an HRT of 2 days than at 4 days (1,814–2,640 mg/L as acetic acid).³¹ Fang and Yu observed an almost 2-fold increase in VFAs production from dairy wastewater when the HRT was increased from 4 h to 12 h, but the increase in VFAs production improved only by 6% when HRT was further increased from 16 h to 24 h.⁵⁵ The production of propionic acid was favored when HRT was increased during acidogenic fermentation of whey (from 20 h to 95 h) and paper mill effluent (from 11 h to 24 h), but the production of butyric acid decreased at longer HRT.⁴⁸

Similar to HRT, SRT also significantly affects VFAs production.^56,57 However, SRT should be adjusted based on the substrate type and how easily the substrate can be hydrolyzed. A short SRT does not provide enough time for hydrolysis of the substrate, whereas prolonged SRT results in VFA consumption by methanogens.

OLR has a significant effect on the concentration and distribution of the VFAs.^54,58,59 During the acidogenic fermentation of pharmaceutical wastewater, Oktem et al. found that increasing OLR to 7,000–13,000 mg chemical oxygen demand (COD)/L/d was followed by an increase in VFAs production. However, a further increase in OLR to 14,000 mg COD/L/d resulted in decreased VFAs concentration (from 3,410 mg/L to 1,370 mg/L as acetic acid).⁵⁹ During organic acid production from starchy wastewater, the relative amount of propionic and butyric acid produced varied according to the OLR, while acetic acid was the major VFA produced over all the OLR ranges. Propionic acid was the second major acid, at a medium OLR of 10,000 mg COD/L/d, but at a higher OLR of 26,000 mg COD/L/d, butyric acid replaced it.⁵⁸

Since both the operating conditions and the substrate type have a significant interactive effect on both the yield and composition of the VFAs, the operating conditions should be adjusted based on both the substrate type and the VFAs of interest.

AD for bioenergy production from organic residues of bioenergy industries

The conversion of renewable bioresources into biofuels generates a significant amount of organic residues (solid or liquid) in both upstream and downstream processes. The actual characteristics of such residues vary significantly based on the feedstocks and processing strategies. Such wastes, in general, are rich in organic matter and nutrients, and thus need to be treated prior to disposal. Hence, a significant potential exists in using these wastes for bioenergy production via AD, with concurrent waste valorization.

Every gallon of bioethanol produced from sugarcane juice, for example, generates about 9–18 gallons of liquid waste known as vinasse.⁶⁰ Vinasse in general is acidic in nature (pH=3.5–5.0) and is rich in organic matter (COD=50–150 g/L).⁶⁰Although vinasse has primarily been disposed of through fertigation, the presence of phytotoxic, antibacterial, and recalcitrant compounds limits its land application. AD of vinasse, however, provides an opportunity to treat the waste and simultaneously produce bioenergy, such as CH₄ or hydrogen (H₂). After AD, the effluent can be recycled back to the ethanol-production plant as process water. Martín et al. reported the successful utilization of vinasse as a substrate for AD for bioenergy generation.⁶¹ España-Gamboa et al. conducted an AD study using a laboratory-scale modified upflow anaerobic sludge blanket reactor (UASB) to treat the vinasse.⁶⁰ The authors reported COD removal of 69% at an OLR of 17,000 mg COD/L/d with concurrent CH₄ yield of 263 L/kg/COD_added.

Similarly, the liquid stream obtained following the ethanol distillation from starch-based feedstocks (e.g., corn and cassava) is commonly known as stillage. Stillage in general is an acidic (pH 4.5) residue high in total solids (TS, 11.4%), volatile solids (VS, 10.4%), and organic matter content (total COD [TCOD], 203 g/L) but low in alkalinity and nitrogen (<1%) content.⁶² Due to the high TS content, the stillage is typically centrifuged to separate the liquid stream (called thin stillage) and solid residue (wet cake). In a corn-based ethanol plant, part of the thin stillage is recycled back as process water, while the rest of the thin stillage is concentrated (to syrup) and mixed with dried cake to produce distiller's dried grains with solubles (DDGS) for use as animal feed. Processing of stillage to DDGS, however, is energy-intensive due to the drying and evaporation steps, which account for about 30–45% of the total energy consumption in a corn-based ethanol plant.^60,63 Thus, utilization of stillage as a substrate for biogas production via AD may enhance the bioenergy yield from starch-based ethanol plants. However, low alkalinity and nitrogen content (i.e., high C:N ratio) may cause digester instability and a low CH₄ yield. Hence, stillage digestion with a substrate (codigestion) rich in alkalinity and nitrogen content, such as cattle manure or poultry litter, may significantly improve digester performance as well as CH₄ yield.

Luo et al. studied the treatment efficiency of an anaerobic sequencing batch reactor at thermophilic conditions (55±1°C) to treat cassava stillage (TCOD, 40–70 g/L; soluble COD [SCOD], 25–35 g/L; suspended solids [SS], 30–45 g/L; and pH 4.0–4.2). The authors reported TCOD and SCOD removal efficiencies of about 91% and 87%, respectively, at an HRT of 10 days, with concurrent specific CH₄ yield of about 220 L CH₄ ^/kg COD_added.⁶⁴ In another study, Zhang et al. integrated ethanol fermentation with AD to recycle the wastewater produced from a cassava-based ethanol plant. The stillage produced from cassava fermentation was anaerobically digested to produce biogas, and the AD effluent was recycled back to the ethanol plant for media preparation.⁶⁵ With proper operation of the anaerobic digester, the authors demonstrated the successful application of AD effluent as process water in an ethanol plant without any adverse effect on ethanol production.

Similarly, seed cake, the solid residue following oil extraction from oil crops (e.g., soybean, rapeseed, and jatropha), is the major solid residue generated in biodiesel plants. Though seed cake (e.g., rapeseed and soybean) is commonly used as an animal feed, seed cake obtained from second-generation energy crops such as jatropha is unsuitable for direct use as animal feed due to the presence of toxic compounds.⁶⁶ Such seed cake, however, is rich in organic matter and, if digested properly, has significant CH₄-production potential. Similarly, macro- and microalgae have been studied as alternative feedstocks for biodiesel production because of their high biomass yield and reduced competition for land and fresh water. The solid residue obtained following lipid extraction from algae is rich in protein and carbohydrate and could serve as a suitable feedstock for biogas production via AD.

Crude glycerol is the liquid byproduct generated in the biodiesel-production process following transesterification. About 10 lbs of glycerol are produced for every 100 lbs of biodiesel production. Crude glycerol is generally composed of 50–60% glycerol, 12–16% alkali soaps and hydroxide, 15–18% methyl ester (biodiesel), and 2–3% water, among others components.⁶⁴ The separation and purification of crude glycerol for industrial applications (e.g., pharmaceuticals and cosmetics) is energy-intensive and costly. Thus, the application of crude glycerol for biogas production via AD could supply heat and electricity for an operating biodiesel plant.

AD has significant potential to recover resources from organic wastes (both solid and liquid) from the bioenergy industries. Such systems generate energy (biogas) that can be used to meet the heat and electricity demand of industry, and treated effluent that can be recycled as process water.

Anaerobically Digested Fiber for Biofuel and Biobased Products Generation

Digestate fiber, a solid residue from the AD of lignocellulosic feedstocks, has been viewed as a low-value product and is commonly used as a soil additive or for animal bedding.⁶⁵ However, recent studies have shown that digestate fiber has better fiber properties (e.g., composition and size) than the input feedstock, such as animal manure or corn stover, and can potentially be used as a feedstock for biofuel and biobased product generation.^11,13,67 More specifically, studies have demonstrated that AD degrades hemicellulose faster than it does cellulose, which facilitates the breakdown of lignocellulosic feedstock's matrix structure. Consequently, fibers are relatively easier to hydrolyze after digestion than the feedstock before AD. In addition, AD significantly reduces particle size. The combined effect of the significant reductions in hemicellulose content and biomass size effectively destabilizes the recalcitrant biomass structure, thus allowing for the solubilization (i.e., via saccharification) of cellulose by commercial enzymes in the downstream processes.^11,12 Moreover, lower hemicellulose content in the AD fiber eliminates the pentose-utilization problem that lignocellulosic biorefineries currently face.¹³ Thus, AD has the potential to act as an effective biological pretreatment technology for lignocellulosic biomass. Also, glucose derived from the hydrolysis of cellulose-rich digestate fiber has several potential applications. It can be used as a substrate for producing drop-in biofuels (via the carboxylate platform) or as a precursor for high-value products such as bioplastics, succinic acid, fungal protein, etc.^1,17,19

Several studies have demonstrated the potential application of digestate fiber for bioethanol production.^11,13 An estimated 120 million dry tons of cattle manure produced annually in the United States can generate about 63 million dry tons of fiber after AD, with the potential to produce more than 1.67 billion gallons of ethanol annually.¹³ Recently, the emphasis has been on biobutanol production over bioethanol production due to butanol's higher energy density, lower affinity for water, and ability to be blended into transportation fuel without the need for engine modifications.⁶⁸ Several Clostridia species, including Clostridium acetobutylicum, Clostridium beijerinckii, and Clostridium pasteurianum, can convert digestate fiber-derived glucose into butanol.⁶⁸ Butanol production by Clostridium species has been reported at 0.235 g/g of initial glucose and 0.247 g/g of initial xylan.⁶⁸

Moreover, the monomeric sugar produced (glucose) can also be biologically converted into carboxylates, which are the precursors for different solvent and/or fuels, such as carbonyls and esters via thermo- and electrochemical processes, and alcohols and alkanes via decarbonation and reduction processes.¹⁷ Additionally, glucose can be used as a substrate to produce lactic acid, which can be further converted to lactate esters (a solvent for the cleaning industry), acrylic acid, or 1,2-propanediol (used in polyester resins and polyurethane).⁶⁹ Polymerized lactic acid has many industrial applications such as to produce biodegradable plastic.⁷⁰ Succinic acid can also be synthesized from glucose via bacterial fermentation, and can then be converted into products such as surfactants for use in detergents.^71,72

Cellulose obtained after the delignification (e.g., via alkaline pretreatment) of digestate fiber can be used as an environmentally friendly, renewable filler in polymer composites such as polypropylene–microcrystalline cellulose composite, to improve strength and heat-resistance properties.⁷³ Cellulose can also serve as an initial substrate to synthesize many cellulose derivatives, such as cellulose esters (cellulose nitrate, cellulose acetate, and cellulose acetate propionate), and cellulose ethers (methylhydroxyethyl cellulose and carboxymethyl cellulose). Because of their mechanical and optical properties, cellulose ester films can be used to produce optical media (e.g., photographic films).⁷⁴ Furthermore, cellulose nitrate and cellulose acetate have been widely applied in filtration membranes, including particle-, micro-, ultra-, and nanomembranes for water purification, food production, and medical processes.^74,75 Similarly, cellulose ethers have several industrial applications ranging from building materials (e.g., methylhydroxyethyl and methylhydroxypropyl celluloses) to milk stabilizers (e.g., carboxymethyl cellulose).⁷⁴ Thus, digestate fiber as a cellulose-rich substrate has several potential applications.

Lignin for Biofuel and Biobased Products Generation

The solid residue obtained following enzymatic hydrolysis of digestate fiber consists primarily of lignin. Lignin provides rigidity to the plant cell wall and is recalcitrant to biological degradation.⁷⁶ Currently, lignin is treated as a low-value component and is either applied to agricultural land as a source of organic matter or combusted to generate heat and electricity.⁷⁷Recent studies, however, have shown the potential of using lignin for producing various high-value biobased products.⁷⁸

Lignosulfonates are obtained from the sulfite process, one of the most commonly used processes for lignin isolation.⁷⁹ Lignosulfonate is already used commercially as a plasticizer in making concrete; it minimizes the amount of water required in concrete mixtures and ultimately results in higher density and compression strength in the concrete.⁷⁹ Because of its binding properties, lignosulfonates can also be used as a binder in animal feeds and solid fertilizer pelletization, as well as in dust treatment on unpaved roads.^79,80

Vanillin (4-hydroxy-3-methoxybenzaldehyde), one of the most commonly used flavoring agents, can also be synthesized by oxidizing lignosulfonates in the presence of cobalt and copper acting catalysts.^79,81 In 2011, about 12,000 tons of vanillin were produced globally, and the majority of the substrate used in vanillin production was derived from lignin monomers, such as coniferin.⁸² Lignin can also be used as a low-cost substitute for polyacrylonitrile in carbon fiber production.⁷⁹ The raw materials for such carbon fiber production consist of lignin (90%) and synthetic polymer (10%) such as polyvinyl alcohol and polyethylene oxide.^76,79,83 Lignin could also be used in bioenergy production via thermochemical pathways such as combustion, gasification, and pyrolysis. However, the higher moisture content of the biomass, especially following enzymatic hydrolysis, could be an issue for thermochemical conversion.

Lignin has already been utilized to produce biobased chemicals as well as biofuels. However, hydrocracking and hydrodealkylation processes for converting lignin into biobased products like phenol and benzene are energy-intensive and require petroleum-based H₂.⁷⁹ AD of different organic wastes for biohydrogen production could provide H₂ for processing lignin into high-value products, thus making the process self-sustainable in terms of energy and substrate requirements.

AD Effluent for Algal Biomass Production

AD effluent in general is rich in nutrients, primarily phosphorus (150–430 mg/L) and nitrogen (1,500–2,200 mg/L).^84

–87 The common practice of AD effluent disposal on agricultural land using it as a soil additive is called fertigation. The excessive application of AD effluent, however, presents several issues, including phytotoxicity and surface and ground water contamination; consequently, land application of AD effluent has become regulated. Thus, AD effluent requires storage for a significant duration and/or transportation over a long distance to find a suitable site and avoid over-application. This incurs high transportation and operational costs for effluent disposal.

As a post-treatment technique, AD effluent can be utilized as a nutrient-rich media for producing algal biomass. This approach accomplishes two goals: producing algal biomass, and removing the nutrients from the AD effluent.⁸⁵ Singh et al. successfully cultivated three strains of microalgae (Chlorella minutissima, Chlorella sorokiniana and Scenedesmus bijuga) in 6% (by volume) AD effluent from a digester fed with poultry litter as feedstock.⁸⁵ The result indicated that S. bijuga has the highest biomass productivity—76 mg/L/d—among the three strains tested. The highest nitrogen and phosphorus removal efficiencies—70% and 34%, respectively—were also observed. Similarly, Yan and Zheng studied nutrient removal from AD effluent by cultivating the microalgae Chlorella sp. Total nitrogen and total phosphorus removal efficiencies as high as 84% and 80%, respectively, were observed under light intensity of 350 μmol/m²/s with a photoperiod of 14 h/d.⁸⁸ The algal biomass produced is generally rich in proteins (40–60% of TS) and lipids (10–75% of TS) and has several potential applications, ranging from feedstocks for bioenergy production to a source of animal feed.^89
–91 The lipid present in the algal biomass has significant potential for biodiesel production, while the protein-rich algal meal, following lipid extraction, can be used for animal feed applications or as co-substrate for CH₄ production via AD. In addition, studies are being conducted for the extraction of high-value nutraceuticals, such as ß-carotene, astaxanthin, and polyunsaturated fatty acid, from microalgae.⁹²

The CH₄ yield of algal residue following lipid extraction has been reported to vary from about 300–600 L/kg VS_added.^89,92 The monodigestion of the residue, however, may cause digester failure due to ammonia toxicity—the most common cause of digester failure while digesting the substrate rich in nitrogen. This could be addressed by codigesting extracted algal residue with a substrate rich in carbon content (e.g., crop residues, dedicated energy crops, wastewater from ethanol and starch-processing plants, etc.). One such co-substrate could be crude glycerol produced during transesterification of lipids derived from algal biomass. The availability of both the nitrogen-rich solid residue (i.e., algal biomass after lipid extraction) and carbon-rich liquid waste (i.e., crude glycerol) make them good cosubstrates for achieving the optimal C:N ratio for codigestion. In addition, crude glycerol can also be codigested with other nitrogen-rich feedstocks, such as cattle manure, chicken litter, and waste activated sludge. Several studies have reported a significant increase in biogas production when crude glycerol was codigested with animal slurry.^93
–95Moreover, there was no negative effect of crude glycerol on the digestate composition, thus not compromising its use as a soil additive. Therefore, an integrated AD and algae-production system offers an opportunity to treat AD effluent with concurrent algal biomass production. However, the processing of algal biomass into biofuels and biobased products is highly energy intensive and involves biomass harvesting, dewatering, and drying, as well as lipid extraction. Technological advancements for efficient processing of the algal biomass offers an avenue to produce both biofuels and biobased products with minimum economic and environmental footprint.

Biogas Utilization

One of the merits of AD for bioenergy production is that the biogas produced has multiple applications. For example, biogas can either be directly used for combined heat and power (CHP) generation or purified and upgraded to biomethane (CH₄ content ≥95%) by removing impurities such as hydrogen sulfide (H₂S), ammonia (NH₃), particulates, moisture, and carbon dioxide (CO₂).⁶² The upgraded biogas could be used as a natural gas substitute for transportation, microturbines, and fuel cells.^96
–98 CHP is the most commonly used biogas-utilization technology.⁹⁶ Electrical efficiencies of about 30–40% and thermal efficiencies of about 40% can be achieved with CHP.^96,99 However, to avoid damage to CHP units, H₂S concentration should be below 250 ppm, and desulfurization is often needed prior to injection into a CHP unit.¹⁰⁰ The H₂S from biogas can be removed by using an iron sponge (ferric oxide), absorption (using alkali solution such as NaOH and lime), or a biological filter. The H₂S concentration in the biogas can also be significantly reduced by injecting a small amount of oxygen (micro-aeration) into the digester. Khanal reported a reduction in H₂S content in biogas from 16% to less than 0.1% by using the micro-aeration technique.⁶² Microturbines (30–300 kW capacity) are an alternative biogas utilization technology for small-scale biogas plants. The electrical and thermal conversion efficiencies of such turbines are reported to be about 28% and 54%, respectively. The low maintenance cost due to few moving parts and low off-gas emissions due to the lower combustion temperature are the major merits of using microturbines. The high capital cost, however, is a major drawback of this technology.^62,96

Biogas, after being upgraded to biomethane, can be used in fuel cells that directly convert CH₄ to electricity.¹⁰¹ Higher fuel-conversion efficiency and low pollutant emissions are major benefits of fuel cell technology.⁹⁶ The electrical- and thermal-conversion efficiencies of a Molten Carbonate Fuel Cell (MCFC) have been reported to be 50% and 40%, respectively. However, due to the significantly high cost, MCFC is not as favorable as other biogas-utilization technologies.^62,96

Low energy-conversion efficiency (e.g., of CHP) and high capital and operating costs (e.g., upgrading to biomethane, microturbines, and fuel cells) are the major shortcomings of existing biogas-utilization technologies. In this context, the conversion of biogas to other higher-value chemicals such as methanol could result in a higher return on investment. Methanol, the simplest alcohol with multiple industrial applications—including in gasoline blends, biodiesel production, and as a carbon source for wastewater treatment—can be produced from biogas either by chemical or biological conversion pathways. Currently methanol is being produced from petroleum-derived CH₄, which is first chemically oxidized to CO₂ and H_2; the CO₂ is then reduced to methanol. This process not only involves redundant steps, but also is costly.¹⁰² Thus, the production of methanol from biogas opens up other applications for biogas without the need to upgrade biogas to biomethane.

Ammonia-oxidizing bacteria (AOB) such as Nitrosomonas europaea are capable of reducing CH₄ to methanol by using ammonia as an energy source. Studies are currently being conducted on the biological conversion of CH₄ to methanol. Taher and Chandran studied the autotrophic conversion of CH₄ to methanol by using a mixed microbial culture of AOB.¹⁰² They observed a maximum specific CH₄-to-methanol conversion rate of 0.82 mg methanol COD/mg AOB biomass COD/d. In other words, maximum methanol production was about 60 mg methanol COD/L within an incubation time of 7 h. The authors point out that the CH₄-to-methanol conversion can be enhanced by adopting proper strategies to feed the substrates (CH₄, NH₃, and reducing equivalents), improving gas (CH₄)–liquid mass transfer, and minimizing the impacts of cosubstrates (CH₄ and NH₃) and product (methanol) inhibition.

Conclusions

An AD-based biorefinery has great potential to serve as a technology platform to convert a variety of low-value feedstocks, ranging from municipal and industrial organic wastes to agricultural and forest residues and energy crops, into high-value biofuels and biobased products with concurrent waste valorization. However, implementation of the AD-based biorefinery concept is still rare. A more comprehensive study supported by research and development is crucial for developing an AD-based biorefinery analogous with today's petroleum refinery.

Footnotes

Acknowledgments

This project is supported by funding from the Sun Grant Western Regional Center at Oregon State University (Corvallis, OR) through a grant provided by the United States Department of Agriculture National Institute of Food and Agriculture under proposal number 2012-03373.

Author Disclosure Statement

No competing financial interests exist.

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