Strategies to boost anaerobic digestion performance of cow manure

The lignocellulose of cow manure hinders

• The lignocellulose of cow manure hinders a good methane yield in anaerobic digestion.• Biological pretreatment (composting) of cow manure is promising in fullscale application.
• Selection of a lignin-poor co-substrate is vital when conducting co-digestion trials.
• Fe-based nano-particles are excellent additives in lab and full-scale applications.• Bioelectrochemical reactor represents future reactor module treating cow manure.

G R A P H I C A L A B S T R A C T
a b s t r a c t a r t i c l e i n f o

Introduction
Livestock is a significant contributor (40%) to the global agricultural markets (WHO, 2017).To meet the increasing demand for meat/dairy products, traditional scattered family-scale livestock farms have been gradually transferred into centralized ones in the past years.Consequently, those farms are 'swamped' by an enormous amount of manure generated continuously.This situation also applies to the European Union (EU), where approximately 1.2 billion tons of manure is generated annually (Scarlat et al., 2018b).Alongside this, cow manure (CM) holds a great share, which stands for more than half of the total amount of manure currently and will further reach around 75% in one decade (Meyer et al., 2018).A direct spread of CM as fertilizer for crop cultivation could be an option.However, CM may contain traces of antibiotics, heavy metals, and pathogens, which not only affect the plants by salt toxicity through the direct application but also the humans via the food chain of accumulated toxins (Leclerc and Laurent, 2017).Moreover, this approach may act as a potential source of water and air pollution.Water pollution, triggered by the overflowing of the slurry store or run-off of the rain, can strongly affect aquatic life in terms of eutrophication (Jahra and Kawahara, 2019;Shivam et al., 2019;Wu et al., 2019).Air pollution is ascribed to the emission of ammonia (NH 3 ) and the greenhouse gasses (GHG) such as carbon dioxide (CO 2 ) and methane (CH 4 ) (Liu et al., 2019;Cai et al., 2019).Great concern should be given to CH 4 , as its global warming potential (GWP) is 8-10 times higher than that of CO 2 (Grant Richard et al., 2015).It was further pointed out by Purdy et al. (2018) that the emission rate of methane enjoyed a triple amplification if manure was left uncovered for over 4 months.The quest for achieving a 40% GHG reduction and 32% improvement of renewable energy installed capacity in 2030 has been set as a policy target in the EU (European Commission, 2018).Anaerobic digestion (AD), in this regard, is gaining more attention as AD can guarantee simultaneous waste disposal and generation of biogas via a series of bioprocesses.AD installations have been widely adopted in dairy/beef sectors using CM to generate biogas (Scarlat et al., 2018b).Concomitantly, the output (digestates) of the AD facilities can be spread as fertilizer with an enhanced fertilizer characteristic and low GHG emission potential (Li et al., 2020a).
Despite the benefit of AD for the energy exploit of CM, its monodigestion performance can be constrained by the initial characteristics of CM, such as low C/N ratio, which may lead to a poor AD efficiency.Therefore, a project was launched by introducing carbonaceous wastes to compensate for the C deficiency of CM (Meyer et al., 2018).However, a geographical survey thoroughly reviewed the biogas production potential from crop residues (carbon-rich waste with high yield worldwide) and manure in the EU accounting for technical, regional, and economic constraints (Einarsson and Persson, 2017).In other words, both wastes were segregated rather than concentrated in some areas, which made the available substrate mixture varied widely between regions (Fig. 1).On the other hand, cow digests the easily-degradable part of the feed (grass or silage) with rumen microbes, making the leftover CM fibrous.These recalcitrant parts of CM (cellulose and lignin) could thus hamper the hydrolysis of CM in AD due to their complex structure (Abbas et al., 2020;Tsapekos et al., 2017).Ample operational experience is needed to decompose solid fractions (especially recalcitrant lignocellulose) in a better manner and reinforce the biogas production efficiency of CM to achieve simultaneous waste diminishment and renewable energy generation via AD (Chen et al., 2020;Xu et al., 2020;Wang et al., 2019b).Intriguingly, CM is so common a substrate in AD, while recent updates of promotions on CM are rarely summarized.Conventionally, co-digestion and pretreatment are the most well-exploited approaches for AD of CM.While recently developed technologies such as various additives (carbon, metal, and biological additives) and innovative AD systems are seldom discussed.Moreover, the pilot-scale application potential of these strategies requires thorough investigation since AD is such a technology inherently practical in the disposal of waste streams in rural or urban areas.Hence, this paper aims to present a holistic study on how to boost the AD performance using CM as the substrate, from both lab-scale and pilot-scale perspectives.

Properties of CM
To better utilize CM through AD, the initial characteristics of CM should be determined.Accordingly, general CM has four distinguishing features: 1) high moisture and ash contents; 2) high lignocellulosic components (equal or more than 50%); 3) fruitful alkaline metals (buffer capacity) and 4) pronounced fermentative and methanogenic microbial guilds (Table 1).Particularly, high moisture (˃70%) discourages CM to directly participate in thermochemical processes to generate energy (Font-Palma, 2019).Hence, the introduction of AD to alternatively exploit the energy potential of CM seems reasonable.High ash contents, however, come either from sampling (contain soil for instance) or from the bedding materials (coarse sand for instance) used in dairy barns (Shen et al., 2015).Moreover, the carbohydrate-rich diet of cows, together with recalcitrant lignocellulosic bedding materials (straws, sawdust, and composted CM) used for cleaning and collection purposes, results in a high lignocellulosic content of CM (Font-Palma, 2019).Additionally, CM possesses pronounced alkaline metals (Ca and Mg) originating from the cow's feed additives.These alkaline metals result in the high buffer capacity of CM in AD.Last but not the least, CM contains various fermentative microbes, making CM an inoculum well-suited for the start-up of anaerobic digesters.All these features indicate that CM can serve as a suitable AD substrate, however, its refractory lignocellulosic compounds may hinder a good AD performance.

Process description
Anaerobic co-digestion refers to a strategy adopted in case the C/N ratio in the targeting substrate is not optimal for mono-digestion.Specifically, for CM, a carbon-rich co-substrate (mainly crop residues) is highly preferred to reach an optimal C/N ratio between 15 and 30.Moreover, researchers who conducted co-digestion of CM and other substrates, claimed that the strong buffer capacity of CM paved the path for enhanced methane yield compared with mono-digestion.In other words, the presence of CM may lead to the stimulation in the digestion of a given substrate (such as mono-digestion of municipal solid wastes (MSW)).Under such circumstances, numerous researchers have focused on this simple yet effective AD intervention approach, and concomitantly, most of them have obtained an improved methane production compared with digesting CM alone.When we checked out the C/N ratio of CM in those peer-reviewed papers, we found out that most of the C/N ratio fell into the optimal range (15-30) for AD (Font-Palma, 2019).This observation indicates the unique character of CM among the different types of manure waste.Indeed, concern still exists due to the high concentration of nitrogenous compounds in manure, which may act as potential inhibitors in AD.However, CM contains a relatively low concentration of nitrogen components compared with frequently used pig and poultry manure (Siddique Md Nurul and Ab Wahid, 2018;Matheri Anthony et al., 2018).Besides, CM is rich in nutrients and can provide strong buffer capacity, and thus, CM seems more robust than other manures in AD (Font-Palma, 2019).Therefore, the alleviation of ammonia inhibition when CM is used in AD seems not that urgent and should not be the priority of co-digestion.Additionally, CM is categorized as lignocellulosic waste due to its high amount of lignocellulose (50% in dry matter), which is relatively low in other types of manure (Kafle and Chen, 2016).Hence, to make full use of CM to produce more methane via co-digestion, attention should be paid to how to improve the degradation of recalcitrant lignocellulose in CM.Taken this focus into account, we reinvestigated these papers and tried to figure out if co-digestion of CM and organic wastes promoted the degradation of lignocellulose in CM.Unfortunately, limited information was documented in these published papers as most of the authors emphasized the improvement of methane yield compared with CM alone.Undeniably, an enhanced methane yield is the ultimate purpose of both engineers and biogas plant operators who would expect payback on the investment of the AD installation.Co-digestion could treat various wastes at the same time, which is also beneficial for regional waste disposal.As a scientific researcher, one would like to dig a step deeper, for example, where does the improved methane yield come from?Does the improved methane yield come from the enhanced degradation of the lignocellulose of CM or does it come from the contribution of the codigestion partner?To answer these questions, we introduced an equation known as a synergistic effect equation (Li et al., 2020a): where i = timespan of AD (d), M mixture , i = simulated methane yield of the mixture at the ith day (mL/g volatile solids (VS)), M CM , i = methane yield of CM at the ith day, Y 1 % = the percentage of CM in the mixture,  Bah et al., 2014;Zhao et al., 2018;Cestonaro et al., 2015;Hjorth et al., 2011 Hemicellulose (%) 14. 05-19.00 20.00-26.70 Li et al., 2020a05-19.00 20.00-26.70 Li et al., , 2020b;;Bah et al., 2014;Zhao et al., 2018;Cestonaro et al., 2015;Hjorth et al., 2011 Lignin (%) 13.97-16.008.09-14.00Li et al., 2020aLi et al., , 2020b;;Bah et al., 2014;Zhao et al., 2018;Cestonaro et al., 2015;Hjorth et al.,  M CS , i = methane yield of the co-substrate at the i th day, and Y 2 % = the percentage of the co-substrate in the mixture.
The difference between the simulated methane yield and the observed methane yield is regarded as the synergy.Moreover, the degradation of lignocellulose in all co-digestion experiments is also discussed here to elucidate the effect of co-digestion better.

Laboratory studies of co-digestion
Wheat straw (WS), which is the second abundant agricultural waste in the world, is a typical substrate used in AD.Mono-digestion of WS may experience a constrained performance due to high lignocellulose content, which hampers the hydrolysis step.Besides, its high C/N ratio (100) exceeds the recommended optimal C/N ratio in AD.Hence, codigestion of CM and WS was an excellent match as indicated by Xavier Cristiane et al. (2015) who replaced 5% fresh weight of CM with WS (shredded and briquetted) and obtained a 29%-31% enhancement in methane yield compared with digesting CM alone.However, in similar research, Risberg et al. (2013) co-digested CM with WS and discovered that there was no apparent enhancement compared with digesting CM or WS alone.Likewise, Li et al. (2015) reported both negative and positive synergy (−3.6%-5.8%)when co-digesting CM and rice straw (RS) compared with digesting CM alone.In contrast to Li's research, Sukhesh Muramreddy and Rao (2019) obtained apparent synergistic effects when CM and RS were mixed, especially at a high CM addition (˃50%, mass ratio).Moreover, a shorter lag phase in methane production was observed in the co-digestion experiments than in CM alone.Among the aforementioned research, only Xavier Cristiane et al. ( 2015) listed the composition of cellulose and hemicellulose before and after AD.According to their statement, an improved methane performance in the co-digestion came from an improved degradation of cellulose and hemicellulose in AD.
Switch grass (SG) is a perennial crop with low fertilization and pest control requirements.It serves as a suitable energy crop for the production of bio-fuel.Zheng et al. (2015) co-digested CM and SG and found an improved methane yield up to 39% compared with digesting the individual substrates alone.Besides, the co-digestion experiments showed an accelerating methane production rate compared with CM alone, but detailed information on the solid component removal was not reported.On this subject, a recently published paper focused on the codigestion of roadside grass (RSG) and CM in a pilot-scale fermenter (André et al., 2019).Two filling strategies of the reactor (layer and mixture) with CM and RSG at different mixing ratios were compared.In both situations, an improved methane yield (24%) was obtained compared with low RSG addition.However, the increased methane yield was not derived from an improved degradation of cellulose and hemicellulose in CM.Most likely, the enhanced methane yield was from the addition of readily fermentable substrates present in RSG.
Aloe peel waste (APW), a common agricultural waste in China which requires proper disposal, was co-digested with CM in AD (Huang et al., 2016).Apparent synergistic effects of the blends were identified throughout the experiment, with CM:APW = 1:3 (mass ratio) reaching the maximum synergy (24.5%).Following this optimal ratio, the same group introduced vermiculite as additives and obtained a further methane enhancement (51.2%).An improved lignocellulose degradation rate brought by the metals in vermiculite was assumed positively correlated with the methane enhancement (Xu et al., 2020).Spent tea waste (STW), a typical surplus organic waste in India, was co-digested with CM at varying ratios (Khayum et al., 2018).They argued that the addition of STW greatly promoted the overall biogas yield, with CM:STW 3:7 (mass ratio) reaching the highest biogas yield (1669 mL kgTS −1 ).Moreover, the methane content was found consistently higher in the co-digests (61.2-71%) than in CM alone (50%), indicating their great energy application potential in household usage.
Sweet potato (SP) is one of the most utilized dedicated energy crops in Brazil for AD.Montoro et al. (2019) found an array of higher methane yields (323-444 L kgVS −1 ) at different CM:SP ratios (4:1-1:1, mass ratio) compared with digesting CM alone (307 L kgVS −1 ).Although no detailed information on the removal of lignocellulosic materials was mentioned, a higher reduction rate of VS was highlighted in the blends than in CM alone.
Exhausted sugar beet cossettes (ESBC) are typical lignocellulosic agri-food wastes in Spain.Aboudi et al. (2016) conducted co-digestion of CM and ESBC in AD.They showed an outstanding increased methane yield (24.7%-25.3%enhancement) by the addition of ESBC equal to or less than 50% (mass ratio).An enhanced VS degradation compared with digesting CM alone was observed eventually.Despite the lack of direct evidence for the increased degradation rate of lignocellulosic compounds, they concluded that hydrolysis and acidogenesis in the blends were balanced, especially in the case where 25% ESBC was added.
Forage radish (FR) is widely used as a top cover crop during wintertime in the US.Belle et al. (2015) sought to determine if additional benefits could be obtained from FR by using it as a co-substrate in dairy digesters.Two trials with high (23% on VS basis, trial 1) and low (13% on VS basis, trial 2) FR addition were comprehensively evaluated in terms of methane performance in field reactors.Comparative methane yields were obtained between high radish addition and CM alone despite an improved VS degradation in the co-digests.Whereas in the low-addition case, a marginal difference was obtained in terms of methane yield, but no apparent difference was obtained in VS degradation among the two trials.Hence, in this case, an inconsistency between hydrolysis and methanogenesis was observed.
Sheep bedding (SB) is popping up as a new source of waste in the sheep farming industry.It is rich in fiber, mainly due to the bedding material (corn stover), which is resistant in AD.Cestonaro et al. (2015) co-digested SB with CM at variable ratios (mass ratio).A negligible enhancement was found in the co-digests compared with digesting CM alone.Additionally, no improvement in lignocellulose removal was obtained throughout the experiment.Alternatively, a recent attempt was conducted by Li et al. (2020a) using sheep manure (SM), which contained much less lignocellulosic components than SB to co-digest with CM.A synergy ranging from 3.5% to 10.1% was observed in the blends.Moreover, an improved degradation of cellulose and hemicellulose was obtained among the co-digests than CM alone, which was ascribed to the alleviation of lignin inhibition in the co-digestion.
Seaweeds (SW) are regarded as promising substrates for thirdgeneration biofuels.These resources are highly abundant in countries with long coasts.The state of the art in co-digestion of SW and CM is mainly on the adaptation of C/N ratios, as well as the alleviation of potential salinity inhibition (sulphur and chloride) of SW.In this sense, Tabassum et al. (2016) tested the possibility of co-digesting SW (Laminaria digitata and Saccharina latissimi) with CM in batch and continuous experiments.In contrast to the expectation, batch AD co-digests presented mostly negative synergy (−15% to −3%) with only one exception (1%) in S. latissimi:CM at 2:1 (mass ratio).Although an enhanced daily methane yield was observed in the co-digestion compared with CM alone in the continuous mode, the improved part may originate from the easily-fermentable fraction of SW instead of CM.
Palm pressed fiber (PPF) is a by-product of the oil extraction of the oil palm fruit industry.Conventionally, PPFs are burned as fuel regardless of the substantial air pollution.Except for open burning, an alternative sustainable approach to make use of PPF is via AD.Since PPF is rich in carbon, Bah et al. (2014) tried to co-digest CM with PPF to maximize the methane yield.No synergy was obtained between co-digestion of PPF and CM, although an enhanced hydrolysis index was modeled in the co-digests compared with CM alone.The authors attributed the higher hydrolysis index to increased degradation of the fat fraction of PPF instead of the improved hydrolysis of lignocellulose.More concrete evidence was calculated based on the information listed in this paper.We demonstrated that co-digestion resulted in a decreased degradation of cellulose and hemicellulose compared with digesting CM alone.Thus, adding PPF to co-digest with CM was unfavorable to hydrolytic bacteria, most likely.Another by-product of the palm oil industry, palm oil mill effluent (POME) was co-digested with CM aiming to boost the overall methane performance in AD.The authors found synergistic effects in the co-digests, especially at a high POME addition (˃50%, mass ratio).Furthermore, an improved VS degradation was obtained in the codigests, but the profile of the removal of lignocellulose was not reported in this paper (Bin Khalid et al., 2019).
Oat straw (OS) is becoming a surplus agricultural waste accompanied by the extensive cultivation of oat in China.Zhao et al. (2018) investigated the feasibility of co-digesting of CM and OS at varying ratios (4:1, 2:1, 1:1, 1:2, 1:4, mass ratio) in a mesophilic batch system.Not only a synergistic effect was identified in the co-digests, but also a pronounced cellulose and hemicellulose removal was confirmed in the codigests compared with CM alone.
Besides lignocellulosic residues, some other organic wastes may also serve as potential co-substrates with CM in AD.Crude glycerin (CG) is a redundant by-product of biodiesel production in Brazil.Simm et al. (2018) co-digested CG with CM in a semi-continuous bioreactor and modeled the profile of methane production and lignocellulose degradation.They claimed an improved daily methane production and lignocellulose removal in the co-digests (CG:CM 5:95 or 10:90, mass ratio) compared with CM alone.Since CG contains no lignocellulosic compounds, the results may indicate an alleviation of lignin inhibition during the co-digestion.
Food wastes (FW), which account for a significant fraction of MSW in urban cities, are a big concern for local authorities.FW contains more readily biodegradable components for fast conversion to biogas, but they have a low buffer capacity, thus easy to acidify.Co-digestion of FW and substrates with complementary characteristics such as present in CM is a proper option as CM could provide enough buffer capacity in AD.Li et al. (2009a) studied co-digestion of FW and CM at different mixing ratios.Eventually, a synergy was found up to 71% at an FW:CM mass ratio 1:1.In a recent study, Xing et al. (2020) used CM as an additive to co-digest with FW (1:3.5 on VS basis) in a dynamic membrane bioreactor.They observed a decreased cellulose crystallinity among the co-digests that promoted the degradation of lignocellulose in CM due to the fatty acids generated from the organic fraction of FW.

Understanding the promotion of co-digestion from the perspective of CM
Clearly, researchers who advocated co-digestion scenarios would always obtain improved methane yield compared with digesting CM alone.Among these papers, however, only half of them (57.6%) reported a synergistic effect in the co-digests (Table 2).In other words, in half of the cases, the enhanced methane yield came from the co-substrate other than CM.On top of that, only a few papers elucidated whether or not the enhanced methane yield came from an enhanced lignocellulose degradation in CM (Table 2).To map the correlation between lignocellulosic contents of the co-digests, synergy, and lignocellulose degradation, we investigated the information provided in these individual cases.A clear trend between the initial lignin content in the system and lignocellulose degradation was identified (Fig. 2).That is to say, in most of the cases, if the input of the cosubstrate introduced less lignin than CM does, the removal of cellulose and hemicellulose of the co-digests would be higher compared with digesting CM alone.The inhibition of lignin in AD was coincidentally reported by Schroyen et al. (2018) as well.Moreover, the low lignin content of the co-substrate could bring about evident synergy, as illustrated in Fig. 2, while an exception reported by Shen et al. (2019) might somehow impair this speculation.Although no concrete conclusion could be drawn due to the lack of information on lignocellulosic components in most reviewed co-digestion scenarios, we tend to recommend the selection of lignin-poor co-substrates for future co-digestion experiments with CM.By doing so, simultaneous methane production enhancement (synergy) and improvements in lignocellulose degradation in CM can be expected, which contributes to a simultaneously improved CM diminishment and energy recovery.

Pretreatments
In general, pretreatment methods targeting the lignocellulosic compounds in CM have been widely studied to overcome the resistance of undigested lignocellulose in AD.Briefly, pretreatments aim to free the lignin fraction through breaking the covalent bonds between cellulose and hemicelluloses, as well as to convert crystalline cellulose into more accessible cellulose (Gao et al., 2013).The following section overviews a variety of pretreatment approaches, which are categorized into mechanical, thermal, chemical, and biological pretreatments.Mechanical pretreatment aims to disintegrate organic particles and/ or reduce the size of solid fractions, thus increase the accessibility of fermentable fractions.An increased surface area renders better contact between hydrolytic bacteria and degradable particles and hence, promotes the subsequent AD process.Angelidaki and Ahring (2000) implied that CM fibers with a size of 1-2 mm (sieve mesh size) had a 16% higher biogas potential than fibers larger than 5 mm.In that study, they introduced maceration, which incorporated the physical chopping, grinding, and blending for the reduction of particle size of CM.By this mechanism, they obtained an improved methane production compared with non-treated CM.As for milling, Taherzadeh and Karimi (2008) stressed that colloid mills and extruders were suitable only for materials with moisture contents higher than 15-20%, whereas two-roll, attrition, hammer, or knife mills were suitable only for biomass with moisture contents of up to 10-15%.The ball or vibratory ball mills are universal types of disintegrators and can be used for either dry or wet materials (Kratky and Jirout, 2011).Hence, the main subject of mechanical pretreatments such as maceration, high-pressure homogenizer, sonication, and milling is to reduce the particle size of CM.
Besides size reduction, other fundamental functions within mechanical pretreatments should be pointed out.It is noteworthy that the influence of maceration comes more from shearing than cutting (Hartmann et al., 2000).Moreover, the crystallinity of lignocellulose in CM might be decreased via maceration (Angelidaki and Ahring, 2000).Another form of mechanical pretreatment, high-pressure homogenization, relies on hydrothermal cavitation provided either by an orifice plate or throttling valve in a liquid flow, which generates a drastic decrease in local pressure to cause cavitation.Subsequently, the created cavities collapse due to the recovery of pressure down the constriction.Consequently, a structural change, followed by a high extent of delignification in CM is realized.Similarly, sonication delivers acoustic cavitation at low frequency (below 40 kHz), which brings about particle disintegration and lysis of microorganisms, depending on the treatment time and power (Carrère et al., 2010).In turn, free radicals (H•, OH•, HO 2 •) prevail at high frequency (higher than 40 kHz), thus facilitate chemical reactions of recalcitrant organic substances into smaller fragments during the treatment (Harris Peter and McCabe, 2015).
Apart from mechanical procedures, mechanical separation, such as inverted phase fermentation (IPF) has been identified recently as an efficient technique for CM pretreatment (Negral et al., 2017).IPF can be regarded as a method that also preserves the endogenous hydrolytic microbes in CM by keeping the entire pretreatment process under anaerobic conditions.IPF results in a separation between the top layer full of solids and the bottom layer rich in the clarified liquid, which is caused by the flotation effect of the gas bubbles (mainly CO 2 ) produced by the hydrolysis of organic matter.Hence, the separated solid and liquid fractions of CM can be digested individually, which can maximize the methane potential of CM.The advantages of mechanical pretreatment include simple implementation and relatively low maintenance costs.Disadvantages include a limited effect on pathogen removal and possible intensive energy input.

Laboratory achievements
Maceration, hydrothermal cavitation (HC), and sonication are the most exploited mechanical pretreatments of CM (Table 3).Maceration of CM fibers down to 2 mm contributed to a 16% enhancement of biogas production, while further reduction to 0.35 mm contributed to a 20% improvement.However, further size reduction was found redundant due to the negligible difference in biogas production (Angelidaki and Ahring, 2000).Similarly, Hjorth et al. (2011) used extrusion to deal with different forms of CM before AD.They concluded that extrusion was only effective on large particles (>0.25 mm), which lead to an enhancement of methane yield of 13% and 28% for screw-pressed solid fraction and raw CM, respectively.Such a phenomenon was further backed up by Hartmann et al. (2000), who found that too small particle size might inversely contribute to the subsequent AD process.
Langone and his colleague employed HC of CM under different pressures (6, 7, and 8 bar).Despite an improved disintegration of CM at elevated pressure (5.8%, 8.9%, and 15.8%, respectively), a small increase of methane yield was obtained in the treated CM (2.7, 4.9, and 5.9%, respectively) (Langone et al., 2018).Zielinski et al. (2019b) applied HC to a mixture of CM and WS (weight basis 2:1) at different energy inputs (up to 8064 kJ kg TS −1 ).An increased soluble COD up to 30%, followed by a maximum 39.4% enhancement of biogas production was recorded at an energy input of 8064 kJ kgTS −1 .The same group scaled up this application in a pilot installation, and could still observe an evident promotion of 16.5% compared with untreated CM and WS mixtures (weight basis 1:1), supporting the soundness of HC in pilot-scale AD (Zielinski et al., 2019a).Zou et al. (2016aZou et al. ( , 2016b) ) attempted to use ultrasonic pretreatment (UP) at various energy inputs and timespans for CM.The particle distribution pattern of CM became more uniform after UP and thus, improved the accessibility of lignocellulose in CM.Ultimately, an enhanced cellulase activity, together with an improved methane yield (15.2%-43.9%)were obtained in samples that underwent UP.These bonuses were also emphasized by Ormaechea et al. (2018), who found an almost double enhancement of methane yield of the sonicated CM in a pilot-scale thermophilic reactor.

Process description
Thermal pretreatment emphasizes the improvement of anaerobic digestibility at a wide temperature range (50-250 °C) (Senol et al., 2020).It breaks down high-molecular substances into their constituents, thus making them available for subsequent rapid conversion into biogas (McVoitte Wilton and Clark, 2019).Meanwhile, pathogens from the waste stream are inactivated after the treatment (Budde et al., 2014).Those merits, together with a low installation and maintenance cost, make thermal pretreatment one of the most exploited methods.Nevertheless, attention should be paid to the temperature and treatment duration to avoid triggering unwanted reactions (i.e., Maillard reaction), which may undermine the AD process (Budde et al., 2014).Hydrothermal, microwave, and steam explosion are typical thermal pretreatment methods adopted for better degradation of CM (Table 3).

Laboratory achievements
It is noteworthy that temperature could impose a significant influence on the pretreatment efficiency of CM and, thus, being the priority for researchers.Nielsen et al. (2004) stated that a positive enhancement  (24-56%) of methane yield of CM could already be achieved at low temperatures (68 °C).Besides, the extension of the pretreatment period from 36 to 108 h was found advantageous to liquid CM.In another study, Budde et al. (2014) sought the optimal temperature range of the thermobaric pretreatment of CM.A gradual improvement of methane yield was identified at escalating temperature, reaching 58% at a temperature of 180 °C.Notwithstanding, toxic by-products (furfural, 5-hydroxymethyl-furfural, and phenolic compounds) increased with rising temperature, which adversely influenced AD (200 and 220 °C).An even lower threshold of temperature was demonstrated by Qiao et al. (2011), who found that 170 °C was high enough to drop the methane yield by 6.9% compared with untreated CM.Hence, to overcome the potential shortcoming of thermal pretreatment at high temperatures, the adoption of moderate heat or a combination of moderate heat and other pretreatment approaches (i.e., chemical pretreatment) could be alternatives.In addition to thermal pretreatment, the microwave may be a suitable replacement for standard ovens.Zielinski et al. (2019c) compared both thermal and microwave pretreatments on blends of energy crops and CM (weight basis 2:1).At the same conditions, both pretreatments showed enhanced lignocellulose solubilization, followed by an improved methane yield.The microwave pretreatment was slightly more effective than thermal pretreatment using ovens.Perhaps, materials exposed to microwave radiation undergo nonthermal modifications as well, such as changes in the structure and function of biological membranes (Jeon and Kim, 2000), changes in enzymatic activity (Banik et al., 2003), and modifications in genetic material (Takashima et al., 2006).Besides hydrothermal pretreatment, steam-explosion is equally applied for the depolymerization purpose.Wet-explosion includes both physical disruption (as in thermal pretreatment) and partly chemical degradation of the biomass (Sorensen et al., 2008).In this sense, Biswas et al. (2012) launched wet-explosion to investigate the AD performance of CM in batch and continuous modes.The highest biogas enhancement of 136% was obtained at 180 °C for 10 min without the addition of oxygen.An average of 75% increment in biogas yield was displayed in a long-term CSTR system.Ahring et al. (2015) implemented oxygen-assisted wet-explosion on feedlot CM in thermophilic AD.The promotion of lignin solubility as well as lignin conversion (44.4%) was identified compared with non-treated CM (12.6%), leading to 4.5 times higher methane yield because of the pretreatment.

Chemical pretreatment 4.3.1. Process description
Chemical pretreatment uses variable acids, alkalines, and oxidants to break down the robust, complex lignocellulosic compounds in CM.The main function of chemical pretreatment is to destroy the rigid lignocellulosic complex by cleaving the lignin-hemicellulose lineage and/or decreasing the crystallinity of cellulose.In this context, the use of strong acids (i.e., HCl and H 2 SO 4 ) is not preferred not only because of its high severity but also the excessive loss of the fermentable sugars.Besides, substantial chemicals are required for neutralization due to the sensitivity of methanogens in AD, which puts an additional economic burden on the overall process.Therefore, the use of diluted acids is preferred in acid pretreatment.Acid pretreatment may also be combined with high temperature, which is known as thermal-acid pretreatment.
Despite the well-being of acid pretreatment, alkaline pretreatment (NaOH, KOH, Ca(OH) 2 , and NH 3 ) stands out as it offers a desirable environment for subsequent AD by preventing pH decline.In addition to the function as described in acid-pretreatment, alkali induces swelling of the lignocellulose and subsequently enhances the accessible area of organic compounds (Carlsson et al., 2012).Among others, applications using Lime (CaO or Ca(OH) 2 ) and ammonia soaking (AS) are notable for their low price, safety, as well as their recycling and reuse potential (Ai et al., 2019;Ramos-Suárez et al., 2017;Mirtsou-Xanthopoulou et al., 2014).
Hydrogen peroxide (H 2 O 2 ) and ozone (O 3 ), are excellent representatives of an oxidative pretreatment.They promote the accessibility of cellulose by eliminating hemicellulose and lignin of the feedstock with highly reactive hydroxyl radicals released through H 2 O 2 and O 3 .Oxidative pretreatment does not generate toxic by-products that might intervene in subsequent fermentation stages.Since oxidative pretreatment cannot remove (toxic) decomposed fractions from lignin, a combination of oxidative and alkaline (ammonia soaking) pretreatments were proposed to provide hydrolyzable fibers containing low lignin concentration for AD (Ai et al., 2019).

Laboratory achievements
Using acids (H 2 SO 4 and HCl) to deal with recalcitrant lignocellulose of CM has been thoroughly studied (Li et al., 2009a;Passos et al., 2017;Yang et al., 2017).Li et al. (2009a) used diluted H 2 SO 4 (1%) at a pH of 6.0 to pretreat CM for 3 days.Lignin, cellulose, and hemicellulose in the treated CM were reduced by 13.1, 9.4, and 28% (dry basis), respectively.Whereas soluble COD increased from 8.7% to 24.5%, leading to more than two-fold increase in methane yield in treated CM compared with untreated CM.Likewise, Yang et al. (2017) used diluted H 2 SO 4 (4%) as the pretreatment reagent and found 75.7% and 43.7% removal for hemicellulose and lignin in treated CM, respectively.An immediate start-up of methane production in AD was also found in the treated case, followed by an enhanced cumulative methane yield (203 mL gVS −1 ) compared with untreated CM (190 mL gVS −1 ).A combination of thermal (100 °C and 37 °C) and diluted HCl pretreatment (0.5%-10%) was adopted in Passos's study.Comparable methane yields in AD, however, were obtained in most treated cases compared with untreated CM (Passos et al., 2017).
As discussed above, alkali can be a proper substitution for acids (Table 3).In Yang et al.'s research, CM was soaked in hot (180 °C) 8% NaOH solution as pretreatment for 0.5 h.They found a considerable (62.9%) removal of lignin, which was higher than the removal after pretreatment with 4% H 2 SO 4 (43.7%).Consequently, significantly higher methane yield was observed in the alkali-treated CM (285 mL gVS −1 ) than in the control (190 mL gVS −1 ) (Yang et al., 2017).In another study, Wahid et al. (2020) carried out alkaline pretreatment on CM using 8% KOH at room temperature (25 °C) for 1 day.Despite an increment in COD solubilization after the pretreatment, a small increase in methane yield in AD was obtained using treated CM (115 mL gVS −1 ) compared with untreated CM (102 mL gVS −1 ).Besides NaOH and KOH, cheap lime (CaO or Ca(OH) 2 ) has been applied in numerous studies (Table 3).Ramos-Suárez et al. ( 2017) used quicklime (CaO) at various concentrations (0.05, 0.10, and 0.15 g gTS −1 ) to pretreat CM.Enhanced COD solubility and subsequent methane yield enrichment (32%) in AD were realized with treated CM.
Somers Matthijs et al. ( 2018) performed an oxidative pretreatment on the digestates of CM before AD.A 30% H 2 O 2 solution in doses of 5, 10, and 30 g H 2 O 2 kgTS −1 were applied for the CM digestates at room temperature.In parallel, the CM digestates were treated with 5, 10, and 30 g O 3 kgTS −1 .Although an enhanced disintegration phenomenon in all pretreatment cases was realized, statistically non-significant increase in methane yield was found in AD compared with untreated CM digestates (p˃0.05).In another study, Ramos-Suárez et al. ( 2017) used peracetic acid (known as PAA, which contains 15% active ingredient and 20% H 2 O 2 ) to dose in CM at different concentrations (0.01, 0.05, and 0.10 g gTS −1 ) at two timespans (6 h and 12 h).The results implied that applying PAA caused a significant increase in solubility of CM, reflected by a higher soluble COD in all treated cases.Meanwhile, they affirmed an enhanced availability of cellulose and hemicellulose at the expense of lignin removal, with the highest dose reaching the highest lignin removal.

Process description
Most mechanical, chemical and thermal pretreatments require intensive energy or chemical input, leading to a harsh temperature or pH change, as well as toxic by-products in extreme cases.Biological pretreatment, however, is performed by the addition of industrial cellulolytic microbes or enzymes to break down the lignocellulosic components in a controlled and mild environment.Biological pretreatment outcompetes other pretreatment methods in terms of low demand for energy and chemicals, with non-toxic output.However, the production of enzymes requires a stable fermentation that might need additional equipment, thus increase the capital cost.

Conventional laboratory achievements
Aerobic pretreatment, such as composting, can be an efficient way to conduct the decomposition of lignocellulosic matter with the assistance of aerobic microorganisms (i.e., white-rot fungi).Composting is beneficial for lignin degradation and, therefore, promotes higher-efficiency AD.Zou and Kang (2018) reported that composting pretreatment resulted in a decrease of lignocellulosic compounds, which provoked the activity of cellulase activity in subsequent AD.They also affirmed that composting pretreatment yielded higher concentrations of VFAs owing to the strengthened activities of the hydrolytic and acidogenic bacteria.Bruni et al. (2010a), however, announced that partial aerobic pretreatment coupled with aerobic inocula (compost from garden waste and fungi from straw silage) did not affect the AD performance of CM fibers.Although the reason was veiled by the author, we inferred that the limited effect of composting in the latter case was due to the origin of fibers which were derived from the effluent of a biogas plant.Since these fibers had already been treated in a biogas reactor, the remaining lignocellulose was more resistant to aerobic degradation than fresh CM.Angelidaki and Ahring (2000) introduced a hemicellulosedegrading bacterium B4 to pretreat CM fibers prior to batch AD.Such implementation exhibited an approximately 30% increase of the methane yield regardless of some solids loss for bacterial growth.Sutaryo et al. (2014) initialized the pretreatment by adding mixed enzymes (pectate lyase, cellulase, and protease) to CM and incubated for three days at 50 °C.A merely 4.44% higher methane yield in AD was demonstrated compared with untreated CM.
The natural wood-decaying capacity of aerobic fungi makes them excellent candidates to be applied as efficient lignocellulose-degraders in biological pretreatment.The advantage of highly-cellulolytic whiterot fungi Trametes versicolor to pretreat CM was depicted by Akyol et al. (2019a).They illustrated an improved methane yield in AD by 10%-18% and cellulose degradation up to 80%.Zulkifli Zulfah et al. ( 2018) carried out pretreatment assays inoculated with Aspergillus fumigatus SK1 or Trichoderma.They described a substantial lignin removal of 60% in Trichoderma-inoculated CM, resulting in a significant enhancement of biogas yield in AD compared with the control.

Two-stage AD and temperature phased anaerobic digestion (TPAD)
Physical separation of the hydrolytic-acidogenic step and methanogenesis step is achieved in two-stage AD.The first configuration (hydrolytic-acidogenic reactor) can be regarded as a sort of pretreatment since the environment is designed favorable for the enrichment of various hydrolytic bacteria (Wang et al., 2019a).Therefore, in this review paper, we consider this step as a biological pretreatment.Coats et al. (2012) established an innovative two-stage mesophilic AD where the first stage served as the pretreatment stage to produce fermented thickened CM for the second stage.The outcome was unexpected since the observed gross biogas yield metrics were generally comparable between two-stage AD and the control (one stage AD) under the same condition.Presumably, the deprive of easily-biodegradable carbohydrates in the thickened CM and relative short hydrolytic retention time (HRT) were the cause for this phenomenon, as argued by the author.
In contrast to Coats et al. (2012), other researchers found two-stage AD advantageous over conventional one-stage AD when treating CM (Demirer and Chen, 2005;Akyol et al., 2016).Moreover, two-stage AD could cope with high solid inflow which was not achievable for conventional one-phase configuration, indicating a higher disposal efficiency and potential cost savings (Demirer and Chen, 2005).Nielsen et al. (2004) examined the performance of TPAD, with the first being a thermophilic pretreatment reactor (68 °C), connected to the second methanogenic reactor operated at 55 °C.The results highlighted the effect of pretreatment as improved hydrolysis was observed, resulting in a 7%-8% enhancement of methane yield compared with the one-stage thermophilic AD reactor.

Combination of different pretreatments
Different pretreatment approaches rely on variable mechanisms to make most of the lignocellulose in CM for AD.A combination of pretreatment steps may provide further enhancement of biogas production.Among these, thermal-chemical pretreatment is most exploited for CM.Yuan et al. (2019) evaluated a sequential process of thermalalkaline and hydrolytic enzymes applied for blends of CM and CS (1:1, mass ratio).Thermal-alkaline, together with enzymatic pretreatment, enhanced the methane yield in AD by 63.64%, whereas the improvement dropped to 31.82% for thermal-alkaline treatment only.Wahid et al. (2020) evaluated the efficiency of ultrasonic, alkaline, and the combination of both pretreatments at various conditions.Applying either alkaline or ultrasonic pretreatment on CM showed little or adverse contribution to the methane yield in AD (102.82,115.47, and 99.47 mL gVS −1 for untreated, alkaline, and ultrasonic, respectively).Whereas the highest methane yield (122 mL gVS −1 ) was highlighted in the combination, which was ascribed to the dual benefits of both pretreatments.Such observation complied well with Jin's research where microwaveassisted chemical pretreatment (NaOH, CaO, H 2 SO 4 , and HCl) of CM presented a significant enhancement than microwave pretreatment alone (Jin et al., 2009).
Ozone and aqueous ammonia (AA) are tagged as attractive pretreatment methods with the pros and cons of each.A combination of both methods was proposed by Ai et al. (2019) based on the unique dual benefit since AA could solubilize lignin, while the presence of ozone in the combined pretreatment oxidized lignin into small organic molecules.Excellent lignocellulose solubilization was verified in the combination, bringing about a significant promotion (55.3% -103.6%) of methane yield in AD compared with AA-treated or ozone-treated CM alone.

Comparison of various pretreatment methods for AD of CM
A systematic evaluation of pretreatment approaches through the methane yield is necessary for choosing the desirable one for CM.Fig. 3 and Table 3 list the current situation of different pretreatments and corresponding efficiency on the enhancement of methane yield.
For researchers undertaking pretreatments, chemical pretreatment (30.8%) is the first choice, followed by mechanical pretreatment (28.2%) (Fig. 3).In general, for individual pretreatment methods, mechanical and thermal pretreatments could boost the AD performance of CM to a rather similar extent, with most of the cases falling in the range between 10% and 58% (Table 3).Undeniably, more extraordinary enhancements of CM could be obtained for chemical pretreatment, reaching up to 120% (Table 3).A drastic structural change of the lignocellulose in CM induced by varying chemical reagents may still interest those who pursue making the most of CM.Furthermore, for different combinations of pretreatments, the most pronounced methane enhancement of CM can be obtained, especially for thermal-chemical pretreatment (thermal-alkaline:4.2 times; wet-explosion assisted with O 2 : 4.5 times) (Tsapekos et al., 2016;Ahring et al., 2015).Whereas, for biological pretreatment, relevant lab studies were poorly documented, and the reported enhancement of CM was rather limited.However, aerobic composting should be highlighted due to its excellent biogas yield enhancement compared with other biological methods (Table 3 and Fig. 3(b)).

Micro-and macro-nutrients
In addition to the above-introduced AD incentives of CM, the supply of micro-and macro-nutrients (MNs) has become an important topic in the agricultural biogas sector.The presence of macronutrients (i.e., N, P, and K) plays fundamental roles by providing sufficient buffering capacity and maintaining activities of microorganisms in AD (Zhang et al., 2018).While micro-nutrients such as Fe, Co, Ni, Zn, and Cu could guarantee a well-functioning of key microorganisms in AD (Abdelsalam et al., 2015;Garuti et al., 2018;Wandera Simon et al., 2018).The presence of these metal ions is essential for the activity of many enzymes, coenzymes, and cofactors that are necessary during AD.Briefly, Fe participates in methanogenesis by acting as the cofactor of various enzymes (formyl-MF-dehydrogenase, hydrogenases, carbon monoxide dehydrogenase), and in acetyl-CoA synthesis (Wood-Ljungdahl pathway) and could also act as a terminal electron acceptor (Romero-Gueiza et al., 2016).Co is a metal-ligand of vitamin B12 (methyltransferase) and enables microbes to degrade methanol.Ni is crucial for coenzyme F420 formation in methanogenic Archaea (Romero-Gueiza et al., 2016).Zn is essential in the formation of methyl coenzyme M and serves as a structural ion in the transesterification factor, while Cu is essential for coenzyme Q and biological electron transport (Bartacek et al., 2008;Fermoso et al., 2008;Metcalf, 2003).

Iron
Iron is recognized as one of the most prominent additives to improve AD performance owing to its conductive properties and low price.Different iron forms are capable of stimulating AD through different mechanisms.Fe(III), for instance, could favor the oxidation of organics into simple molecules by self-reduction.Moreover, the presence of Fe oxides could also promote AD via direct electron transfer by establishing an electrical syntrophic relationship between microbial communities such as Geobacter and Methanosarcina, Trichococcus and Methanosaeta (Kato et al., 2012;Baek et al., 2015).Whereas the presence of Zero-valent Iron (ZVI) is beneficial for hydrolysis and, subsequently, supports methanogenesis by acting as an electron donor (Abdelsalam et al., 2017a).Nonetheless, an overdose of Fe(III) should be prevented as Fe(III) reduction is thermodynamically more favorable than methanogenesis (Romero-Gueiza et al., 2016).
Preeti Rao and Seenayya (1994) realized that FeSO 4 was an efficient accelerant of AD of CM.Increased conversion of volatile fatty acids (VFAs) into methane was achieved when FeSO 4 was dosed in the system.The introduction of 20 mM FeSO 4 triggered the enrichment of the methanogenic population from 46 × 10 7 cell ml −1 to 15 × 10 10 cells ml −1 , and they obtained 40% higher biogas yield.Later, Abdelsalam et al. (2015) studied the impact of FeCl 3 in a batch AD of CM.They confirmed a faster start-up of methanogenesis, followed by an improvement of the methane yield (21.2%) for reactors receiving 10 mg/L FeCl 3 .In a subsequent study, Yun et al. (2019) investigated the additional benefit of different Fe salts (Fe 2 (SO 4 ) 3 , Fe(NO 3 ) 3 , FeCl 3 , and FeCl 2 ) in AD of CM that also received other MNs.They reported a shorter digestion time (15-18 days) in Fe salts groups in comparison to the control group (20 days).More importantly, FeCl 3 and FeCl 2 at tested dosages contributed to an increased COD t removal (58.1-69.3%)and thus, resulted in evident enhancement of biogas yield (2.7-6.4%)compared with the control.

Niobium (Nb)
Recent advancements using Nb-based chemicals as MNs have been underlined by Zhang et al. (2017).They studied the influence of various forms of Nb (Nb 3.49 N 4.56 O 0 .44 , NbO 2 , and NbN) in a batch AD and linked the superior AD performance to the presence of these MNs.On the one hand, an improved biogas yield was identified in most cases, with the highest biogas yield (522.7 mL gVS −1 ) obtained when NbO 2 was dosed at 60 mg/L, which was 27.7% higher than the control.On the other hand, an improved degradation of lignocellulose was realized in reactors receiving Nb-based molecules, suggesting the stimulatory effect of these metals on microbial profiles, especially on hydrolytic and methanogenic guilds.

Mixed metals
In addition to the supply of single metal MNs, mixed metal MNs have been added to AD as well.In this matter, Xu et al. (2019) applied limonite (Fe 2 O 3 (61.65%),SiO 2 (16.45%), and Al 2 O 3 (4.09%)) at different concentrations (1%, 5%, and 10%) in a dry batch co-digestion of CM and RS to investigate the overall performance.The addition of limonite enhanced methane yield in an inversely dose-dependent manner.In comparison to the control reactor, reactors dosed with 1%, 5%, and 10% limonite improved methane yields by 29.6%, 11.8%, and 15.5%, respectively.Moreover, the addition of limonite (1%, 5%, and 10%) resulted in limited cellulose removal, with 1% limonite reaching the highest cellulose degradation (52.5%) compared with the control (45.8%).At the same time, hemicellulose removal improved with increasing dosages.Likewise, Han et al. (2019) applied steel slags (CaO, SiO 2 , Fe 2 O 3 , MnO, MgO, Al 2 O 3 , P 2 O 5 , and others) as accelerants in AD of CM.They declared that not only improved CODt removal (52.7-56.0%)but also improved methane yield (90.7-153.7%)were obtained.Thermogravimetry analysis of the digestates (weight loss after incubation between 200 and 400 °C.steel slag: 24.6-30.5%;control: 33.5%) suggested an increased degradation of cellulose and hemicellulose in reactors supplied with steel slag.Lu et al. (2018) established an innovative Iron Oxide−Zeolite System (IZs) and successfully tested its influence in two-stage AD processes fed with CM and RS.The reactors exhibited an extraordinary AD performance in terms of lignocellulosic components degradation, VFAs generation, and methane yield.In particular, they argued that higher concentrations of VFAs in the IZs-fed acidogenic phase were owed to a better degradation of the lignocellulosic biomass with respect to the control reactor.In addition, they verified the effect of IZs on the mitigation of ammonia concentration (22.0-40.5% lower than the control) due to the adsorption capacity of zeolite in IZs.Furthermore, the stimulatory role of IZs on microbial activity (hydrolytic, acetogenic, and cellulolytic bacteria) was suggested by the enrichment of Clostridia and Bacteroidia in AD reactors containing IZs.

Carbon-based accelerants and composited accelerants
Besides metal additives, bio-based carbon additives made from lignocellulosic residues (sawdust, walnut shell, corn cob, and waste cardboard) were developed by Yun's group as well (Yun et al., 2018;C. Wang et al., 2020).Instead of being used as a co-substrate, the powder form of these residues offer a new opportunity on improving the methane yield of CM.The substrates can accumulate on the large surface area (Brunauer-Emmett-Teller (BET): 580-824 m 2 g −1 ) and increase the microbial degradation of these substrates.Besides, the employed biobased carbon material is typical amorphous carbon and mainly comprises of a small-sized graphitic carbon framework having a superior electrical conductivity.This unique characteristic facilitates direct interspecies electron transfer between fermenting bacteria and methanogens, and accelerate acetate metabolism and biogas production.They concluded that an improvement of biogas yield by 30-70% was achieved by these mechanisms compared to the reference system.Moreover, an improved degradation of cellulose and hemicellulose was inferred from thermogravimetry analysis of the digestates (weight loss after incubation between 250 and 500 °C; carbon additives: 47.2-51.0%;control: 56.3%).Similarly, Wang et al. (2019b) assessed the application potential of two types of bio-based carbon accelerants (aloe peelderived and acorn shell-derived accelerants) with a modified porous structure in CM-based AD.A strengthened interspecies hydrogen transfer between acetogens and methanogens was observed on the surface of the added accelerants.Hence, an enhanced CODt removal (57.4-67.7%),followed by an improved biogas yield (5.4-42.0%),was accomplished.Alternatively, Zhang et al. (2018) combined various low-cost composite accelerants (consist of urea, bentonite, activated carbon, and plant ash) to maximize the methane production of CM in batch AD.A higher biogas yield (485.7-681.9mL gVS −1 ), as well as more pronounced methane content in the biogas (63.0-66.6%),were achieved in reactors with accelerants compared with the control (59.4% and 361.9 mL gVS −1 ).Moreover, the digestates from accelerant-introduced reactors showed superior characteristics in terms of stability, safety, and fertility.More recently, Chen et al. (2020) proposed a novel carbon-based Cocomposite accelerant (Co/C, CoO/C, and Co 3 O 4 /C) in AD of CM.Apparent enhancements of methane yield and CODt removal for reactors receiving accelerants were identified (Experimental: 576-585 mL gVS −1 and 68.48-71.11%;Control: 435.8 mL gVS −1 and 50.74%).Moreover, they innovatively conducted first-principle density functional theory calculation to illustrate the enhanced methanogenesis (direct interspecies electron transfer) induced by the added carbon-based composites, shading lights for the development of functional accelerants for AD.

Micronutrients nanoparticles as AD additives
Together with the utilization of MNs in AD, supplementation of corresponding nanoparticles (NPs) is gaining increasing attention.Compared to atomic or bulky counterparts, nano-sized materials have superior physical and chemical properties due to their quantum-size, mesoscopic, and small object effects.Moreover, NPs possess a high surface area to volume ratio promoting the accessibility of active sites, where many reactions take place (Hsieh et al., 2016).Alternatively, NPs could play as electron donors/acceptors and cofactors of important enzymes to enhance the methane yield (Dehhaghi et al., 2019).

Fe and Fe x O y NPs
Previously, researchers observed that ZVI NPs could enhance hydrolytic fermentation by activating the microbial population (Ganzoury Mohamed and Allam, 2015).That is to say, at reasonable concentrations, ZVI NPs stimulate hydrolysis-acidification microbes by, for example, disrupting other microbes' cell membranes that are relatively more susceptible.The cell lysis could lead to a considerable release of metabolites and proteins, which in turn stimulates the growth of the hydrolyzing and acidifying populations (Dehhaghi et al., 2019).However, an overdose of ZVI NPs might undermine AD through similar mechanisms to the beneficial microorganisms such as disruption of cell integrity, which requires proper regulation of their addition.Moreover, the presence of ZVI NPs could influence the chemical and physical surroundings of methanogens by manipulating pH, ammonia-nitrogen concentration, and VFAs (Dehhaghi et al., 2019).The modification of pH can be realized through oxidation of Fe 0 to Fe 2+ using, for example, water as an oxidant to generate iron oxide or cover the surface of NPs with oxyhydroxide (Suanon et al., 2017).Meanwhile, the produced oxyhydroxide could adsorb ammonia-nitrogen.Finally, ZVI NPs could also promote the enrichment of hydrogen-consuming microorganisms and the consumption of produced VFAs (Suanon et al., 2017).Focusing on its utilization, Abdelsalam et al. (2017a) revealed the effectiveness of ZVI NPs on CM, reaching 37.6-59.5% enhancement regarding methane yield.They also ascribed the improved methane yield to the stimulatory effect of Fe 0 since it reduced carbon dioxide into methane through autotrophic methanogenesis.
Recently, iron oxide nanoparticles (INPs) have received intensive investigations because of their super-paramagnetism, high coercivity, and low Curie temperature.Moreover, their non-toxic and biocompatible characteristics were appealing in AD (Abdelsalam et al., 2017a).Farghali et al. (2019) studied the impact of INPs via a series of batch tests.They affirmed the positive effect of INPs on CM at tested dosages (20 and 100 mg/L) by realizing a 10.5-19.1% higher methane yield in AD, together with an increased VS degradation (3.4-15.2%)than the control.They attributed such enhancement to 1) an improved production of extracellular polymeric substances (bacteria), which provided cell protection against microbial cytotoxicity; 2) the uptake of INPs by methanogens which benefited methane-producing enzymes.Meanwhile, a sharp reduction of H 2 S (53.0-57.9%)was reported with respect to the control, which was due to the formation of precipitation brought by INPs, such as ferrous sulfide (FeS).Later on, the same group compared different iron NPs (waste iron powder (WIPs) and INPs) at varying concentrations (100, 500, and 1000 mg/L) in AD.The results indicated that improved kinetics and VS reduction were identified in reactors with iron NPs, leading to improved methane yield (18.4-56.9%)compared to the control.Moreover, they depicted the superiority of WIPs over INPs in terms of methane yield, with the highest methane yield (221.69 mL gVS −1 ) achieved by WIPs at 1000 mg/L, which represented a 56.9% enhancement compared to the control.They ascribed the difference to the fact that INPs tended to aggregate and settle at the bottom of digesters.At the same time, WIPs (consist 80% of Fe 3 O 4 ) remained freely moving and ensured good mass transfer between the organics and bacteria.Finally, they proved a net profit (up to USD 42.07 m −3 CM) by using WIPs instead of INPs, highlighting its field application potential (Farghali et al., 2020).

Other metal (oxide) nanoparticles
Juntupally et al. ( 2017) comprehensively evaluated different metal NPs (Co 3 O 4 , Fe 3 O 4 , NiO, and MoO 3 ) as additives for enhanced methane generation from CM in one-and two-stages AD.In both systems, improvement of biogas yield, together with an improvement of biogas quality (70-80% methane) was underlined when additives were present.Specifically, Fe 3 O 4 NPs showed the best results, reflected by its earliest biogas peak value and the highest biogas yield (0.16 L gVS reduced −1 ) at the end of the experiment (HRT of 20 days).However, two-stage reactors receiving NiO NPs reached the highest biogas yield (0.3 L gVS reduced −1 ), which was the highest among the tests.Apparently, physical separation of AD into acidogenic and methanogenic phases in the presence of NiO was beneficial, reflected by higher biogas yield among two-stages reactors than single-stage reactors.The authors didn't hypothesize on the mechanisms of the beneficial effect of the metal NPs.We support the idea that NiO NPs stimulated the growth of hydrolytic and acidogenic bacteria because the highest VFAs concentration was obtained in the acidogenic reactor (1925 mg/L).The well-being of Ni NPs was highlighted as well by Abdelsalam et al. (2016) who claimed that reactors with Ni NPs (2 mg/L) generated the highest biogas and methane production (512.2 and 304.1 mL gVS −1 for biogas and methane, respectively) followed by Fe 3 O 4 , Fe, and Co NPs.
Then, Li et al. (2018) tested the nano-scale of four types of transition metal compounds (TMCs) (HfC, SiC, TiC, and WC) in a mesophilic batch AD of CM.The AD configuration with TMCs presented a pronounced biogas yield (463-499 mL gTS −1 ) and COD degradation rate (58.62-78.90%)compared with the control (294 mL gTS −1 and 46.99%, respectively).The mesoporous structure of these NPs was most likely responsible for the increase in the activity of the microorganisms by improving the fermentation environment.Moreover, all the digestates from AD systems receiving TMCs had higher fertilizer values, which qualified them as a standard bio-fertilizer.Therefore, the application of TMCs in AD of CM simultaneously increased the conversion rate of CM into biogas and improved the fertilizer characteristic of the digestates, which laid the foundation for the waste-energy nexus in agricultural sectors.Similarly, Jia et al. (2020) applied Ti-sphere coreshell structured NPs in AD of CM or CM blends (CM and APW) with the presence of a magnetic field.Ultimately, Ti-based NPs increased the biogas yield by 27.12%-65.53%for mono-digestion and 8.47%-35.89%for co-digestion systems under the optimized magnetic field intensity (5 mT), respectively.
Z. Wang et al. (2020) outlined the merit of tungsten (W)-based NPs (WC, W 2 N, and W 18 O 49 ) in AD of CM based on experimental evidence and theoretical calculation.Theoretical inspection of the electrical structures of W-based NPs demonstrated their excellent electronic transmission capacity, laying the foundation for the enhanced methanogenesis (direct interspecies electron transfer).Meanwhile, the co-existence of the electron-donor bacteria (Alphaproteobacteria, Bacteroidia, and Synergistia) and the electron-acceptor archaea (Methanomicrobia and Methanobacteria) was identified, underpinning the promotive effect of W-based NPs.The introduction of metal (oxide) NPs in AD, however, doesn't always bring about sound outcomes.According to Luna-delRisco et al. (2011), the toxicity of CuO and ZnO NPs emerged as long as they were present in AD.Furthermore, such a detrimental effect was dosage-dependent.In other words, the inhibition effect of CuO and ZnO increased at elevated concentrations, leading to complete inhibition at 330 and 246 mg/L for Cu and Zn, respectively.In line with their statements, the high cytotoxicity and ability of CuO NPs to cause DNA damage and oxidative lesions were underlined by Karlsson et al. (2008).They concluded that most likely, the toxicity came from different surface properties and thus surface reactivity ("Trojan-horse type Carriers"), which could deliver toxic Cu 2+ inside cells.Whereas, for ZnO NPs, the toxicity might be based on the released soluble Zn ions (Karlsson et al., 2008).Abdelsalam et al. (2017b) reported a small concentration range between optimal and toxic Co concentration when dosed in AD fed with CM.At 1 mg/L, Co NPs were the best option (+101.3%improvement of methane yield), while 2 mg/L Co NPs were the worst (−12.6% improvement of methane yield).Other metal oxides, such as Fe oxides, can also jeopardize AD if overdosed.For instance, Barnes et al. (2010) reported that the concentration of Fe oxides higher than 0.3 g L −1 inhibited biological sulfate reduction.

Bioaugmentation as biological additives
Bioaugmentation aims to enhance hydrolytic reactions by adding microorganisms empowered with the capacity to degrade lignocelluloses.That is to say, bioaugmentation relies on the abilities of the injected bacterial community to encode either specific cellulolytic enzymes (i.e.cellulase, β-glucosidase, xylanase), or multienzyme complexes (i.e.cellulosome) (Tsapekos et al., 2017).Moreover, bioaugmentation using fungi depends on other specific enzymes to conduct depolymerization, such as peroxidases and laccase (Akyol et al., 2019b).A positive contribution of bioaugmentation was shown in an investigation on thermophilic cellulolytic bacteria Caldicellulosiruptor lactoaceticus.The study was carried out in a two-stage continuous reactor fed with CM.The first phase was inoculated by C. lactoaceticus, followed by methanogenesis in the second phase.Compared to the control, a 5.5% increase in methane yield was provided in the bioaugmented reactor (Nielsen et al., 2007).
Similarly, bioaugmentation with Clostridium thermocellum and Melioribacter roseus was examined by Tsapekos et al. (2017) in both batch and continuous modes treating lignocellulosic wastes (CM accounted for 90% of total VS).Particularly, for reactors inoculated with C. thermocellum, the methane yield was boosted by 13.7% in the batch AD.While in the continuous reactor, a slight increment (7.5%) was identified in the bioaugmented reactor.However, bioaugmentation with C. thermocellum didn't alter a lot the indigenous microbial communities, indicating that the added pure culture was washed-out ultimately.Despite that, C. thermocellum can still be used to extract the residual methane potential of CM, reflected by an improved cellulose removal in the bioaugmented reactor (bioaugmented: 30.9%TS; control: 33.7%TS).In contrast, M. roseus contributed little to the methane enhancement, with a 7.3% enhancement in the batch mode and no effect in the continuous mode.Alternatively, Li et al. (2020b) used a complex microbial culture as the bioaugmentation dosage to inoculate a continuous AD reactor fed with CM.Accordingly, the bioaugmentation dosage was rich in not only cellulolytic bacteria, but also 'cellulolytic archaea' Bathyarchaeota (97% within Archaea).The results indicated that the injected bioaugmentation dosage promoted the daily methane yield by 24.3%, and was caused an improved cellulose reduction (14.7%) compared with the control.When bioaugmentation stopped, its influence on the hydrolysis and methanogenesis sustained, reflected by an improved daily methane yield (11%) as well as an enhanced cellulose reduction (15.8%) compared with the control.Presumably, complex microbial cultures were advantageous over pure ones since the former could provide robust metabolisms, as suggested by the authors.
Besides cellulolytic microorganisms, the feasibility of aerobic brownrot fungi in the hydrolysis of CM was demonstrated by Myint and Nirmalakhandan (2009) in leach-bed reactors.In that study, the maximum VFAs production from reactors with brown-rot fungi was 0.152 g gVS −1 , which was significantly higher than the control (without brown-rot fungi, 0.132 g gVS −1 ).Consequently, a pronounced soluble COD generation was obtained in the bioaugmented reactor (0.186 g/g dry manure) compared with the control reactor (0.172 g/gdry manure), which was due to enhanced hydrolysis of CM.
Microorganisms in herbivorous' digestive systems, known as rumen microorganisms, are considered as efficient lignocellulose degraders.Specifically, members of the rumen fungi (phylum Neocallimastigomycota) were efficient fiber-degrading microorganisms and represented a promising bioaugmentation source (Akyol et al., 2019b;Ince et al., 2020).In this context, Yildirim et al. (2017) examined the bioaugmentation with a mixture of four isolated rumen fungi species (Orpinomyces sp., Piromyces sp., Anaeromyces sp., and Neocallimastix frontalis) at various inoculation dosages (5%, 15%, 20% (v/v)).The results indicated that AD reactors with a bioaugmentation dosage of 15% achieved the biggest boost of biogas yield (up to 60%) due to the enhanced biodegradation of lignocellulosic compounds in CM.Furthermore, the same group investigated bioaugmentation with a microbial consortium enriched from rumen fluid in batch AD.They elucidated that bioaugmentation stimulated the growth of Ruminococcus flavefaciens and Ruminococcus albus, which in turn contributed to improved cellulose degradation and subsequently, 36% enhancement of methane yield compared with the nonbioaugmented AD reactor (Ozbayram et al., 2018a).More recently, Ince et al. (2020) co-inoculated rumen fluid and seed sludge in AD fed with CM and barley.They elucidated an 18.9% increase in methane yield compared with the control.They also reported the enrichment of cellulolytic Rikenella exclusively in co-inoculated reactors, highlighting that cellulolytic bacteria were present in the rumen fluid.

Comparison of various additives
Additives in different forms have been successfully adopted to promote AD performance of CM (Table 4).Accordingly, some trends can be summarized for appropriate additive selection.Metal (oxide) supplements have shown promising results by enhancing digester performance, revealed by the stimulatory effect on microorganisms (i.e., capacity to improve biomass uptake rate) (Table 4).Specifically, Fe and Fe x O y seem to be the most studied additives (either as bulky agents or NPs) since both Fe 0 and Fe(III) can boost solubilization of organic substances and subsequent digestion occurs at relatively low cost.Meanwhile, other trace metals (Co, Ni, and Nb) have been shown as effective additives, but their detailed roles in AD (i.e., the effect on microbial activity) require more investigation.Alternatively, a rough costeffective substitute to metal (oxide) supplements is the introduction of carbon-based materials made from organic waste.By doing so, not only an improved digester performance can be achieved, but also enhanced fertilizer value of the digestates can be realized.Furthermore, NPs are emerging as promising additives owing to their extraordinary features, as discussed earlier.However, a systematic evaluation of optimal dosages of these NPs is necessary since relevant discussions are hardly present in the literature.For biological additives, the supplied microbial cultures with high hydrolytic capabilities facilitate the hydrolysis of lignocellulosic compounds of CM and ultimately reach similar goals as inorganic additives do.Nonetheless, uncertainties related to bioaugmentation still exist nowadays.For instance, it is ambiguous if a single addition could guarantee the effect of bioaugmentation or if continuous doses are necessary to maintain the bioaugmentation benefits.Answering these aforementioned questions, researchers should turn to sophisticated microbial sequencing techniques.Meta-transcriptomics, metaproteomics, and meta-metabolomics enable a thorough investigation of communities, up and down-regulation of metabolic pathways and the flux of metabolites from substrates to products when metal and non-metal additives are introduced in AD.
6. Innovative AD systems 6.1.Single electrode-assisted and magnet-assisted bio-electrochemical reactors Bio-electrochemical reactors allow researchers to influence the microbial activity, especially methanogens to boost the methane yield using an external electrochemical system (Sasaki et al., 2011).Hence, Qu et al. (2014) initially investigated the effect of electric polarity (cathode or anode) at 2500 mV on AD of CM in single electrode-assisted fermenters.They observed that electric polarity could impose a strong influence on biogas production as well as biogas quality by promoting the growth of methanogens.Reactors equipped with a cathode attained 79.4% enhancement of biogas yield with high purity of methane (77.9%) in comparison to the control group without electrode.At the same time, reactors with the cathode inserted showed the highest lignin (56.2%) removal, which was astonishing since lignin was regarded as the least degradable compound among the lignocelluloses.The results were further backed up by the presence of less crystalline cellulose (5.9%) in electrode-assisted reactors than in the control (11.2%).Finally, this novel system could generate more additional methane energy compared with the low electric energy consumption, inspiring its promising application in the methane fermentation industry.
Based on the aforementioned phenomenon, the same group further developed a system called Electricity and Magnet-assisted microelectrolysis Fields (EMF) to treat CM in AD.The joint merits of this technology were briefly described as follows: 1) A large amount of active free radical HOˑ was generated by microelectrolysis (Fe\ \C electrodes) which could cleave the glycosidic bonds between glucose units in cellulose, reducing the degree of polymerization; 2) The presence of Fe was beneficial to the propagation of microbial guilds and facilitated the synthesis of methane, while the formation of a biofilm on the surface of the Fe\ \C electrodes improved the overall biological activity; 3) A weak magnetic field was favorable for microbial growth, microbial enzyme activity, and microbial cell membrane permeability, hence increased both mass transfer rate and (bio)chemical reaction rate.Based on these advantages, they elucidated that the highest lignin and cellulose removal (23% and 36%, respectively), methane content (87%) and methane yield (151 mL gVS −1 ) could be achieved simultaneously when AD was equipped with EMF set at optimal condition (voltage 0.5 V under magnetic field).Furthermore, metagenomic sequence analysis revealed that cellulose-degrading bacteria Fibrobacteres aggregated around the electrodes and grew well (control: negligible, experiment: 11-18%) under loading EMF, which facilitated the decomposition of lignocellulose (Qu et al., 2020).

Biostimulation (laser irradiation)
Biostimulation with laser light targets the inoculum instead of the turbid CM due to the limited penetration of laser light.This technology relies on an external stimulator to activate the microorganisms in the inoculum to enhance the AD performance.Abdelsalam et al. (2018) aimed to verify the feasibility of laser irradiation by dealing with rumen fluid inoculum prior to AD.Although the biogas and methane production rates were inversely proportional to the irradiation time, they conceived an improved AD performance induced by laser irradiation (26-30% higher methane yield).Eventually, the highest methane yield (367.9 mL gVS −1 ) and organic solids degradation (55.2%) were achieved when the inoculum was exposed to laser light for 0.5 h at 532 nm.The authors hypothesized that the presence of laser light with a specific wavelength (400-540 nm) could stimulate the methylcoenzyme M [2-(methylthio)ethanesulfonic acid (CH 3 -SCoM)] to form methane.
7. Feasibility of various incentives in full-scale AD 7.1.Economic feasibility and environmental consideration of co-digestion From an economic point of view, AD co-digestion is believed beneficial as long as the co-substrate is regionally accessible.Otherwise, the additionally produced biogas brought by the co-substrate should offset its extra cost (i.e., transportation fee) to make AD profitable.Existing economic feasibility estimations of the co-digestion scenario were rather mature and variable (Aui et al., 2019;Usack et al., 2018).In this regard, Usack et al. (2018) evaluated the economic viability of codigestion implementation in terms of co-substrate selection as well as a mixing ratio.A ubiquitous phenomenon that co-digestion generated more net present value compared with digesting CM alone was identified.On top of that, to guarantee an economic benefit, the percentage of the co-substrate should be as high as possible as long as it didn't sabotage the overall AD process.However, such an approach would inversely contribute to decreased COD removal and concomitantly, harm the on-farm environment.Taking the co-digestion criteria of CM and FW as an example, they demonstrated an inconsistency between net present value and GWP at a high mixing ratio.In other words, increased profitability came at the expense of increased environmental injuries at the farm-scale.Despite that, total life-cycle emissions, which consisted of all the indirect emissions occurring outside the farm, were reduced through co-digestion compared to mono-digestion of CM.Such an advantage was mainly ascribed to the environmental benefits gained from replacing local grid electricity and commercial fertilizer with biogas and nutrition derived from the co-substrates.

Economic feasibility of pretreatment
Whether or not a certain pretreatment is energy-saving (costsaving) depends on the energy input and benefits from the additional production of methane.Zielinski et al. (2019c) stated that, for thermal pretreatment (microwave) of CM and energy crop codigests, only medium temperature (150 °C) contributed to a significantly higher net energy output (4.2 Wh gVS −1 ) compared with the control (3.75 Wh gVS −1 ) (P˂0.05).It is suggested that microwave equipment should consume less than 2.65 kJ gVS −1 to achieve a positive energy balance, which is easier to accomplish on a large scale than on a laboratory-scale.They also promoted HC as an energysaving approach for the pretreatment of CM and WS co-digests in full-scale applications due to the energy bonus (61.05 kWh d −1 ) observed from a family-scale AD installation (2.5 m 3 ) (Zielinski et al., 2019a).Budde et al. (2016) theoretically estimated the energy balance using thermalbarical pretreated CM in an established fullscale AD nexus.They claimed a possibility to obtain more thermal energy from the pretreated CM than was necessary for the pretreatment itself.Moreover, they emphasized the importance of using concentrated CM (solid CM) on a shorter energy payback time (9 months), which was indicated by Zielinski et al. (2019c) as well.Likewise, Ormaechea et al. (2018) reported a net energy increase (50%) by applying low-energy ultrasound (520 kJ kg TS −1 ) to CM in a pilot-scale AD facility.
For the mechanical pretreatment of CM employing extrusion, Hjorth et al. (2011) depicted a positive energy budget (7-26% electricity increase) over the non-treated CM, suggesting its great potential for full-scale application.Hartmann et al. (2000) calculated that a 0.9-3.8%increase in biogas production would be enough to cover the maceration expenses in a Danish full-scale biogas plant using CM as the feedstock.They showed that a biogas yield enhancement of 25% using the macerated CM taken from the same unit was possible, indicating the feasibility of maceration in full-scale AD.
The economic benefit of thermal pretreatment was validated in various studies.Cano et al. (2014) reported a net profit of 4.7 € t −1 CM without considering energy integration, while the benefit went up to 10.3 € t −1 CM with energy integration (recovering heat to produce steam in a co-generation plant).Budde et al. (2016) incorporated a model with a full-scale AD facility to investigate the additional benefit brought by thermal pretreatment.They claimed that the annual profits ranged from 3763 € a −1 for solid-liquid CM (180 °C) to 60,253 € a −1 for solid CM (160 °C) whereas liquid CM generated losses of up to 23,199 € a −1 .
Concerning chemical pretreatment, the main obstacle preventing it from upscaling to the full-scale application was the high price of reagents despite a pronounced improvement of methane yield obtained at laboratory-scale experiments (Passos et al., 2017;Ramos-Suárez et al., 2017).To make chemical pretreatment economically-feasible in Chile, Passos et al. (2017) estimated that the market price of HCl acids should be below 330 $ ton −1 , provided the selling price of the methane injected to the national grid was set as 162.7 $ MW h −1 , and the cost of electricity for industries was 86.4 $ MW h −1 .Otherwise, the current price of HCl acids (660 $ ton −1 ) would definitely impair the stakeholder's investment passion.This also applied to alkali addition (Ramos-Suárez et al., 2017).Unless the reagent-related operation costs (18,765 € a −1 with reagents and 7537 € a −1 without reagents) dropped, the improved energy production couldn't compensate for the capital investments.Therefore, to make chemical pretreatment economically viable, future experiments should either use cheap reagent (limestone, for example) or reuse the reagent utmost to minimize the overall chemical input.
Unlike well exploited mechanical, thermal and chemical pretreatments, biological pretreatment of CM is less studied at full-scale.Nevertheless, the full-scale application potential of aerobic pretreatment (composting) was recently highlighted by Bremond et al. (2018).Based on compost addition, a microbial consortium (Bacteriometha™) rich in hydrolytic microorganisms sold by a French company called Sobac (Lioujas, France) was tested in full-scale AD treating CM.By mixing the additives with CM at aerobic conditions for 3-15 days, trials on two full-scale plants displayed higher energy production and ensured economic interest despite the purchasing fee (Sobac, n.d.).Furthermore, the potential benefit brought by aerobic fungi shouldn't be overlooked.More recently, a market product called Optimash® AD-100 was released, which was an enzyme blend from Trichoderma reesei and Myceliophthora thermophile.Results implied that the direct introduction of the enzyme culture in a full-scale plant lead to a 10% decrease in maintenance costs for the digester (Bremond et al., 2018).Since the relevant laboratory studies on aerobic composting and aerobic fungi of CM are poorly stated, future investigations on this aspect are highly recommended.

Economic feasibility and environmental consideration of various additives
The economic benefit of trace element addition to CM-based fullscale AD is poorly documented.Therefore, we try to deduce it from a case study presented by Garuti et al. (2018), who studied trace elements (TE) supplementation (Mo, Ni, Co, and Se) in a full-scale facility receiving mixtures of CM and CS.The first implementation of TE dosage undoubtedly lead to a prompt enhancement of daily methane yield in AD (23.1-38.5%),which might have brought some economic benefit.However, the methane production bonus failed to persist and lasted for merely three days.Moreover, the repeated introduction of TE (two more times) didn't promote the methane yield further and remained at the previous baseline.Hence, the supplementation of TE might be economically-infeasible from this point of view.Nonetheless, the addition of TE as precautions to prevent potential process inhibition (organic acids accumulated from 3229 to 4530 mg CH 3 COOHeq L −1 ) should be highlighted.Otherwise, the depletion of TE would have induced process imbalance and loss of investment when AD proceeded (Garuti et al., 2018).Therefore, the addition of TE could somehow be regarded as a money-saving strategy to overcome VFAs accumulation and, thus, maintained a healthy AD process (Garuti et al., 2018).Additionally, the environmental benefit regarding GHG mitigation of TE can't be neglected, which was underlined by Garuti et al. (2018) and Hijazi et al. (2020a).For instance, Garuti et al. (2018) stressed that the residual methane potential (RMP) of the digestates from the TE sufficient stage (17.8Nm 3 CH 4 tVS −1 ) was significantly lower than that from TE starving stage (58.3Nm 3 CH 4 tVS −1 ).Thus, an adequate supply of TE gave environmental benefits by the reduction of RMP, hence mitigating GHG emissions in the atmosphere from the digestates stored in uncovered tanks and from the application of the digestates as a soil fertilizer.
The current economic feasibility analysis of carbon-based accelerants is lacking, which deserves an estimation due to its substantial improvement of biogas yield (Yun et al., 2018).Hence, we simulate an economic analysis based on Yun et al. (2018)'s laboratory output and data obtained from a case study (Wang et al., 2011).In our estimation, waste corn cob is used as the source and the washing detergent is H 3 PO 4 to generate carbon-based accelerants.According to Wang et al. (2011), an average electricity output for farm-scale AD facilities receiving 69 t CM d −1 was 5295 kWh d −1 , equal to an average electricity revenue of 958 $ d −1 (0.181 $ kWh −1 ).Hence, to boost the AD performance of CM (69 t d −1 ), 124.2 kg of carbonbased accelerants should be added, which will consume at least 124.2 kg of waste corn cob and 372.6 kg of concentrated (85%) H 3 PO 4 as input (based on Yun's research).Subsequently, if we assume that the methane content remained unchanged and the enhancement of biogas (40.8%) in the laboratory-scale experiment also applies for the farm-scale facilities, then additional electricity revenues of 391 $ d −1 out of 69 t CM will be achieved.H 3 PO 4 can be purchased online at 0.5 $ kg −1 (Alibaba™), which means 186.3 $ d −1 is spent on chemical input.The remaining profit margin is 205 $ d −1 , but should also deduct the price of waste corn cob and electricity consumed by heating devices.The waste corn cob can be purchased online at 0.58 $ kg −1 (Alibaba™) or can be collected from the farmland, which may require additional transportation fees and labor costs.If purchased, then another 72 $ d −1 will be deducted from the net profit, leaving 133 $ d −1 to cover the rest of the expenses.Since the capital cost of the heating machine is fixed, we conclude that the addition of carbon-based accelerants derived from waste corn cob is economically feasible for the long run at farm-scale AD.To speculate on the environmental impact of carbon-based additives, we depend on the qualitative and quantitative analysis performed by Yun et al. (2018).On the one hand, they reported an enhanced CODt removal by adding carbon-based accelerants (59.9%) compared to the control (29.6%), resulting in improved thermal stability of the digestates (control: 70.54%; carbonbased accelerants: 63.22%).On the other hand, excellent stability and prolonged coexistence of carbon-based accelerants in the digestates were revealed (Yun et al., 2018).
For low-cost composites, a similar calculation can be conducted as done for carbon-based additives (Yun et al., 2019;Wang et al., 2011).According to Yun et al. (2019), the combination of urea (0.3 wt%), diammonium phosphate (0.3 wt%), citric acid (0.5 wt%) improved the biogas production by 38.9% compared with the control check.According to Wang et al. (2011), the average electricity revenue treating 69 t CM daily was 958 $ d −1 (0.181 $ kWh −1 ).Then, if we assume the improved biogas yield in the lab also applies for large-scale AD, the additional revenue will be 372.7 $ d −1 .Considering the total prices of the aforementioned additives (0.48 $ kg −1 for urea, 0.50 $ kg −1 for diammonium phosphate, and 0.45 $ kg −1 for citric acid.(Alibaba ™)), the initial input for additives treating 69 t CM is 357.8 $.Hence, there is a narrow profit margin generated (14.9 $ d −1 ).However, the digestates coming from the AD installation receiving additives have a reasonable fertilizer value, which can be applied as a soil amendment (Yun et al., 2019).Such an 'invisible' benefit should also be included and thus, the addition of low-cost composites could also be regarded as a profitable incentive in the large-scale AD of CM.
Economic estimation of the addition of NPs (ZVI) in CM-based AD installation was conducted by Dehhaghi et al. (2019).They concluded that the price of NPs for a feasible AD scenario shouldn't exceed 118.88 $ kg −1 , and any further improvements in the cost-effective production of ZVI NPs would translate into corresponding higher profits.After sending a query for the latest price of ZVI NPs at the online store (Alibaba™), they returned that ZVI NPs could be purchased at 96 $ kg −1 , which would generate better profit margins for biogas-plant operators.Likewise, a theoretical economic estimation based on Li et al. (2018) and Wang et al. (2011) is also illustrated here.According to Li et al. (2018), the addition of SiC NPs (0.025 wt%) can boost the biogas yield by 69.7%.The market price of SiC is 20 $ kg −1 (Alibaba ™), if we assume 1 t CM needs to be treated, then the enhanced electricity revenue from 1 t CM is 9.67 $ (here we assume the enhancement of biogas yield also applies for pilot-scale AD) (Wang et al., 2011).Thereafter, we deduct the input of SiC (5 $) needed for 1 t CM, the rest profit is still considerable (4.67 $ t −1 CM treated ).From an environmental perspective, Hijazi et al. (2020b) perceived the environmental benefit brought by various NPs (Co, Ni, Fe, and Fe 3 O 4 ) via life-cycle analysis (LCA).Accordingly, the environmental impacts were categorized into GHG emissions, eutrophication, acidification, ozone layer depletion, and human toxicity potential with GHG emissions, the foremost indices for biogas installations.The outcome of the research indicated that the lowest GHG emissions were achieved by adding Co NPs (0.0225 kg CO 2 eq.MJ elect −1 ), followed by Ni NPs (0.0276 kg CO 2 eq.MJ elect −1 ), Fe 3 O 4 NPs (0.0290 kg CO 2 eq.MJ elect −1 ), Fe NPs (0.0336 kg CO 2 eq.MJ elect −1 ), compared to the control (0.0366 kg CO 2 eq.MJ elect −1 ).Meanwhile, Co NPs showed the lowest acidification (1.499 × 10 −5 kg SO 2 MJ elect −1 ) and eutrophication (6.1 × 10 −6 kg phosphate eq.MJ elect −1 ) impacts on the environment, allowing AD to expand its environmental protection boundary.However, concerns of extensive implementations of NPs still exist since NPs could cause health complications of humans (or animals) via inhalation, skin contact, and ingestion (European Commission, 2014).Once absorbed, NPs might reach susceptible organs (i.e., brain) by crossing the bloodbrain barrier or cell boundaries and might induce neurodegeneration diseases such as Huntington's disease (Dehhaghi et al., 2018).In some instances where NPs are released into the environment (i.e., use of NPs-containing digestates as soil fertilizer), these compounds may exert biocidal or biostatic effects on microbial communities through the aforementioned mechanisms.Additionally, some fungal species (such as wood-decay fungi) may degrade NPs into some antimicrobial metabolites, affecting microbial ecology.In light of that, research efforts are highly recommended to thoroughly assess the use of NPs in agricultural AD sectors, especially their impact on human toxicity.
Frankly speaking, there are hurdles for future applications of biological additives.Briefly, the laboratory-scale continuous experiments couldn't present a substantial enhancement of methane yield (5%-11%), which required a persistent feeding of bioaugmentation dosage (Nielsen et al., 2007;Tsapekos et al., 2017;Li et al., 2020b).Thus, constant input of bioaugmentation culture is mandatory, which adds additional fees to the overall running costs.What's more, a limited enhancement of the methane yield may extinguish the investor's passions to invest in bioaugmentation.To overcome the limitation, an alternative approach, such as immobilization of augmenting microorganisms to retain them within the reactor, could be further developed.Recent achievements regarding these proposals have been tested by different researchers who claimed promising results (Ta et al., 2017;Georgiou et al., 2005).However, to the best of our knowledge, relevant laboratory trials haven't been established for CM yet, not to mention full-scale applications.Qu et al. (2014) emphasized the promising application of bioelectrochemical fields for industrial purposes based on the negligible energy consumption of the system running at 2500 mV (no circuit produced due to single polarity) versus a substantial increment of energy output.A simultaneous lignocellulosic compounds removal and methane production improvement not only boosted the running profit but also resulted in improved substrate rheology, and therefore a reduction in stirring costs inside the digester (Ramos-Suárez et al., 2017).Although no field data were documented in that paper, this system might be an example for future designs of novel reactor types and should be further exploited in the agricultural AD scenario.

Economic feasibility of innovative AD systems
According to Abdelsalam et al. (2018), the main drawback of the application of laser irradiation came from the limited irradiation capacity for dense CM.

Summary, future perspective and concluding remarks
Existing strategies have been successfully applied to improve methane production of CM in laboratory and pilot-scale anaerobic digesters, such as (1) co-digestion with organic wastes; (2) varying pretreatment approaches; (3) introduction of chemical and biological additives; (4) bio-electrochemical fields (Fig. 3).Taking into consideration the economic feasibility and environmental benefits, different treatment configurations are discussed and rated, as shown in Table 5. Thermal and biological pretreatments stand out for their economic and environmental sustainability at full-scale facilities (Table 5).In addition, newly-developed trace element based NPs and bio-electrochemical fields may render a promising application potential regarding their excellent lab achievements and desirable estimation of full-scale practice.For the EU where the biogas market is rather mature, a new trend emerges where the produced biogas is upgraded into biomethane (˃95% CH 4 ).The demand is growing as a result of support schemes for applications in transportation or injection into the gas grid (Scarlat et al., 2018a).In this context, Scarlat et al. (2018a) announced that Europe the world's leading producer of biomethane as a vehicle fuel with 160 million m 3 used for gas stations in 2015.Therefore, for the developed market, future studies should not only focus on the practical application of AD incentives, but also concentrate on inexpensive biogas upgrading, energy storage, and transportation approaches to make AD competitive to substitute fossil fuels (Pellegrini Laura et al., 2018;Heubeck et al., 2007).Whereas, for agricultural sectors in developing countries (like China) where simple AD facilities are installed, households expect stable energy for cooking or heating (Luo et al., 2020).Provided that mono-digestion of CM can't provide surplus biogas, co-digestion, composting pretreatment, and the supplement of waste carbon, urea, and waste Fe power can be alternative inexpensive approaches.Future studies should also prove the feasibility of these incentives in small and medium household AD to stress the concern in the rural economy (Luo et al., 2020).Furthermore, AD is an important participator in 'circular agriculture' where the generated agricultural wastes are disposed and the AD digestates can be reused on-land.Great attention should be paid to additives, as the digestates can be qualified as commercial fertilizers (Huang et al., 2016;Zhang et al., 2018).It is noteworthy that AD plays a vital role in a sustainable society.However, an efficient AD framework containing researchers, individual stakeholders, and the government is urgently required.In other words, researchers are encouraged to work in fields to flexibly choose AD incentives to satisfy the stakeholders' interest case by case.Individual stakeholders should be aware of the importance of 'sustainable society' and get basic trained to run the AD facility themselves.The government can build up websites or mobile apps for individuals to report their running problems or for lab researchers to broadcast their tailor-made AD incentives in different cases.Policymakers should also pass legislation to guarantee the interest (by giving subsidy) of individuals who run AD and enable a benign cycle in a sustainable economy.
Concluding remarks are summarized as follows: (1) The synergy equation should be introduced to verify whether or not an enhanced methane yield of co-digestion scenarios comes from an improved degradation of CM when CM is involved.More importantly, the addition of lignin-poor co-substrate could promote the degradation of lignocellulosic compounds of CM. (2) Mechanical and thermal pretreatments are currently most suitable for CM, either at laboratory, pilot, and full-scale.(3) Biological pretreatment, especially aerobic pretreatment (mixing enzymes from aerobic fungi) of CM, offers a full-scale application possibility but requires comprehensive laboratory-scale investigations first.(4) Implementation of NPs additives in full-scale AD of CM is promising in terms of economic and environmental benefits.However, researchers should concentrate more on the human-related impact of NPs and reach a comprehensive agreement on the proper use of NPs in AD. (5) Bio-electrochemical reactors deserve more investigations regarding pilot and full-scale AD of CM.

Funding
This work was supported by the Chinese Scholarship Council (CSC).

Fig. 1 .
Fig. 1.Estimated available crop residues and cow manure in the EU (source files are derived from Einarsson and Persson, 2017).

Fig. 3 .
Fig. 3. Summary of reviewed AD incentives (a) and their maximum methane enhancement potential (b).

Table 1
Basic information of cow manure.

Table 2
Summary of anaerobic co-digestion of cow manure and organic wastes.

Table 2
(continued) Bah et al., 2014;Li et al., 2020a;Zhao et al., 2018;Simm et al., 2018;Cestonaro et al., 2015.se and hemicellulose), and synergy in different studies.Data is derived fromBah et al., 2014;Li et al., 2020a;Zhao et al., 2018;Simm et al., 2018;Cestonaro et al., 2015.Note: For subgraphs of AD co-digestion of CM and SB (bottom left) and AD co-digestion of CM and CG (bottom right), the information of lignin wasn't provided by the authors, but we could infer from the author's statement that SB is rich in lignin whereas CG contains no lignin; SB: Sheep bedding; CG: Crude glycerine; PPF: Palm pressed fiber; SM: Sheep manure; OS: Oat straw.

Table 3
Summary of different pretreatment methods on anaerobic digestion of cow manure (used as sole or main substrate).
Mesophilic batch No significant enhancement for individual pretreatment, while combined pretreatment improved the methane yield by 19.6% Wahid et al., 2020 CM Microwave+ thermal chemical Mesophilic batch Using microwave + thermal alkaline (CaO, NaOH) generated the highest methane yield (450 mL/g CM) Jin et al., 2009 CM fiber Aqueous ammonia, O 3 , and combination of both Mesophilic batch Combined aqueous ammonia and O 3 significantly increased biogas production by 6.2-8.8%compared with O 3 alone, while 55.3-103.6%compared with aqueous ammonia alone Ai et al., 2019 Y. Li, J. Zhao, J. Krooneman et al.Science of the Total Environment 755 (2021) 142940

Table 4 (
No data presented by the author but can be inferred directly from the figure.h Cellulose and hemicellulose degradation enhancement is inferred based on qualitative analysis (thermogravimetry analysis).
f Biogas enhancement.g

Table 5
Overall evaluation of different incentives adopted in anaerobic digestion of cow manure.(Note: more '+' indicates better performance.)Y. Li, J. Zhao, J. Krooneman et al.Science of the Total Environment 755 (2021) 142940