A Critical Review on the Various Pretreatment Technologies of Lignocellulosic Materials

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 925-935

producing highly digestible feedstock, and little or no

biomass size reduction (8). Although many pretreatment

techniques have been developed by scientists, there is no

technique that to have all the above mentioned futures.

Various developed techniques for pretreatment of

lignocellulosic biomass have been categorized in Figure

cultivation of this plant is faced with several difficulties

(24). Based on the above mentioned explanations, it is

necessary to find an alternative way to produce biofuels.

Lignocellulosic biomasses including cereal straws which

are widely cultivated worldwide may consider as a

suitable feedstock for biofuel production. Cereal straws

are usually burned by farmers which produces a large

amount of air pollutants (25). Several inexpensive

lignocellulosics agricultural waste are accessible such as

wheat straw, rice straw, softwood, switch grass, salix,

willow, timber species, rice hull, and sugarcane baggase

as a feedstock for biofuel production. In some countries

that have a wide agricultural activities the agricultural

wastes have a good potential to produce biofuels.

Different biofuel production yields can be obtained using

various crops. The cost of biofuel production depends on

the cost of agricultural wastes, transportation method of

agricultural wastes from farms to factories, and

processing technology. Also, government policies about

biofuel production can be considered as an influential

factor on biofuel cost. The cost of biofuel production and

market price are other factors which are effective on final

biofuel cost. Seasonal availability of agricultural wastes

depend on the type of it; for example cotton stalk is

available between January and March whereas maize

stalk is available from August and December.

Information about seasonal availability of agricultural

wastes is very important to guarantee the feedstock

availability all over the year for the biofuel factory. The

most important part of plant cell wall is composed of

lignocellulos. Lignocellulos is a natural and complex

composite including lignin, hemicellulose, and

biopolymers— cellulose (26, 27). Some other materials

such as ash can be found herewith lignocellulosic

biomass. Lignocellulosic biomass is a heterogeneous

composite of lignin and carbohydrate polymers which

contains up to 75% of carbohydrates (based on dry

weight) (24). Lignocellulosic biomass contains complex

sugars; therefore, it is not readily converted to biofuel.

Also, it is contains several polysaccharides including

hemi-cellulose and cellulose which require to be

converted to the monosaccharide. Lignin, hemicellulose,

and Cellulose are strictly associated with each other so

this association can almost stop the access to the

hydrolytic agents. The lignin should be eliminated or

modified to access the hydrolytic agents using different

chemical or biological techniques (28). Cellulose which

is also known as cellobios is considered as a polymer of

glucose. The cellulose structure aid to have a tightly

packed polymer chains, resistant to depolymerization,

and highly crystalline structure (29). Another

carbohydrate component which is found in the

lignocellulosic biomass is hemicellulose. This compound

is composed of 5 and 6 carbon sugars that has an

amorphous, branched, and random structure. Both the

hemicellulose and cellulose are polymers of sugars;

therefore, they are considered as a potential source of

fermentable sugars. Both the hemicellulose and cellulose

can be simply processed into other various products (30-

. There also some reports about generation of inhibitory

substances within the process of pretreatment (9).

Pretreating several particular biomasses causes

production of toxic compounds (9). The most important

toxic compounds generated during pretreatment

processes of lignin are aromatic compounds, ketones,

aldehydes, organic acids, and furans (10). Pretreatment is

an influential technique on other steps of biofuel

production. Pentose and hexoses can be converted to

inhibitory

substances

such

furfural

and

hydroxymethylfurfural within pretreatment processes.

The amount of produced sugar and biofuel and be

significantly decreased when toxic compounds are

presented in the environment.

Effecting parameters

.1 Delignification

Elimination of lignin from lignocellolusic biomasses

(such as woody tissue) using different natural enzymatic

or chemical methods is known as delignification.

Nowadays, many methods have been introduced to

produce energy in various forms such as pyrolytic biooil,

biodiesel, and biogas (5, 11-14). Bioethanol production is

the most important way to produce energy from

ligninocellulosic biomass compared to other energy

sources such as biogas, pyrolytic oil, and biodiesel (10).

Abundance and low cost of lignocellulosic biomass is the

main reason for selection of this type of biomass to

produce different energy sources like bioethanol.

Lignocellulosic biomass has low amount of oil and that’s

why this type of biomass is not suitable for production of

biodiesel. Biogas are usually utilized for generation of

thermal or electricity energy and it is not used as a

vehicle fuel. Biodiesel is another important technology of

energy conversion. Trans-esterification process of animal

fats and vegetable oils is a way to generate biodiesel.

Biodiesel is mostly achieved from specific type of plants

such as rubber seed (15), coconut (10), mahua (16),

tobacco seed (17), castor (18), Eruca sativa (19),

pongamia (20), and jojoba (21, 22). Also usage of some

other plants like palm oil, sunflower, flax,, pongamia,

jatropha, and mustard have been reported to produce

biodiesel (10). Lignocellulosic biomass do not has oils

that are necessary for biodiesel generation. Although

some crops such as mustard has oils they are used for

animal feed. Fresh oil of vegetable has a viscosity

between 28 and 40 mm /s. Direct usage of vegetable oil

as diesel engine fuel is not suitable and can make

problems for the diesel engines including deposits

formation and injector coking arising from poor

atomization due to its high viscosity (23). A study

showed that the present economics of ethanol produced

based on molasses could not be appropriate for

commercial blending of ethanol in petrol. This study also

demonstrated that nearly 736.5 million ton of sugarcane

is require if Indian government aim to have 10%

blending of biodiesel and diesel fuel. It means that Indian

government need to have 10.5 million hectare for

sugarcane cultivation (24). Sugarcane require a large

3). Although, many difficulties have been introduced

for biofuel production, existence of lignin is the most

important one. Lignin is a very stable biopolymer built

from three cross-linked phenylpropane units of p-

coumaryl alcohol, coniferyl alcohol and sinapyl alcohol

(34) which are bonded together with over two-thirds

being ether bonds (C–O–C) and the rest C–C bonds (34).

amount of water to grow (20000–30000 m /ha); therefore,

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2020, Volume 8, Issue 2, Pages: 925-935

Conventional hydrotermal techniques

High pressure microwave pretreatment

pretreatment at atmospheric pressure

Wet oxidation

Microwave assisted technique

Steam explosion

Ammonia fiber explosion

Liquid hot water

Biological technique

Acetic acid

Acid technique

Phosphoric acid

Sulfuric acid

Pretreatment

Techniques

Alkaline technique

Sodium hydroxide

chloride, bromide, and thiocyanate ion

-butyl-3-methylimidazolium

Ionic liquid technique

1-butyl-3-methylimidazolium chloride

-ethyl-3-methyl imidazolium diethyl

-ethyl-3-methylimidazolium-acetate

,4-butanediol

Glycerine

Organic solvent

Mechanical technique

Figure 1: Categorization of pretreatment techniques (35)

Lignin is hydrolyzed using cleavage of the ether

bonds that are catalyzed by water molecules and

hydrogen or hydroxyl ions (36). Ash content of

lignocellulosic biomasses is usually composed of

minerals such as sodium, magnesium, calcium,

aluminum, and silicon. Extractives are other negligible

compounds found in the lignocellulosic biomass that

include minerals, salts, phytosterols, phenolics, fatty

acids, resins, and fats. Delignification using biological

techniques has three main steps: (a) first step is

lignocelluloses as recalcitrant structure to reactive

cellulosic intermediates; (b) enzymatic hydrolysis in

which cellulose hydrolyze using reactive intermediates to

fermentable sugars such as xylose and glucose; and (c)

fermentation process that can produce biofuel or some

bio-based compounds like succinic acid or lactic acid

(37-40). The delignification of ligninocellulosic biomass

is illustrated in Figure 2. Ligninocellulosic biomass can

be biologically or chemically degradable after removal of

recalcitrant compounds from the biomass (41). There are

several explanations for the importance of delignification

lignocellulose

pretreatment

that

can

convert

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 925-935

which are introduced by many researchers (9, 42). An

ideal delignification must have following futures: very

limited production of sugar; very limited amount of

Types of pretreatments

A costly step in lignocellulosic biomass conversion is

pretreatment. This step can be done by chemical,

physical or hybrid techniques which are generally energy

intensive. The most important techniques for

pretreatment are presented in the next sections.

residual lignin; producing

a very high degradable

cellulose, very low amount of required energy, low

required cost; and ability to recover nearly all

carbohydrate. The delignification process is the most

important and difficult steps in biofuel production. It was

reported that delignification has an intensive effect on all

next steps in the biofuel production (9). Although lignin

should be eliminated using pretreatment techniques for

enhancement of biofuel production, recent studies show

that lignin may be used in construction industry, dust

particle controls, cleaning metallic surfaces, chemical

.1 Microwave assisted technique

Using microwaves is another way for pretreating of

lignocellulosic

biomass.

Microwaves

electromagnetic wave with wavelength between radio

waves and infrared radiation. Microwave radiation for

pretreatment of lignocellulos in comparison with

hydrothermal technique has many advantages such as

lower the reaction activation energy, enable more rapid

product formation, and reduce the reaction time

remarkably (47). Such advantages make microwave

technique as a promising technique for pretreatment of

lignocellulosic biomass. The conventional pretreatment

techniques such as autohydrolysis, steam explosion, and

diluted acid pretreatment must be done at high pressure

and temperature. These pretreatment techniques are able

to break complex chemical bonds of lignin-carbohydrate,

lignin, and hemicellulose. In this condition cellulose can

be exposed to cellulase attacking. Usually,

lignocellulosic biomasses should be heated between 160

and 250˚C. The heat can be provided using indirect heat

conduction or injection of high pressure steam. When the

conventional pretreatment techniques are utilized, the

row lignocellulosic biomasses should be crushed to small

pieces due to saving energy. Hemicellulose is converted

to furfural and even to humic acids during the

pretreatment process which is adverse to fermentable

sugars recovery and conversion. Microwave technique is

able to heat lignocellulosic biomasses quickly and

uniformly. When microwave technique is applied for

pretreatment of lignocellulosic biomass, the above

mentioned difficulties can be avoided. Pretreatment

using microwave was firstly introduced by Ooshima et al.

coatings, agro farmland improvements, and

industry (10).

paint

.2 Enzymatic hydrolysis

Using an optimized enzymatic treatment process

such as utilization of accessory enzyme including laccase

and xylanases may decrease the concentrations of the

enzymes required. It can also improve the cost-

effectiveness of biofuels generation. The lignocellulosics

materials digestibility using enzymes depends on the

level of crystallization, lignification, and acetylation

processes (43). An extensive delignification is needed for

achieving

suitable digestibility regardless of

crystallinity and acetyl content. Delignification and

deacetylation cause enzymatic hydrolysis and

crystallinity not to stop. This does affect initial

hydrolysis rates but does not significantly affect sugar

yields (43). Studies showed that the size of

ligninocellulosic biomass (except large chunks) has not

an significant influence on enzymatic digestibility of

bagasse (10), switch grass (43), corn stover (44). It has

been reported that the effectiveness of the absorbed

enzymes and enzyme adsorption are influential factors

on enzymatic hydrolysis rate (45). The effectiveness of

the enzyme can be enhanced after lignin removal since

non-reproductive adsorption site is eliminated using

raising access to cellulose and hemicellulose

(1984). After that a remarkable progress on using

microwave pretreatment techniques such as designing

particular microwave vessels for loading biomass and

new microwave reactors has been observed (7). Usage of

high boiling solvent and microwave technique was also

used by (48). They showed that the yield of levoglucosan

from fast pyrolysis of corncobs could 189 times increase

when corncobs were pretreated by microwave at 150 W

for 18 min and glycerol as high boiling solvent.

Additionally, the results demonstrated that the alkali and

alkaline earth metals and ash could be effectively

removed from corncobs using microwave and glycerol

(holocellulose). The covalent linkages and physical

binding between lignin and hemicellulosic grass cell

walls affects enzymatic hydrolysis (46). Therefore,

enzymatic hydrolysis of wheat straw for biofuel

generation is bottle-necked by the limitation of their

efficacy and the cost of enzymes (13).

(48).

Enzimatic →

hydrolysis to

hexoses

Conversion to

biofuel

Celloluse

Hemicelloluse

Lignin

Enzymatic →

hydrolysis to

pentoses

Recalciterant

Conversion to

biofuel

part

Lignocellulosics

biomass

fractionation

Biodegradable

Energy/Fuel/Ch

emicals

part

Figure 2: Fractionation of lignocellulosic biomasses (35)

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2020, Volume 8, Issue 2, Pages: 925-935

Salt ions have more movement in electric field

compared with pure water; therefore, the heat of salt

solution can increase faster to the expecting temperature.

Studies on the pretreatment using salt solution and

microwave radiation demonstrated that solution

containing salts can be heated sooner than that without

salt ions (49). (50) applied calcium chloride under

microwave radiation for pretreatment of corn stover.

They could find that residence time and temperature

were the most effective parameters on the corn stover

enzymatic digestibility. The optimal temperature and

residence time with solid-to-liquid ratio (w/v) at 10%

were 162˚C and 12 min, respectively. They reported that

under optimum condition enzymatic hydrolysis ratio was

by two groups of scientists (55, 56). They demonstrated

that ratio of hydrolysis of polysaccharides to soluble

sugars was enhanced by 5.23% compared to that with

conventional heating. Ferric sulfate was used for

pretreatment of bamboo using microwave as source of

heating (57). The results illustrated that the ratio of

cellulose enzymatic hydrolysis was raised from 52.72%

to 72.15% by pretreatment using microwave.

3.2 Alkaline technique

The removal of some recalcitrant compounds

including various uronic, acetyl groups, lignin, and acid

substitution from lignocellulosic biomasses is an

essential duty of a pretreatment technique. The cellulose

0.66%. Although higher amount of calcium chloride

and

hemicelluloses

solubilization

alkaline

can decrease the required temperature for pretreatment of

corn stover, it was difficult to separate it from solution

after finishing the process (50). Pretreatment of

lignocellulosic biomasses can be efficiently pretreated by

pretreatment technique is not effective as much as other

pretreatment techniques such as hydrothermal or acid. In

the other hand this technique needs lower operation

temperature but it require a longer residence time

(between hours and days). Generally, sodium hydroxide

have been employed as a basic catalysts (58). In an

alkaline pretreatment sodium hydroxide solution was

used to improve generation of biogas from residues of

herbal-extraction process (59). They demonstrated that

72.1% of raw materials weight was reduced during the

biogas generation. Sodium hydroxide solution was used

for pretreatment of microcrystalline cellulose (60). They

reported that the performance of pretreatment can be

increased at higher alkalinity and temperature. Their

results showed that the samples could be completely

converted to glucose. The theoretical investigations

revealed that the maximum yield of ethanol was around

59% for the samples pretreated by sodium hydroxide.

The method of Box and Behnken Design was applied to

study the influence of substrate concentration, resident

time, and alkali concentration and on the hydrolysis of

polysaccharides to soluble sugars of rice straw and hulls

(61). The outcome demonstrated that the best

pretreatment conditions were substrate concentration 30

g/L, heating time of 22.50 min, and alkali concentration

2.75% (61). Orthogonal design was used by (62) to find

the optimum condition of wheat straw for bioethanol

production. The highest yield of bioethanol production

was achieved with heating time of 15 min, 10 Kg sodium

acid (51). Microwave can be used as

a suitable

alternative for increasing temperature in acid

pretreatment technique. Using microwave radiation not

only could decrease the resident time but also could

increase the performance of pretreatment.

Unlike lignocellulosic biomass pretreatment at

atmospheric pressure microwave, high pressure

microwave should be done in closed reactors. The range

of temperature in this type of reactors is between 150 and

50 ̊C . In high pressure microwave technique, higher

temperatures can reduce the resident time and increase

performance for pretreatment. Using the microwave

pretreatment under high-pressure require a more complex

reactor. A dynamic microwave pretreatment reactor was

employed by (52). This dynamic pretreatment reactor

could use theoretically for continuous pretreating of

lignocellulosic

biomasses.

Since

lignocellulosic

biomasses are insoluble in water, they cannot be easily

pumped into the pipes. Lignocellulosic biomasses should

be crushed to small particles and then suspended into a

large volume of a liquid such as water. Next it can be

transferred using pipes. It was reported that a ration of

liquid–solid as much as 50:1 is suitable for transferring

lignocellulosic biomasses using pipes. Application of

such

a big liquid–solid ration can increase the

consumption of energy by a microwave reactor which it

reduce the economic viability of pretreatment (52). A

continues microwave reactor was designed for biomass

pretreatment in a pilot scale and its power of microwave

was linearly varied from 0 and 6 kW (53). Their highest

capacity of pretreatment using this microwave reactor

was 5 kg/h. They used a transport belt to transfer the

biomass toward the microwave reactor which was a good

idea for solid materials including corn stover. The ratio

of liquid to slid and also the require energy to transfer

corn stover was reduced using the transport belt. This

microwave reactor needed high boiling organic solvent

since it should be run at atmospheric pressure. Using

high boiling organic solvent increased the cost of this

type of reactor.

hydroxide/m , and 80 g biomass/kg.

3.3 Ionic liquid technique

Nowadays, using ionic liquids with microwave

radiation for pretreatment of lignocellulosic biomass has

been attracted attention of a large number if scientists.

Ionic liquids showed a very high solubility of biomass

which made it perfect for lignocellulosic biomass

pretreatment. Dissolution of cellulose using ionic liquids

was investigated by (63). This ionic liquid was composed

of many cations and anions such as 1-butyl-3-

methylimidazolium, chloride ion, bromide and

thiocyanate (63) reported that nearly 25% of cellulose

was able to dissolve in 1-butyl-3-methylimidazolium

when it was heated. A ionic liquid containing 1-ethyl-3-

The effect of using various heating sources for

pretreatment of switchgrass was investigated by (54). At

methylimidazolium-acetate,

imidazolium diethyl,

1-ethyl-3-methyl

1-butyl-3-methylimidazolium

the same temperature (190

chloride, and 1-allyl-3-methyli midazolium-chloride

have been intensively investigated due to their notable

cellulose dissolution ability (64). The pretreatment of

lignocellulosic biomass using ionic liquids is and

expensive technique. Also, it needs a large amount of

energy to recycle pure ionic liquids. Furthermore,

pretreatment of switchgrass using microwave could

achieved an enzymatic hydrolysis ratio of 58.5% that

was 53% greater than that by conventional heating

source. An acerbic organic solvent was applied as

absorbing medium of the microwave to pretreat softwood

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2020, Volume 8, Issue 2, Pages: 925-935

viscosity of ionic liquids is gradually increased that

makes it challenging to handle.

can be used for increasing the temperature. The highest

yield of surge reduction was achieved as much as 48.3 g

for each 100 g pretreated hyacinth in an environment

containing 1% sulfuric acid environment. In this

condition 96% of surge was converted (51). Starch-free

wheat fibers was pretreated in a low concentration of

sulfuric acid (71). They found that using microwave as

heating source is more effective than steam for the acid

pretreatment. The effects of temperature on the structure

of lignocellulosic biomasses with a 0.2 M sulfuric acid

was studied (72). They showed that the increasing of

temperature had positive effects on the destruction of

biomass structure during acid pretreatment. The pH of

water somewhat depends on temperature; for example:

.4 Organic solvent technique

Since lignocellulosic biomasses have a high boiling

point at atmospheric pressure, many high boiling organic

solvents such as glycerine and 1,4-butanediol are usually

applied for biomass pretreatment. Usage of high boiling

organic solvents at high temperature can break the bonds

between biomass and lignin and they finally are

dissolved into the organic solvent to be recovered as a

by-product. The residual materials can be easily

degraded after removal of lignin. During this process

organic solvent can be recycled which is considered as

an advantage of organic solvent technique. Moretti et al.

pure water has a pH of 6.99 at 25 ̊C . The pH of pure

water at 150, 200 and 250 C would be decreased to .5.52,

(2014) used different organic solvents (phosphoric acid,

glycerol, and water) for pretreatment of bagasse. They

could found that this technique is more effective to

hydrolyze sugarcane bagasse when glycerol was used as

solvent (65). They also found that when glycerol was

used as their solvent for pretreatment, the yield of sugars

reduction was 12 times higher than water and phosphoric

acid. The experiments showed that 5.4% and 11.3% of

xylan and lignin, respectively were degraded during

pretreatment of sugarcane bagasse using glycerol which

might be the reason of high reducing sugars in this

technique (65). It was reported that simultaneous usage

of glycerine can be used for pretreatment of rice straw

5.16 and 4.88 (73). This phenomenon is due to presence

of weak acids in the water. Therefore, conventional

hydrothermal pretreatment techniques or microwave

pretreatment can be partially categorized as acid

pretreatment techniques (7).

The degradation of hemicellulose can be increased

when acid is used for pretreatment of lignocellulosic

biomasses. It is reported that up to 100% of

hemicellulose can be eliminated when acid technique is

used (74). Plant cell wall contains two important

compounds: hemicellulose and lignin. These compounds

have been linked by three main co-valent bonds for

formation of lignin–carbohydrate complexes (75).

Lignin–carbohydrate complexes are considered as

recalcitrant compounds in lignocellulosic biomass. The

degradation of hemicellulose causes rearrangement of

lignin molecular structure. In this case cellulose is

exposed by cellulase enzyme; therefore, the rate and

proportion of cellulose degradation will be enhanced (76).

Present of sulfur oxide shows some particular advantages

as hydroysis catalyst over sulfuric acid. Sulfur dioxide

can be added either with the steam or ahead of reactor. It

is better distribute sulfur dioxide through the biomass to

have a uniform reaction and decrease the cost. It was

reported that hemicellulose and cellulose hydrolysis in

many types of lignocellulosic biomasses such as corn

cobs, wheat straw, sugarcane bagasse, aspen poplar chips

and pine sawmill residues in present of sulfur dioxide

can be done at temperatures 150 ˚C and 190 ˚C,

respectively.

(7). The reduction of sugars in this way was 2 times

higher than the control. It was reported that temperature

was an effective parameters on pretreatment performance

at more than 160˚C while it is not effective under lower

than 100˚C. Three different solvents was applied by (66):

alkaline glycerol, aqueous glycerol, and water as solvent

for pretreatment of rice husk and corn straw. They

reported that using glycerol as the high boiling solvent

could partially remove lignin from rice husk while

alkaline glycerol was not able to remove lignin. They

also reported that using either glycerol or alkaline

glycerol as organic solvent is effective for removal of

lignin from corn straw (66). Excitingly, the maximum

yields of hydrolysis of polysaccharides to soluble

sugars were achieved for both rice husk and corn straw

pretreated using alkaline glycerol. Analysis of corn straw

structure showed that pretreatment cause a dramatic

change on its structure. These results illustrated that the

type of organic solvent should be selected based on

application of biomass (66).

3.7 Biological techniques

Biological pretreatment techniques are considered as

one of the important techniques because it uses natural

microorganisms or their enzymes (77-79). Biological

techniques have many advantages such as no obligation

to recycle the chemical compounds after pretreatment,

low downstream processing costs, no or minimum

inhibitor formation, simple operating conditions and

equipment and low energy consumption (80, 81).

Therefore, biological pretreatment is an inexpensive, safe,

and environmental friendly technique for pretreatment of

biomass. Value added products may be produced during

biological pretreatment of lignocellulosic biomasses

which make it an economic technique. Biological

pretreatment of lignocellulosic biomasses is able to

convert lignin into different simpler compounds which

can be utilized as the starting materials for production of

syringaldehyde, benzoic acid, cinnamic acid, vanillic

acid, vanillin, and phenolic acids (82). Also, biological

pretreatment of microalgae can produce carbohydrates,

.5 Acid technique

The degradation speed of lignocellulose pretreatment

such as liquid hot water, hydrothermal and un-catalyzed

steam explosion can be increased using catalysts (67, 68).

Different bases and acids have been applied for

lignocellulose pretreatment (69). Some acids including

phosphoric acid, acetic acid, and sulfuric acid have been

commonly utilized as catalysts (8). Additionally,

legnocellulosic biomasses at high-temperature processing

were able to release various acids such as acetic acid that

might be used as the catalyst for autohydrolysis (70).

Pretreatment of lignocellulosic biomasses at acidic

condition could convert hemicelluloses into soluble sugar;

therefore, the cellulose could be simply degraded due to

changing in the structure of biomass. Increasing of

temperature can enhance the speed of hemicelluloses

conversion in dilute acid pretreatment. Several methods

such as burning natural gases or microwave techniques

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 925-935

proteins, and, lipid derived compounds. These compound

can be applied to produce livestock feed, fine chemicals

for pharmaceuticals, and food supplements (83). The

combination of biological pretreatment and bio-refinery

can enhance the sustainability during generation of

biofuels. This combination showed an environmental

friendly and cost-effective way to produce biofuel (84).

Although the technique of biological pretreatment has

potential of implementation on large scale, it has not yet

been used for commercial purposes due to some reasons

such as low downstream yields, loss of carbohydrates,

and long pretreatment time (85). Several studies have

been carried out on biological pretreatment of

lignocellulosic biomass and microalgae which shows a

promising results compared to the results achieved from

using biomass without pretreatment (86, 87). Although

the number of studies on biological pretreatment of

lignocellulos is very higher than microalgae, utilization

of this technique is more effective for microalgae. Zabed

et al. reported that microalgal composition and structure

are more appropriate for biological pretreatment;

therefore, this technique is more effective for microalgae

and (a) required energy for milling (99, 100). There are

many kinds of milling processes including hammer

milling, disk milling, ball milling, and vibratory milling

used to increase enzymatic hydrolysis (101). The reports

shows that vibratory ball milling is a more effective

technique compared to ball milling technique to decrease

cellulose crystallinity of aspen and spruce chips (10).

Also, disk milling that is able to produce fibers is more

effectively increases hydrolysis of cellulose compared to

hammer milling which produces finer bundles (100).

3.9 Pyrolysis

Lignocellulosic biomass can be pretreated using

pyrolysis technique. Cellulose can be decomposed into

residual char and several gaseous products at temperature

more than 300 ̊C . it should be noted that the rate of

cellulose decomposition at temperatures less than 300C

is very low. It was reported that hydrolysis of

lignocellulosic biomass pretreated by pyrolysis technique

using a 1 N sulfuric acid can convert 80–85% of

cellulose into reducing sugars with more than 50%

glucose (102).

(88). Lignocellulos have high quantity of lignin in their

cell walls while microalgae have protein, lipid, and

starch, without any lignin (89). Sometimes lignocellulos

is pretreated for removal of lignin to enhance cellulose

digestibility (90). In addition, rupturing the cell walls and

the macromolecules hydrolysis can be the main reason

for pretreatment of microalgae (91, 92). The reasons for

pretreatment of lignocellulosic biomasses can be changed

depending on the types of biofuels and the biological

pretreatment method that is applied. For instance, biogas

generation using an anaerobic reactor goals to hydrolyze

3.10 Steam pretreatment

In steam pretreatment technique, Lignocellulosic

biomass can be pretreated with high pressure saturated

steam (0.7 to 4.8 MPa) at temperatures from 160 to

240 ̊C . Several reports showed that pretreatment using

steam is able to hydrolyze the hemicelluloses and modify

the lignin. It can also enhance the surface area and reduce

the degree of polymerization and the cellulose

crystallinity (103). Corn stover pretreatment using steam

with and without sulfur dioxide was studied by (74).

There are some reports to show that existence few

amount of xylanases can have a great effect on xylose

production. These reports demonstrated that the glucose

yield rose from 69% to 94% (9, 74). Similar results have

been also reported by other scientists (9, 104, 105).

Approximately, the overall yield of glucose and xylose

during pretreating corn stover with steam at 190 ˚C for 5

min along with sulfur dioxide were 90% of and 80%,

respectively (106). The extended delignification, with

increasing temperature, strongly affects the strength

properties (107). It is possible to produce ethanol from

lignocellulosic biomass by steam pretreatment,

enzymatic hydrolysis and fermentation. An important

factor to have a cost-effective production of ethanol is

the sugar yields, from both cellulose and hemicellulose

(74). One of the advantages of using steam pretreatment

is that it can rapidly increase the temperature without

excessive dilution of the resulting sugars (8). The rapid

increases of pressure aids in defibrillating the cellulose

bundles which improves the cellulose availability for

fermentation and enzymatic hydrolysis (108). Stem

pretreatment has two main steps: auto-hydrolysis and

depressurization. Within auto-hydrolysis step, high

temperature is used which can improve the acetic acid

formation from acetyl groups connected with

hemicellulose. This step leads to hemicellulose

hydrolysis. The acetic acid formed further catalyzes the

hydrolysis of the hemicelluloses. In depressurization step

the size of biomass particle are reduce which increase the

cellulose enzymatic accessibility. It was reported that

steam pretreatment is an efficient technique for corn

stover biomass (109). Steam pretreatment is able to

efficiently breakdown the lignocellulosic structure,

defibration, depolymerization of the lignin components,

the macromolecules without expecting

delignification process for lignocellulosic biomass (93,

4). There is no enough technical information for using

a separate

biological pretreatment of lignocellulosic biomass in

large scale; therefore, more studies should be done to

collect these technical information.

.8 Mechanical techniques

Several kind of techniques are introduced to reduce

the size of lignocellulosic biomass including milling,

grinding, shredding, and chipping for increasing its

digestibility (95). Mechanical pretreatment technique can

enhance the specific surface area and reduce the degree

of cellulose crystallinity and polymerization (96). The

size of materials in various mechanical techniques is

different. Chipping can produce materials with size of

between 10 and 30 mm. grinding and milling have a

better performance and they can produce materials with

size of between 10 and 30 mm. Two factors are

important in mechanical comminution: first, biomass

characteristics; second final particle size. These can

determine how much energy would be needed for the

above mentioned comminution. The needed energy for

ligniocellulosic biomasses such as hardwoods is higher

than agricultural wastes (97). Although some reports

shows that the biofuel production can be enhanced when

the lignocellulosic biomass was milled (98), it is not

cost-effective due to its high energy consumption on

large scale. Many researches have also investigated that

milling after chemical pretreatment can decrease (e) the

generation of fermentation inhibitors, (d) liquid to solid

ratio, (c) energy needed for mixing of pretreatment

slurries, (b) cost of separation of solid liquid because the

pretreated lignocellulosic biomass is simply separated,

Journal of Environmental Treatment Techniques

2020, Volume 8, Issue 2, Pages: 925-935

and hydrolysis of hemicellulosic fraction (110). The

main benefits of using steam pretreatment are: (a) this

technique no need to hazardous chemicals, (b) steam

pretreatment has a low negative environment impacts,

and (c) it shows a high energy efficiency (111). In the

other hand, steam pretreatment technique has two

disadvantages: incomplete disruption of lignin and

production of some toxic chemicals within the process. It

seems that among physical pretreatment techniques for

straw pretreatment, steam technique is the best chose

since it partly hydrolyzes hemicellulose and enhance the

enzymatic digestibility of cellulose remaining in biomass

residues (112).

resident time, the amount of pressure, water loading,

temperature, and ammonia loading (119). This technique

has many advantages including: (a) no toxic compounds

are produced during the process, enhanced enzyme

production (e.g., cellulase), and high surface area (40,

120). Additionally, this technique has an important

disadvantage. It is not able to remove high amount of

hemicelluloses that can increase the accessibility of

enzyme and the yield of final sugar (40).

Conclusion and future recommendation

Although several different techniques such as

microwaves, biological, alkaline, ionic liquid, organic

solvent, acid, mechanical, pyrolysis, steam, wet

oxidation, ammonia fiber explosion and liquid hot water

have been developed for the biomass pretreatment, many

obstacles are still needed to be overcome to use these

techniques for industrial application. The surface area is

the most important effective parameter on pretreatment

process. It was demonstrated that single pretreatment

technique is not able to obtain a very high performance

for the biofuel production. Therefore, the studies must be

focused on combined pretreatment techniques. Also,

study on the energy consumption of pretreatment

techniques and the feasibility of using such techniques is

another necessary topic that should be intensively

investigated. Certain corps can be efficiently pretreated

using some particular pretreatment techniques due to

their added advantage over others. Although lignin

should be eliminated using pretreatment techniques for

enhancement of biofuel production, recent studies show

that lignin may be used in construction industry, dust

particle controls, cleaning metallic surfaces, chemical

coatings, agro farmland improvements, and paint

industry.

.11 Liquid hot water

This technique applied hot water at high pressure to

maintain its liquid form for degradation enhancement of

the lignocellulosic matrix. Liquid hot water technique

uses at temperature between 160 ˚C and 240 ˚C. Also,

resident time in this technique is between few minutes

and an hour. It was reported that 88% to 98% of xylose

recovery can be obtained using liquid hot water without

needing acid or chemical catalyst which make it an

environmental friendly and cost-effective techniques for

pretreatment of lignocellulosic biomasses (8). The

disadvantage of this technique is high energy and water

consumption.

.12 Wet oxidation

Wet oxidation is

a technique in which the

lignocellulosic materials are treated with water and air or

oxygen at temperatures more than 120 ˚C (113). The

toxic furaldehydes and phenol aldehydes formation is

reduced when alkali and wet oxidation is combined with

each other (114). Since lignin and hemicellulose are

solubilized, baggase cellulose content increases within

wet oxidation technique (115). A main difficulty in the

fermentation of dilute acid hydrolyzates is the inability of

the fermentative microorganism to withstand inhibitory

compounds formed during pretreatment, and usually a

detoxification step is needed to improve hydrolyzate

fermentability (116). Similar results could also be

obtained for fermentation of the rice hulls and wheat

straw dilute acid hydrolyzates (117). The inhibitor

problem can reduce when alkaline peroxide pretreatment

was used for rice hulls. It was reported that combination

of wet oxidation and base readily oxidizes lignin for

wheat straw can facilitate the enzymatic hydrolysis of

Ethical issue

Authors are aware of, and comply with, best practice

in publication ethics specifically with regard to

authorship (avoidance of guest authorship), dual

submission, manipulation of figures, competing interests

and compliance with policies on research ethics. Authors

adhere to publication requirements that submitted work

is original and has not been published elsewhere in any

language.

Competing interests

The authors declare that there is no conflict of

interest that would prejudice the impartiality of this

scientific work.

polysaccharides

(114).

Furfural

and

hydroxymethylfurfural were not generated within the wet

oxidation technique. Carboxylic acids and dissolved

hemicellulose can be directly used as nutrient source by

fungal growth. It has been demonstrated that rice hull

hemicellulose is able to be hydrolyzed using a single

preparation of xylanase (viscostar) after a pretreatment

alkaline peroxide (118).

Authors’ contribution

All authors of this study have a complete contribution for

data collection, data analyses and manuscript writing.

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