Waste Generate In Malaysia Biology Essay

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Every people are generating wastes every day, either in low or high quantity. Most highly generated waste is the municipal solid wastes which comes from the residential area where peoples are stayed there. These generated waste is always ended up with buried them at the landfill site which is not environmentally method to be used. Nowadays, wastes have been used extensively in research works where the researcher tries to make use of the wastes to produce high-value added product, such as the biofertilizer, biofuel and others.

According to EPA (2009), waste can be defined under the Environment Protection Act 1993, is any discarded, rejected, abandoned, unwanted or surplus matter, whether or not intended for sale or for recycling, reprocessing, recovery or purification by a separate operation from that which produced the matter. The other definition of waste from EPA (2009) is anything declared by regulation (after consultation under section 5A of EPA regulation and rules) or by an environment protection policy to be waste,

The concept of waste is actually anything or something that is neither a liquid nor a hazardous type of waste that want to be disposed after it is broken or cannot being used anymore in the future. The things need to dispose includes packaging, green waste, dirt/rubble, waste wood and other materials. Tsai and Kuo (2010) in their journal states that wastes can be differentiate into different categories which can be seen in Figure 2.1.

C:\Users\Zul\Desktop\general waste.jpg

Figure 2.1: Categories of (a) waste and (b) general waste pursuant to the Waste Disposal Act in Taiwan (Source: Tsai and Kuo, 2010).

2.1.1 Types of waste produced

There are many types of waste been produced that contribute to the highest amount of waste being generated. Examples of waste being produced are municipal solid waste industrial waste (e.g. manufacturing), hazardous waste, biodegradable municipal waste, agricultural waste and much more. Each of these wastes that been generated have different composition within it. So, the handling of those wastes to dispose is different from each other.

For example, most of composition of municipal solid waste (MSW) which is including household and commercial wastes is primarily the organic materials especially carbonaceous and nitrogenous matters. To treat those kind of wastes, suitable methods should be applied to make sure that the disposed of wastes in better ways. Another example is the hazardous wastes which mainly consist of toxic and hazardous materials such as mercury, boric acid and others that certainly in unstable condition in the natural environment.

2.1.2 Waste generate in Malaysia

In order to determine what types of waste is generated by peoples in Malaysia, the research have been conducted in area of Kuala Lumpur. In 2010, the population of Kuala Lumpur city area has reached 1.33 million people by the conducted research by Malaysia Statistical Board in year 2009. With the increment of population growth rate of 6.1 percent each year, it have been estimate that the waste generated around Kuala Lumpur area is about 2,000 tonnes per day, where each person generates the waste on average of 1.2 kilograms a day.

Table 2.1 indicates the composition of municipal solid wastes from various studies and sites in Malaysia and Figure 2.2 shows to us the composition of municipal solid waste that been generated from Kuala Lumpur area.

Figure 2.2: Composition of municipal solid waste from Kuala Lumpur (Source: Iwan et al., 2012)

From Figure 2.2, we can conclude that most of generated waste is mainly food wastes which are organic wastes that can be utilized for another uses. Second most highly produced waste is the plastic which is difficult to be degrading in nature. If the plastic is buried inside the landfill, it is sure that the plastic will not degrade completely even though buried for long time ago.

Table 2.1: Municipal solid waste compositions from various studies and sites.

Component

20011

20012

20023

20033

20044

20055

20056

20077

20108

Food waste & organic

68.4

32

56.3

37.4

49.3

45

47.5

42

43.5

Mix plastic

11.8

16

13.1

18.9

9.7

24

-

24.7

25.2

Mix paper

6.3

29.5

8.2

16.4

17.1

7

18.5

12.9

22.7

Textiles

1.5

3.4

1.3

3.4

-

-

2.13

2.5

0.9

Rubber & leather

0.5

2

0.4

1.3

-

-

-

2.5

-

Wood

0.7

7

1.8

3.7

-

-

4.41

5.7

-

Yard wastes

4.6

-

6.9

3.2

-

-

2.72

-

-

Ferrous

2.7

3.7

2.1

2.7

2

6

-

5.3

2.1

Glass

1.4

5.5

1.5

2.6

3.7

3

-

1.8

2.6

Pampers

-

-

-

2.1

-

-

3.81

-

-

Others

2.1

1.9

8.4

5.3

18.2

15

21.93

2.6

1.8

Total

100

100

100

100

100

100

100

100

100

Sources: (1) Hassan et al. (2001); (2) Wan Ramle Wan A. Kadir (2001); (3) Nazeri A.R. (2000); (3) S. Kathirvale et al., (2003); (4) JICA (2004); (5) Ministry of Housing and Local government's website (2005); (6) Bukit Tagar Sanitary Landfill (2005); (7) Muhammad Abu Eusuf et al., (2007); (8) Siti Rohana M. Yatim (2010).

2.1.3 Landscape waste

Landscape waste or sometimes called as yard waste can be defined as the accumulation of biodegradable waste normally plant materials including leaves, grass clippings, pruning, branches, brush, garden material, weeds, tree limbs, and other as the result of care of the landscape area. The demolition debris such as concrete, wallboard, lumber, or roofing materials that may introduce during renovation of landscape area does not included as the landscape wastes.

Generally, landscape wastes is considered as a part of biomass by-product which produces from landscape works whereby it mostly cover a large area of organic matter that present in environment. Examples of landscape waste that easily obtained in nature are fallen leaves, grass clippings, herbaceous pruning, brush and chipped woody matter.

Currently, l8% of total solid waste generated nationally comprises of landscape waste (Surender and Reddy, 2007). Recently, the landscaping wastes has been used for raw material in producing such good product such as bio-fertilizer from composting processes which suitable be used in natural environment. Therefore, the proper way of treating this waste should be construct carefully to make sure the waste is not become the burden to the private authority.

2.1.4 Municipal sewage sludge

Generally, municipal sewage sludge (MSS) as in Figure 2.3 can be defined as the wastes generated during treatment of domestic sewage before being released back into the nature. MSS usually produced in the form of solid, slurry, or liquid residue that been dumped from domestic and industrial sector.

MSS is produced as the by-product of primary, secondary or advanced wastewater treatment. During these treatments, the wastewater is treated depend on what types of treatment that need to be done, separating those sludge and wastewater by sedimentation process. MSS usually consists of 90 to 99% of water content and accumulation of settleable solids from those wastewater treatment processes. To make use of this sludge, it needs to be treated first by the dewatering process for other purpose such as land application or else.

http://www.prwatch.org/files/images/biosolids1.jpg

Figure 2.3: Municipal sewage sludge (Source: http://www.prwatch.org/news/2011/05/10700/

dont-be-duped-sewage-sludge-industrys-compost).

Commonly, peoples always think that MSS cannot be utilized as they are already wasted. To throw away MSS, there are several methods that people always done such as carried out aerobic or anaerobic digestion process, burn in the incinerator and deep buried in landfill. Beside from that particular activity, some people aware about this problem and used MSS as the substrate in carry out composting process. Composition of MSS can be benefit or harmful to environment, depend on excessive uses or MSS under unstabilized condition to be applied.

Natural phenomenon that causes MSS to be unstabilized is due to the presence of pathogenic organisms, excessive heavy metal content and other trace constituent within it. Particular handling of MSS containing pathogenic organisms should be done in order to avoid and reduce the risks to human and nature. Before they can use MSS for compost, these wastes need to be in stabilized form where they need to undergoes digestion processes over certain period of time. This action is needed in order to reduce the amount of organic matter as well as pathogenic organisms that could be present there.

Most composition that present in MSS produced from wastewater treatment is mainly organic component which beneficial to the environment. Nutrient likes phosphorus, nitrogen, magnesium, calcium, sulphur and other essential microelements is very important nutrients can be obtained from MSS especially for plants and soil fauna to live. The plants need nitrogen and phosphorus as the source for them to grow well in nature. The soils can be very good in soil amendment properties for plant growth due to the improvement of physical and chemical properties of soils from MSS applications.

Previous research that been conducted by Abdullah et al. (1985) using MSS as fertilizer for sweet maize. The result obtained shows that the total yield of sweet maize production is higher compared to the control ones. Abdullah et al. (1985) also conducted the research on utilization of MSS as potting media for horticulture crops such as chrysanthemum and jasmine shows that with the correct ratio, the growth of those horticulture crops can be same as using chemical fertilizer.

In 2001, Hassan A.H.H. conducted experiment on composting using selected organic sludge such as food factory sludge, palm oil mill effluent (POME) sludge, landfill leachate and MSS. The experiment is conducted by using 75-L rotary drum modified from cement mixer as the composter reactor to mixing and aerates the compost continuously. As the result, it prove that by using MSS compared to other organic sludge, the decomposition rate of compost can be achieve about 60% and can be complete stabilized in 40 days only (Hassan A.H.H., 2001).

For Malaysia as the developing country, the usage of MSS is not wide as for develop country like American, Europe and others. To obtain MSS in Malaysia, Indah Water Konsortium (IWK) Sdn. Bhd. is the public authority that responsible to collect it first from the treatment of wastewater. There are two institution that currently used MSS as the substrate for their research work which is Universiti Putra Malaysia (UPM) and Universiti Teknologi Malaysia (UTM). There are several projects that have been done by IWK to utilize MSS to make use of waste in terms of beneficial to nature. Examples IWK projects with collaborations of UPM and UTM (IWK website, 2012) that been done are:-

MSS as fertilizer for various crops;

MSS as soil amendment;

Co-composting of MSS & municipal solid waste;

Application of MSS in forest rehabilitation & regeneration;

Dewatering of MSS by two stage integrated technique;

Drying of sludge for building material application.

By these collaborations research work, its indicate that the research conducted in Malaysia is finally concern about using those waste in order to produce good and high quality product that can be used back into the environment.

2.2 Composting

Country especially the develop ones tend to produce waste as much as they want to generate economy. With this kind of attitude, there is the possibility where one day the country will face the problems in handling the wastes in such amount. Nowadays, developing country applied concept of composting in order to reduce the amount of wastes have been generate every day for future generations.

Zheng et al. (2007) states composting is the way of environmental-friendly waste management method for tackling the disposal problem of organic wastes generated such as sewage sludges and municipal solid waste. During the composting process being carried out, the microorganism that present will be destroyed and the organic matter will be stabilized with the supplement of appropriate nutrient, carbon source and moisture content as well as the air flow. Figure 2.4 shows the illustration of composting process from raw materials, inputs and outputs of the composting.

E:\UPM\MASTER\PROJECT\THESIS\pic\illustration compost process.JPG

Figure 2.4: The diagram illustration of composting process.

The end product which is stabilized compost can be used on soil with high content values to improve the soil fertility and also provide to plant the good nutrient. The compost is turned into a dark brown, powdery material called humus. The processes occurring in a compost pile are similar to those that break down organic matter in soil. Stabilized compost actually is good because it can reduce the use of chemical fertilizer in plantation, conserve the natural resources and improve the production of plant in healthy condition.

Applications of composting processes are it provides a partial solution to an issue of great concern in many communities whereby the landfills are filling up and other waste disposal options are becoming ever harder to find due to the generation of waste in larger quantities. With composting also, it provides a way not only of reducing the amount of waste that needs to be disposed of, but also of converting it into a product that is useful for gardening, landscaping, or house plants.

2.2.1 Composting process

The composting process can be carried out in many different ways. Each part of composting processes contributes the important role to make sure that the effectiveness of the processes leads to production of good compost. Seed culture which provides the effective microorganisms (EM) plays importance role for compost process. The microbes will use carbohydrates as well as nitrogen sources from the compost material to be used as their energy source, so they can reproduce more colonies and for their activity itself. To produce good quality of compost product, oxygen intake and percentages of moisture content should be controlled properly. According to Crawford (1986) research, the by-product of composting such as water, carbon dioxide and some energy is generated during the process. The energy that been released is used for microbial growth and their movement, while the remainder is given off as heat energy. The composting process is drawn and being illustrated as in Figure 2.5.

Figure 2.5: Inputs and outputs of composting (Source: Idris et al., 2010)

Digestion or degradation of complex organic matters into simpler components can be done by two modes of decomposition either aerobic or anaerobic degradation. Usually, the most significant method of degradation being used is the aerobic ones. Fungi, actinomycetes, bacteria and molds plays the dominant roles whereby EM will utilize organic materials by rapid decomposition such as carbonaceous component, nitrogen, phosphorus and other required nutrient for them to build cell protoplasm (Gotaas, 1956). New synthesized product is being produced due to the activity of EM. Compared to aerobic mode, anaerobic decomposition is characterized by low temperatures, unless heat is applied from an outside environment. Normally, anaerobic process have the problem where the degradation of organic matters is much slower compared to aerobic composting and also there is the presence of bad smell immediate products such as strong ammonia smell as well as rotten egg.

2.2.2 Types of composting

B

A Types of composting can be divided into two categories; (a) non-reactor and (b) reactor systems (Idris et al., 2010). Non-reactor system means that the composting process is been carried out outside a reactor. Fine examples of non-reactor systems are static piles and windrows (Figure 2.6). C:\Users\Zul\Desktop\vertical reactor.JPGC:\Users\Zul\Desktop\Aerated static pile composting.jpg

Figure 2.6: Types of composting processes;-

(A) Non-reactor system - static piles (Source: EPA, 1995),

(B) Reactor systems (Source: http://compost.css.cornell.edu/MSWFactSheets/msw.fs2.html).

In static piles system, the wastes are placed into heaps and to enhance the composting process, the perforated pipeline for air to be blown into the heap should be done to provide good aeration to the composting materials. There is no turning or agitation step should be provided once the static piles are formed. Once the piles have been formed and the air supply and its distribution for compost is sufficient and good enough, the decomposition of those organic materials can be completed in three to five weeks only. Different from static pile, the windrows system is used for the larger quantities of waste and can be considered as a low-cost method of composting due to their simplicity in design. The manual mechanically aeration should be applied on the compost to make sure that the decomposition of materials in composting process is being carried out completely.

For reactor system, it allows the decomposition of the organic wastes to be done in controlled environment within the vessel itself. The process of reactor system or enclosed system is much faster compared to the non-reactor system where the feeding of organic matters can be made depend on the requirement of the system itself. The efficiency of this system is also maintained by injection of air or insulation of the vessel. The comparison of both systems is shown in Table 2.2.

Table 2.2: Comparison of non-reactor and reactor system.

Type of composter

Non-reactor system

Reactor system

Turned windrow

Aerated pile

Forced aeration with agitation

Forced aeration without agitation

Capital cost

Low

Low for small system; high for larger system

Very high

High

Operating cost

Low

High

Low

Low

Control of air supply

Restricted

Complete

Complete

Complete

Requirement for subsequent drying

Not required

Not required (self-drying)

Not required

Small drying required

Sensitivity to climate change

Sensitive

No

No

No

Land requirement

Very high

High

Very low

Low

(Source: Idris et al., 2010).

2.2.3 Parameters

To make sure that the composting process run effectively, there are several environmental parameters should be properly controlled such as C/N ratio, particle size, moisture content, aeration, temperature, and pH. Table 2.3 is summarized of the optimum conditions for rapid composting.

Table 2.3: Ranges of optimum condition for rapid composting.

Parameters

Value

References

C/N ratio of feed

25-30:1

Biddlestone et al., 1987

Particle size

10-50 mm

Obeng and Wright, 1987

Moisture content

50-60 %

Biddlestone et al., 1987

Aeration

0.6-2.0 m3 air/day/kg

Hassan et al., 2001

Temperature

50-70 oC

Biddlestone et al., 1987

Agitation

Every 5-10 minutes

Hassan et al., 2001

pH control

pH 6.0-8.0

Verdonck, 1998

(Source: Idris et al., 2010)

2.2.3.1 Temperature

Temperature is one of factor to be control in the composting processes. Usually, temperature is selected is on ranges of 50-70oC which is in thermophilic temperature with 60oC is the most suitable ones according to Haug research (1980). This range is favourable because it will increase the efficiency of compost process as well destruction of pathogenic organisms and weed seeds. High temperature (70-75oC) at prolonged will destruct some of beneficial microbial action and nitrogen loss in increasing trends due to ammonia being vapourized.

2.2.3.2 C/N ratio

Carbon and nitrogen sources are important for microorganism for their living. Carbon source that been obtained most comes from organic wastes being used as a source of energy by metabolic oxidation of microorganism and also in synthesis of cell wall as well as other cellular structure component. Meanwhile, nitrogen source is used as major constituent of the proteins, nucleic acids, amino acids, enzymes and co-enzymes necessary for cell growth and function.

Initial C/N ratio also will affect the time to carry out composting (Crawford, 1986). Different microorganism required different amount of C/N ratio which can preferentially favour the population of microbial. According to Griffin (1985), bacteria colony can utilize materials with narrow C/N ratio of 10-20:1, while fungus can use materials with wide C/N ratio of 150-200:1. Expected ratio of carbon should be more than the nitrogen ratio because at lower ratios, nitrogen will be supplied in excess and will be lost as ammonia gas, causing undesirable odours for compost. Higher ratios mean that there is not sufficient nitrogen for optimal growth of the microbial populations, so the compost will remain relatively cool and degradation will proceed at a slow rate.

2.2.3.3 Moisture content

Previous studies that been conducted by U.S. composting council. (1997) the moisture content level is between 40-60% by weight to be maintained throughout the aerobic composting period. Most of microorganisms need 40-60% of moisture to survive in such environment, depending on the nature of materials that to be composted. If the moisture level is too much e.g. exceeding 80% moisture level, it will create anaerobic conditions for aerobic microorganism which makes these microbes cannot degrade those materials and killing them. Even if the amount of moisture is below 40%, the degradation of organic matters by EM still occurs in aerobic conditions but in slow pace of time. Usually, the moisture level is controlled in composting process using of water sprinkler.

2.2.3.4 Microorganism

The uses of consortia of microorganism seem much better in the case of composting, rather than uses of pure culture of organisms. The microorganism that comes from garbage as well as sewage sludge normally contain many types of bacteria, actinomycetes and fungi which may working together effectively in degrade those organic matter that present in the compost. The required microorganism especially thermophilic bacteria play major roles in break down readily organic waste as well decomposing of protein while actinomycetes and fungi usually decompose cellulose and lignin components (Hassan et al., 2001).

2.2.3.4 Mixing and aeration

Mixing and aeration are very important steps in composting. Mixing step provides the sufficient oxygen transfer rate to the compost for aerobic activity for speed up the composting processes. The mixing rate is being determine by the types of composting being run as the machines being used for turning or mixing purposes. The aeration system is being used when it involve a large-scale composting including windrow pile. Hassan et al. (2001) suggested that the mixing with constant slow speed or intermittent mixing every 5-10 minutes or the combination of forced air and less frequent mixing is best method in order to produce good quality of compost product.

Aeration is step in which can help to reduce the moisture level content in composting materials. By aeration process, the oxygen transfer rate of compost is at higher level where oxygen just not necessary for aerobic metabolism, respiration of microbes and also help in oxidizing of various organic molecules that present in the moisture (Idris et al., 2010). There are several factor that contribute to consumption of oxygen in composting mass such as; (a) the state process, (b) temperature, (c) degree of agitation of the mass, (d) composition of composting mass, (e) particle size of the mass and others.

2.2.3.5 pH

A pH between 5.5 and 8.5 is optimal for compost microorganisms to carry out their function to degrade the wastes. Verdonck (1998) studies shows that the optimum pH levels are 6.0-8.0 for composting processes and 4.0-7.0 for the end stabilized product. The aerobic bacteria survive well at a pH range of 6.0-9.0. Actinomycetes grow in ranges pH of 5.5-9.5, whereas fungi develop within much wider pH ranges from 3.0 to 9.5.

As bacteria and fungi digest organic matter, they release organic acids during initial stages of decomposition. The resulting drop in pH to acidic conditions is favourable for the growth of fungi and the breakdown of lignin and cellulose. Usually the organic acids become further broken down during the composting process. If the system becomes anaerobic, however, acid accumulation can lower the pH to 4.5, severely limiting microbial activity. In such cases, aeration usually is sufficient to return the compost pH to acceptable ranges. As composting proceeds, the organic acids become neutralized, and mature compost generally has a pH between 6 and 8.

2.3 Lignocellulosic biomass

Lignocellulosic biomass can be catagorised as the plant biomass which comprised of cellulose, hemicellulose and lignin as the major component of cell wall. Other components which is involved with formation of cell wall is small amount of pectin, protein, ash and extractive such as soluble non-structural sugars, nitrogenous material, chlorophyll as well as waxes (Fengel and Wegener, 1984; Jorgensen et al., 2007). Table 2.4 shows the composition of lignocellulosic material inside plant based on the study that has been carried out by Sjostrom (1993).

Table 2.4: Composition of plants.

Composition

Range (%)

Cellulose

40

Hemicellulose

20±30

Lignin

20±30

(Source: Sjöström, 1993)

Basically, the general structure of plant and lignocellulosic biomass is the cell wall which mainly builds up from the primary building block which is lignocellulose component. The cell wall structure is the most difficult to be degrade due to the build-up component included cellulose, hemicellulose as well as lignin which are related to each other to make the cell wall difficult to be degrade by both biological and chemical treatment. Therefore, it is essential to carry out the degradation of lignocellulosic components for the operation of global carbon cycle (Figure 2.7) that been involved naturally or being engineered.

Figure 2.7: Global carbon cycle (Brown, 1985; Colberg, 1988).

Figure 2.8 shows to us the schematic structure of lignocellulosic materials. From the figure provided, we can know that lignin act as the wall covering for cellulose and hemicellulose component due to the not-defined form of hemicellulose and difficult component to access by cellulose.

Figure 2.8: Schematic diagram of lignocellulosic materials (Source: Conde-Mejia et al., 2011).

2.3.1 Cellulose

Cellulose is one of the structural components of the primary cell wall of green plants. Some species of bacteria also produce and secrete the cellulose to form biofilms. Application of cellulose nowadays is being used in many prospects such for industrial as well as renewable energy sources for the future. For industrial use, the wood pulp and cotton produced tonnes of cellulose mainly from its industry. For the paperboard and paper making, cellulose is mainly being used from a smaller extent to produce other good product such as cellophane and rayon. Renewable energy as alternative fuel source can be obtained from the studies that been carried out to converting the cellulose into biofuel such as cellulosic ethanol and others. Normally, cellulose can be presented in form of lignocelluloses or partly purified in the form of pure cellulose (e.g. cotton), or mixed with other materials (e.g. citrus wastes) (Talebnia et al., 2008).

2.3.1.1 Structure of cellulose

Generally, cellulose is an organic compound with the molecular formula (C6H10O5)n, a polymer consisting of a linear polysaccharides chain of several hundred to over ten thousand of β(1→4) linked D-glucose units. The bonding between the cellobiose units is joining together by hydrogen bonds. The cellulose structure has crystalline parts and amorphous ones in the polymeric structure and in addition existing of several types of surface irregularities (Cowling, 1975; Fan et al., 1980). With the presence of this heterogeneity, it makes the fibres capable of swelling when partially hydrated, and then the micro-pores and cavities become sufficiently large enough to allow penetration of larger molecules to be occurred including enzymes. Figure 2.9 shows to us the structure of cellulose and the cellobiose subunit.

Figure 2.9: Chemical structure of cellulose (Source: Kumar et al., 2009)

According to Ha et al. (1998), the 'elementary and microfibrils' are formed when the cellulose chains are strengthened and packed together by inter and intra-chain hydrogen bonds that been formed. The adjacent sheets of cellulose chain that overlie to each other are held by weak Van-der Waals forces. These fibrils then is being packed together by hemicelluloses, different polymers of sugars as well as pectin and covered by lignin. The bundle of microfibril will form the macrofibril. With this characteristic, it makes very difficult to degrade those cellulose by using physical or chemical treatments.

2.3.2 Hemicellulose

Hemicelluloses are generally found in association with cellulose in the secondary walls of plants, but they are also present in the primary walls. The main features that differentiates between those two structures is that hemicellulose has a lower molecular weight compared than cellulose, and branches with short lateral chains consisting of different sugars, which are easy hydrolysable polymers (Fengel and Wegener, 1984).

2.3.2.1 Structure of hemicellulose

As a part of plant cell walls which is about 20-30%, hemicellulose can be known as a complex, branched and heterogeneous polymeric network, based on pentoses such as xylose, rhamnose and arabinose, hexoses such as glucose, mannose and galactose, and sugar acids. The structure of hemicellulose is either in form of a homopolymer or a heteropolymer with the linkage by short branches, β-(1,4)-glycosidic bonds and occasionally β-(1,3)-glycosidic bonds (Kumar et al., 2009; Kuhad et al., 1997). Figure 2.10 shows to us the structure of repeating unit of hemicellulose.

C:\Users\Zul\Desktop\hemicellulose.JPG

Figure 2.10: Structure of hemicellulose (Source: www.bio.miami.edu/dana/226/226F09_3. html)

Compared to cellulose which has 5,000-10,000 repeating units, hemicellulose just consists of 150 repeating unit only, this shows hemicellulose has low molecular weight compared to cellulose. Other features that can display both differences is hemicellulose have a random, amorphous and branch structure with little resistance to hydrolysis (Sjöström, 1993; Morohoshi, 1991; Ademark et al., 1998; Mod et al., 1981; O'Dwyer, 1934). The study that been conducted by Laureano-Perez et al. (2005) proven that hemicellulose acts as a connection between the lignin and the cellulose fibres and gives the whole cellulose-hemicellulose-lignin network more rigidity for the plant cell wall.

2.3.3 Lignin

Lignin can be considered as one of the most important component for plant cell walls building. Lignin is the component of cell wall which is an organic substance that binding the cells, fibres and vessels in plants. According to studies that been conducted by Argyropoulos and Menachem (1997), this component act as an integral cell wall constituent that provides the plant strength and resistance to microbial degradation as well as chemical degradation. Lignin also is the most abundant carbon source biomass that been produced after cellulose component. Nowadays, lignin has been used in the making of biofuel as it can be categorized as the renewable biomass source that easily to be collected. Therefore, more studies should be conduct in order to prove that lignin can be degraded by the biological approach in the future

2.3.3.1 Structure of lignin

The structure of lignin is a very complex molecule constructed of phenylpropane units linked in a three dimensional structure which is particularly difficult to biodegrade. Like cellulose and hemicellulose, lignin also is made from carbon, oxygen, and hydrogen and being classified as phenolics. However, these elements are arranged differently so that they are not classified as carbohydrates. There are chemical bonds that joining together between lignin and hemicellulose and even cellulose (Taherzadeh, 1999; Palmqvist and Hahn-Hägerdal, 2000). With this structure, it can be considered that lignin is the most recalcitrant component of the plant cell wall. Higher the proportion of lignin within the cell wall makes the resistance to chemical and enzymatic degradation of lignin is much more difficult. Figure 2.11 shows the structure of lignin that present in the plant cell walls.

Figure 2.11: Chemical structure of lignin (Source: Morandim-Giannetti et al., 2012).

2.4 Ligninolytic microorganism

Nowadays, the biological approach using enzymatic degradation is more preferable in order to solve the problem of degrading those biomass which contributed to the mass of waste produced especially lignin. Common microorganisms that been chosen for this purpose is litter decomposing fungi and Phanerochaete chrysosporium or its common name is white rot fungus (Hatakka, 2001). There also other microorganism that doing the degradation of lignin such as ascomycetes, mitosporic, brown rotting and mycorrhizal fungi and some bacteria (Daniel and Nilsson, 1998; Hatakka, 2001). Kirk and Farrell (1987) proved that lignin is decomposed considerably under the aerobic condition and lignin losses are negligible when the degradation is carried out under anaerobic environments.

2.4.1 Bacteria

The bacteria that been used for ligninolytic degradation is not well studied before. Therefore, the potential for bacterial ligninolytic in degradation of lignin is still largely unexplored and many novel ligninolytic enzymes will be discovered by other scientists. Table 2.5 below shows several examples of bacteria that been found for the lignin to degrade.

Table 2.5: Examples of ligninolytic bacteria.

Species

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