1 Sources of Material

1.1 Industrial Wood Cutting Waste

There are many sources of biomass particles and sawdust here that could be obtained. It may come from wood processing facilities to make solid wood planks and poles after the cutting process. It was estimated that for every 1000 board feet of lumber produced, 1 tonne of by-products would be created. Woody biomass in the form of sawdust, shavings, slabs and edgings is gathered in sawmills comprising 75% wood and 25% bark portion [1]. Some names are wood fibre, wood flakes, wood chips, wood powder and wood swiths, as shown in Fig. 1.

Fig. 1
6 photographs. These are wooden logs, wood wool, wood chips, firewood wood chips, wood sawdust, and wood shavings.

Different forms of woody biomass [1]

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1.2 Manufactured Particles

Biomass sawdust can be classified into wood and non-wood biomass material. Biomass particles may be obtained by intentionally reducing the biomass size for specific usage. Wood and non-wood biomass from the plant parts, including logs, branches, tree stumps or stems, were purposely chipped or ground into a smaller size. Certain types of trees such as the Kenaf, Jute, Flax and Hemp are planted for their fibres, called fibre crops. These plants are considered non-woody biomass materials.

Size reduction of biomass could be made by chopping, chipping, grinding, and milling to perform solid disintegration. After chipping, the grinding process will break the wood into smaller particles through mechanical stress: impact, compression, shear and attrition. Intercellular and intracellular failure will loosen the fibre bonding, making smaller wood particles. Different grinding mills can be utilised, including; impact mills, roller mills, ball media mills, air jets mills and shearing attrition mills [2].

2 Energy Production

The most basic usage of sawdust is for energy production. Using wood-based materials for energy production has a tremendous environmental advantage. Energy generation using woody materials promotes the carbon cycle. When woody material is burned, it releases energy and carbon dioxide. The carbon dioxide from the environment is deposited back into plants during photosynthesis. The tree will be cut down and again generates carbon dioxide before being absorbed by other trees. As new trees grow, the cycle repeats over and over; that is the carbon cycle [3]. It differs from petroleum fuel burning, where carbon release happens one way without recapturing. Energy generation from wood sawdust can be done through 3 main categories, which are:

  1. i.

    Direct burning

    Energy generation by directly burning forest biomass is a part of the global carbon cycle. It is considered carbon neutral because the same amount of CO2 is released and captured in tree burning and growth [4]. Sawdust may be burned in its original powder form or compressed into pellets and briquettes for easy transportation and handling [5].

  2. ii.

    Heat treatment

    Heat treatment was done to wood sawdust to turn it into charcoal and activated carbon. Pyrolysis involves the thermal degradation of biomass by heat in the absence or limitation of oxygen. Charcoal (solid), bio-oil (liquid) and fuel (gas) products are the output of this process [6]. The solid product, char or charcoal, is commonly compacted into pellets and briquettes for energy storage. Pyrolysis was usually done by heating the sawdust between 300 and 700 °C [7]. Heat treatment at a temperature above 700 °C is called the activation process, producing activated carbon. Considering the high cost of making activated carbon, pyrolysis temperature is enough for energy storage char production.

  3. iii.

    Bioethanol production

    For bioethanol production, three significant steps should be conducted to the sawdust:

    1. a.

      Raw material pre-treatment

    2. b.

      Hydrolysis of pre-treated raw material

    3. c.

      Fermentation of hydrolysed material [8].

Sawdust is a lignocellulosic material which contains lignin, cellulose and hemicellulose. Pre-treatment of sawdust is needed to delignify the material. Delignification is essential to weaken the bond between cellulose and hemicellulose. It is a crucial step to ensure the success of the hydrolysis process. Pre-treatment techniques can be grouped into four categories, including acid pre-treatment (dilute acid, steam explosion, organosolv), neutral pre-treatment (liquid hot water), alkaline pre-treatment (sodium hydroxide, ammonia fibre expansion, soaking in aqueous ammonia) and ozonolysis [9]. Hydrolysis of cellulose and hemicellulose will produce fermentable sugars, including glucose, xylose, arabinose, galactose and mannose. Lastly, fermentation of reducing sugars will produce bioethanol [10].

3 Composite

Sawdust is a very suitable material for composite. Because of its tiny particle size, it can be mixed uniformly with polymer and adhesives. Different kinds of wood composite, such as particleboard, fibreboard and wood plastic composite, can be produced using sawdust as the main component.

Particleboard and fibreboard are made from mixtures of wood particles and fibres with adhesive or binder, respectively. Particleboard and fibreboard save a lot of wood waste, turning them into valuable, solid wood alternatives. Particleboard is prepared by mixing sawdust with 6–12% loading of thermosetting adhesives such as urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF), phenolic resins (PF and TF), and isocyanates (pMDI). Larger particles are commonly placed at the core, while smaller particles are placed at the surface. Mixed resin-sawdust are flat-pressed at a high temperature between 100 and 180 °C for the resin to cure, forming a flat panel. The density of commercial particleboard is commonly set at 650–700 kg/m3.

Fibreboard utilises wood fibre which is produced with the pulping process. Thermochemical pulping is usually employed to separate wood particles into individual fibres. Separated fibres were mixed with 2–3% of thermosetting binders and flat-pressed at a high temperature to cure. Fibreboards are categorised according to their density which is Medium Density Fibreboard, MDF (750–800 kg/m3) and Hardboard (900–1100 kg/m3) [11].

Wood plastic composite is the combination of wood and plastic or polymer. The thermosetting or thermoplastic polymer can be used as the matrix, the material that encapsulates the wood particles and holds them together. Thermosetting polymers are commonly available in liquid form. A polymer such as epoxy, unsaturated polyester and polyurethane can be directly mixed with sawdust using a simple mixer. Mixtures were laid into the mould, degassed using a vacuum bag and left to harden. An example of thermosetting wood plastic composite is the sisal fibre, and wood flour composites were made using unsaturated polyester thermosets by Marcovich et al. [12].

To produce wood plastic composite using thermoplastic as the matrix, the processes that might be involved include extrusion, injection moulding, and compression moulding or thermoforming (pressing) [13]. The plastic and wood were heated and blended when the thermoplastic melted. The sawdust-melted polymer mixture is then shaped before cooled down to harden. Wood plastic composite has many advantages, especially in automotive industries. The presence of sawdust in plastic composite increases the mechanical strength and reduces the material weight of automotive parts. Therefore it causes a reduction in vehicle fuel consumption, lowers production cost, increases passenger safety and improves the vehicle's interior parts’ biodegradability [14]. Some wood composites made using sawdust are tabulated in Table 1.

Table 1 Wood composite based on sawdust
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4 Animal Feed

Sawdust is originally a non-edible substance for animals. Sawdust for animal feed is possible with pre-treatments required to make the produced animal feed pellet digestible. An example of research on sawdust as fish feed has been done by Sharma et al. [25]. A blend of protein-rich yeast Candida utilis, enzymatically hydrolysed sulphite-pulped spruce wood, and enzymatically hydrolysed brown seaweed was prepared. The results of the salmon feeding trial showed that the wood could replace parts of a conventional fishmeal diet without harmful effects.

The effect of wood kraft pulp feed on cow’s digestibility, ruminal characteristics, and milk production performance has been done by Nishimura et al. [26]. The use of kraft wood pulp feed for lactating dairy cows made the same rumen fermentation characteristics as those in cows given a large amount of roughage. The milk productivity was also unchanged when using the new feed type.

Rabbit feed using sodium hydroxide-treated sawdust has been done by Omole and Onwudike [27]. Food consumption of food containing sawdust treated with 6% solutions of sodium hydroxide is higher than consumption of sawdust treated with 0–3% sodium hydroxide. Rabbits given sawdust treated with 4 or 5% sodium hydroxide showed the best weight gain and efficiency of feed utilisation than that given sawdust treated with 0 or 1% sodium hydroxide solutions.

A minimal number of research has been done on using sawdust as animal feed. Digestibility of the sawdust becomes the main issue, while pre-treatment of the sawdust makes the process uneconomical and costly to be done commercially.

5 Adsorbent

Sawdust can be used as an adsorbent to remove contamination from water. The unmodified wood sawdust is the lowest cost material used to clean up dyes, oils, toxic salts, pharmaceutical substances and many more from the environment. The mechanism of contaminant adsorption onto the sawdust surface involves ion exchange and hydrogen bonding. The cell walls of sawdust mainly consist of cellulose and lignin, where there are abundant hydroxyl groups which can contribute to a high number of binding sites.

There are many factors affecting the adsorption process of contaminants by wood sawdust. Solution temperature, contact time, initial concentration of pollutant, adsorbent dosage and solution pH are among the determinant of adsorption effectiveness. Commonly, too high in temperature will decrease adsorption. A longer contact time allows the contaminant to be adsorbed onto the adsorbent surface. Higher adsorbent dosage made the water cleaning process faster, but for economic reasons, optimum dosage should be investigated to prevent wastage. The pHzpc, or the point where the adsorbent is in neutral charge. The surface charge of the adsorbent is positive when the solution pH is below the pHzpc value, while it is negative at a pH over the pHzpc. The adsorption depends on the charge of the adsorbate. For example, a positively charged metal ion will be more attracted to the adsorbent when the pH of the solution is above pHzpc, due to the negative charge of the adsorbent [28].

Suitable treatments can improve the adsorption performance of wood sawdust. Modification can be done to increase the number of pores or increase chemical binding sites on the surface of the adsorbent. For example, modification using alkaline solution on the sawdust of deciduous softwood–poplar and coniferous softwood-fir were found to result in adsorption increment of 5 times for copper ions and 15 times for zinc ions [29]. In another research on potassium hydroxide, KOH-modified poplar sawdust also showed an increment in metal ion adsorption, up to 94.3% for copper ion and 98.2% for zinc ion, compared to unmodified poplar [30]. A list of sawdust used for the adsorption of contaminants is tabulated in Table 2.

Table 2 List of sawdust used for adsorption of contaminants
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6 Nanocellulose

Nanocellulose is a material extracted from the plant cell wall. It is cellulose in nanometer size, high strength, excellent stiffness and large surface area [41]. Nanocellulose can be categorised into three main types; nanocrystalline cellulose, nanofibrillated cellulose, and bacterial nanocellulose [42].

  1. i.

    Nanocrystalline cellulose

    Nanocrystalline cellulose consists of highly crystalline particles. The extraction was commonly done using acid hydrolysis/heat-controlled techniques. The crystals from cellulose fibres were extracted by hydrolysis of amorphous cellulose regions using sulphuric acid, resulting in highly crystalline particles with dimensions depending on the source of raw material. Sulphuric acid hydrolysis gives a negative charge of sulphate half-ester groups onto the surface of the particles, creating electrostatic repulsion between particles that prevents agglomeration [43].

  2. ii.

    Nanofibrillated cellulose

    Nanofibrillated cellulose is bundles of cellulose chain molecules stretched with a long and flexible cellulose nanofibre network. It has a size of 1–100 nm, with alternating crystalline and amorphous domains along the structure. It is produced by delamination of wood pulp through mechanical force after chemical and enzyme treatment. Nanofibrillar cellulose, cellulose nanofibre and cellulose nanofibril are used for microfibrillated cellulose and are commercially available [44]. The surface of nanofibrillated cellulose can be modified through polymer grafting, adding coupling agents and many more.

  3. iii.

    Bacterial nanocellulose

    Bacterial nanocellulose is generated by the bacteria such as Gluconacetobacter, Sarcina, Aerobacter, Agrobacterium, Alcaligenes, Acanthamoeba, Achromobacter, Azotobacter, Rhizobium, Escherichia, Pseudomonas, Salmonella and Zooglea [45]. It has various biomedical applications due to its biocompatibility and non-toxic properties [46]. Because bacterial nanocellulose is not extracted from wood materials, it will not cover extensively in this chapter.

Nanocellulose has various applications, from the engineering and electronic sector to biomedical purposes. Nanofibrillated cellulose has the potential to be utilised in nanocomposite making. Nanofibrillated cellulose can be solution cast, in situ polymerisation or through melt mixing for blending with thermoplastics. In biomedical applications, nanofibrillated cellulose is used in medicine for drug delivery in human blood. Cellulosic nanofibres were also used as a material for the development of nanocomposite scaffolds which match the strength of the original ligaments or tendons. Due to its large surface area, cellulose nanofibril also has the potential to be used as an adsorbent for contaminations from water. In the papermaking industry, the strong affinity of cellulose nanofibre with cellulosic pulp and their ability to form a 3D network made them an excellent additive to increase paper strength [47].

7 Agricultural Cultivation Media

In the agricultural sector, sawdust is commonly used as mushroom cultivation media. Mushrooms can be grown on any sawdust as the media. However, different compositions of sawdust type will affect the mushroom's growth rate and number of fruiting bodies. In South East Asia, rubberwood sawdust is commonly used as it can be found abundantly in rubber plantation agricultural activity [48].

The growth performance of Pleurotus ostreatus from various agricultural wastes mixed with rubber tree sawdust in Malaysia has been done by Ahmad Zakil et al. [49]. Different agricultural wastes were mixed with rubberwood sawdust, including the oil palm empty fruit bunch, oil palm press fibre sugarcane bagasse and corn cob. Results showed the highest total average yield of P. ostreatus was 207.96 g/bag on when using the combination of sugarcane bagasse and rubberwood sawdust at a ratio of 1:1.

Research has also been done on the cultivation of oyster mushrooms (Pleurotus ostreatus) using Moso bamboo sawdust showed a faster growth rate of 3–7 days than conventional cultivation media. When mixed with rice bran, it gives an even better yield of 97.9 ± 3.9 g/bottle and fruiting bodies at 33.6 ± 4.2 no/bottle [50].

8 Clay, Cement, Concrete and Building Materials

Sawdust is commonly added to building materials to reduce its original weight. Low thermal conductivity, high sound absorption and good sound insulation are among the wood characteristics favourable for building material [51]. An attempt to produce lightweight concrete using a mixture of sawdust and cement has successfully created a block of concrete that can withstand more than 2.8 N/mm2, despite the increment in water absorption due to the presence of the sawdust [52]. The incorporation of sawdust into unfired clay blocks showed that up to 2.5% of sawdust could be added to make a good block with particle sizes ranging between 600 and 425 μm [53].

Sawdust has been tested for sand replacement in cement blocks for up to 15% content. Volumetric shrinkage and concrete density decrement were observed, with an increment of water absorption when the sawdust percentage was increased. Testing on two types of concrete prepared, the natural weight concrete, NWC and lightweight concrete recorded 34 MPa and 21 MPa compressive strength, respectively. Therefore, these concretes can be used for structural applications. Up to 21.42% reduction in the heating, ventilation and air-conditioning (HVAC) energy was also recorded in sawdust added concretes [54]. High thermal insulation is favourable in tropical countries as it minimises heat transfer from the outer side of the building into the occupied space.

9 Liquefaction

Biomass liquefaction is a technique of dissolving woody materials into liquid form for obtaining biofuels, bio-based materials and chemicals. Various wood liquefaction techniques include hydrothermal liquefaction, organic solvent liquefaction, co-solvent liquefaction, microwave-assisted liquefaction and plasma electrolytic liquefaction [55].

9.1 Hydrothermal Liquefaction

Hydrothermal liquefaction of sawdust is conducted near (subcritical) or above (supercritical) the critical point of water, which is 374 °C, under 22.1 MPa pressure. Before liquefaction, pre-treatment is necessary to facilitate access to the internal structure of wood for fractionation. Physical action (pulverisation and irradiation), chemical treatment (alkali, acid, organosolv and ammonia explosion), or combination of them as physicochemical (steam explosion, carbon dioxide explosion and wet oxidation) and biological pre-treatment (enzymolysis). General parameters of hydrothermal liquefaction are conducted at a low-temperature range between 200 and 400 °C and high pressure between 5 and 25 MPa. Sawdust will be converted into a bio-oil fraction, a gas fraction and a solid residue after the liquefaction process [56].

9.2 Co-solvent Liquefaction

Hydrothermal liquefaction can be done with co-solvent to assist the liquefaction process. The hydrothermal liquefaction of rice straw with methanol co-solvent has been done by Yerrayya et al. [57]. The presence of methanol as a co-solvent to water at 50:50 has improved the yield of bio-crude production up to 36.8 wt.%. Another hydrothermal liquefaction of rice straw with glycerol as co-solvent. Using 5 wt% of Na2CO3 as catalyst and 260 °C heating temperature for 1 h, the process yielded 50.31 wt% of bio-oil and 26.65 wt% of solid residue [58]. Meanwhile, hydrothermal sugarcane bagasse at 320 °C reaction temperature, 15 min reaction time and 10 wt% potassium hydroxide, KOH as catalyst produce the highest amount of bio-crude of 36.3 wt% [59].

9.3 Organic Solvent Liquefaction

Organic solvent liquefaction can be done at temperatures 240–270 °C without catalysts. The temperature could be lowered to around 80–150 °C with acidic catalysts. Organic solvent liquefaction produces very high yields of solvent solubles of about 90–95% based on the lignocellulosic weight. The conventional liquefaction of wood takes several hours of treatments with a higher temperature required at 300–400 °C with or without catalysts. Conventional liquefaction also yields less, around 40–60%, due to the conversion of the woody material into gaseous compounds [60].

The possible chemical compounds that can be produced from the organic solvent liquefaction are resins such as phenolic resins (novolac and resol type), polyurethanes and epoxy, which are commonly used in wood composite making [61, 62]. An experiment on the effects of phenol, ethylene glycol (EG) and ethylene carbonate (EC) solvents shows phenol as the optimum solvent for bamboo liquefaction with a yield of up to 99%. The liquefaction utilises hydrochloride acid at a temperature of 180 °C in autoclaves for different reaction periods [63].

9.4 Microwave-Assisted Liquefaction

As its name mentioned, microwave-assisted liquefaction is the method to increase liquefaction yield by combining chemical processes and microwave irradiation. Using microwave heating brings the energy directly into the target object due to the applied electromagnetic field. Microwave heating ensures faster heat penetration into the bulk material. Therefore the reaction time can be reduced, which may also reduce the cost of operation [64].

Microwave-assisted liquefaction has been researched by Xue et al. [65] using polyethylene glycol (PEG-400) and glycerol mixture as liquefying solvents together with 97 wt% sulphuric acids as a catalyst at a reaction temperature of 140 °C. A high liquefaction yield (97.47%) was achieved in just 5 min to obtain the polyol. In their research, the polyol produced was made into polyurethane resin.

Another research used pinewood particles as the raw material, with polyethylene glycol and glycerin (70/30 w/w) as the liquefaction reagent. The solvent-to-wood ratio was 7:1, and the liquefaction was done using two types of catalysts, which were 3% of sulphuric acid and phosphoric acid. Liquefaction was done at 150 °C with the help of microwave irradiation. It was found that using microwave heating, the wood meal completely dissolved in just 2 min, compared to 30 min using conventional heating [66].

10 Conclusions

Sawdust has a wide variety of potential uses, from energy generation to chemical extraction. Sawdust cannot be treated as trash but as a new source of income generation for the company. It is also a sustainable way of using wood resources as efficient as possible to guarantee the future of our forests. Hopefully, this book chapter has given some insights into the usefulness of wood sawdust.