With the identification of potential sources of environmental impact and their mechanisms at different stages of the timber production process, the following methods can be applied to tackle the associated contemporary challenges.
Changes in energy sources and consumption pattern
As energy sources and consumption patterns are critical towards overall environmental impacts of energy consumption practices, environmentally friendly energy sources should be promoted. For example, fossil fuel based energy such as energy generated from coal, has more adverse environmental impacts than that of non-fossil based energy sources. Similarly, anthropogenic emissions due to fossil fuel have comparatively higher emission and negative environmental impacts, than that of biogenic emission from burning wood materials (Bergman and Bowe 2008). Therefore, while choosing energy sources for the timber production process, there needs to be proper care in the use of renewable energy instead of fossil fuel-based energy techniques. Even if fossil fuel based energy source are to be used, efforts must be made to use as little energy as possible.
Use of Sawmill by-products as a thermal energy
Instead of leaving the sawmill products within the premises of sawmills, and creating environmental hazards, they could be collected and used for producing thermal energy to reduce environmental impacts. This would help to minimize the reliance on offsite fossils fuel to some extent and promotes the production of bioenergy at the sawmill site. For example, the sawdust could be recycled into a bio-briquette. Such bio-briquettes have even higher heating value ranged from 14.88 up to 16.94 MJ/kg, than that of the briquette made from other substances (Lela et al. 2016).
Improved sawmilling and sawing machinery
Improved sawmilling techniques, machinery and manufactured products help reducing the environmental hazards and human health problems (Harms-Ringdahl et al. 2000) and ultimately contribute to environmental sustainability in numerous ways (Gaussin et al. 2013). The use of recent technology and safety procedures could be helpful in these regards.
Laurent et al. (2016) conducted environmental assessment of a wood manufacturing industry and established environmental profile of the company so that company continue to maintain its environmental integrity as well as environmental profile of different wood products it manufactures.
First, improved and new varieties of machinery instead of old and obsolete one help reducing the wood waste, thereby reduce environmental impacts, while increasing the working efficiency in terms of time, energy and efforts. Second, hazardous energies related to machinery use can be minimised as safety and precautionary measures such as lockout system. The lockout measures is a step-by-step procedure, carried out by authorised employee to prevent inadvertent machine energization or the release of stored energy, which is in practice in Canada and the United States (Poisson and Chinniah 2016). Third, and most importantly, workers health and safety, and ergonomic measures have to be taken into account while planning and executing the sawmilling operation in the field (Jones and Kumar 2007, 2010).
Improved energy efficiency in drying system
Wood drying is the key to controlling wood quality of final products, and it consumes up to 90% of the processing time in hardwoods and more than 70% of primary processing cost, with the use of significant amounts of heat and energy (Goreshnev et al. 2013). The supplied heat is primarily used for the drying process, which is carried out in a drying kiln. Lead-time and wood quality are the major priority before energy consumption while producing the lumber (Anderson and Westerlund 2014). Therefore, the introduction of improved drying processes including simple yet environmentally friendly drying process would be beneficial to reduce the environmental impacts while ensuring the quality of final products. For example, solar drying provides opportunity as an alternative method of drying timber, while using renewable solar energy to address the shortcomings associated with fossil fuel based drying process. In addition, solar systems use the energy from sun, which is abundant, inexhaustible and nonpolluting (Akinola 1999; Akinola et al. 2006; Kumar and Kishankumar 2016), thereby has little environmental impact (Belessiotis and Delyannis 2011), unlike other forms of fossil fuel based drying methods. However, external factors such as air temperature, air velocity geographic locations, and relative humidity influence the potential drying rate. Yet, it has advantages over open-to-the-sun or air drying techniques, because the solar dryer traps solar energy to increase the temperature of circulating air and ensures the required equilibrium moisture content (EMC), enhanced shelf life, value addition, and quality enhancement (Helwa et al. 2004; LayThong 1999). These features can be further complemented by the controlled air humidity and other drying conditions, even with the use of water sprayers in some cases. However, there might still be chances that productivity is affected by weather condition such as rainfall, cloud cover, and less predictable outcomes than that of industrial kilns (Haque and Langrish 2005).
Solar kiln drying is usually affected by geographic and climatic conditions. For example, the temperature inside the kiln is affected by the ambient temperature and solar radiation (Hasan and Langrish 2014; Phonetip et al. 2017a). Areas with low humidity offer a productivity performance for solar kilns (Ong 1997). According to Phonetip et al. (2017b), decreasing the relative humidity (RH) level to 40% can dry boards faster than when the conditions are maintained at 60% RH. Taking advantage of a low ambient RH could result in several benefits, such as lowering the consumption of water and energy.
A study by Phonetip et al. (2018) described a method that used the combined tools of GIS and Fuzzy theory to identify the most suitable locations for solar kilns based on variables of geographical and climatic conditions and restricted areas, using an example location in Vientiane, Laos. This method can be applied to different geographical regions and local climatic seasons.
Therefore, in order to improve efficiency and reduce the environmental impacts, various kind of solar drying are in practice, such as integral, distributed and mixed type solar dryers based on the mode of utilization of solar heat, and greenhouse system, external collector, and mixed mode solar drying depending on greenhouse systems. Currently, enhanced solar timber kilns can also be used with characteristic features of solar energy storage with independent heating, integration of an air heater in the storage and in the drying chamber, and management of different drying cycles based on the quality control of the products (Ugwu et al. 2015).
Overall, solar drying has more environmental advantages due to shorter drying time, and better drying quality than that of air-drying. Similarly, it requires, low operating costs and lower training manpower, along with the chances of having EMC in broad range of climates, and ultimately constitutes an environmentally friendly technique due to its reliance on renewable resources and low environmental impact.
Studies on improving energy efficiencies have shown that if available state-of-the-art technologies are applied in drying kilns, it could reduce the heat consumption by about 60% (Anderson and Westerlund 2011, 2014; Johansson and Westerlund 2000). Moreover, a study by Anderson and Westerlund (2014) using the Torksim simulation program has further reported that energy recovery technologies in the sawmill industry could save considerable amounts of energy and biomass for the other purpose. According to the authors, use of a heat exchanger, mechanical heat pump, and open absorption system are the major energy recovery technologies. For instance, open absorption system is the most effective which will reduce energy consumption by 67.5%, whereas mechanical heat pump could also decrease a significant amount of energy usage and result in a large heat surplus in the drying system. However, the latter requires high consumption of electricity. In contrast, use of heat exchanger technology contributes only a marginal increase in energy efficiency of 4–10% depending upon the sawmill condition and drying scheme. Therefore, findings of such studies mainly related to the result of higher energy efficiency from open absorption system should be promoted to reduce the energy use and GHG emission, increase the efficiency, and minimize the environmental impacts.
Use of environmentally friendly chemicals
There is a growing trend towards environmentally friendly preservatives to reduce the environmental impacts while improving the durability of timber products. In this context, environmentally benign wood preservative systems can be developed with proper combination of an organic biocide with metal chelating and/or antioxidant additives (Schultz and Nicholas 2002). That will not only enhance protection of wood against fungi as compared to the biocide alone, but also consequently, help reduce the environmental impacts especially on land and water resources. Physical barriers have been accepted as alternative non-biocidal wood protection method in India as they reduce leaching and subsequent negative impacts of wood preservative components to the organisms in vicinity (Sreeja and Edwin 2013).
Policy and legislative measures to ban the use of toxic preservatives, and growing awareness on using less toxic and more environmentally friendly preservatives would be another way to reduce the environmental impacts (Lin et al. 2009). For example, a number of toxic preservatives such as CCA, cresote, and preservatives based on volatile organic solvent (VOC), are restricted in Europe and the USA. Instead, use of environmentally friendly preservatives such as copper-organic preservatives replacing CCA, CCB and CCP preservatives, microemulsion water-dilutable concentrates with organic fungicides and insecticides, and water and solvent-based coloured preservatives replacing creosote, have emerged to fill the gap (Coggins 2008; EU 2006). Therefore, stringent environmental policies will have to be practiced to reduce the use of harmful chemicals in wood preservatives, as practiced under Biocidal Products Directives within European Union (Hingston et al. 2001) and restricted pesticidal use of three primary heavy duty wood preservatives (“HDWPs”) under Environmental Protection Agency, USA in 2008 (Tomasovic 2012).
Australian Government Department of Agriculture and Water Resources (2016) accepts certain permanent preservative treatments as biosecurity treatments for use on certain timber products and timber packaging. For a timber preservative treatment to sufficiently address biosecurity risks and be accepted as a biosecurity treatment by the department, it must meet the following requirements:
suitable treatment application methods, preservative penetration zone requirements, preservative retention requirements and accepted preservative formulations.
As biochemical adhesives have 22% fewer environmental impacts than that of petrochemical adhesives (Yang and Rosentrater 2015), use of biochemical adhesives should be encouraged. For example, Pizzi (2006) have identified bio-based adhesives such as tannin, protein, carbohydrate, lignin, and unsaturated oil to maintain both environmentally friendly alternatives and efficient traditional adhesives of the timber industries. Consistent with these findings, Navarrete et al. (2012) conducted a comparative study between the emission from particle board produced with UF and the natural adhesives and found that there was at least seven times higher emission of urea formaldehyde than that of biochemical based adhesive such as lignin and tannin. Yet, the impacts from these biochemical adhesive is quite significant, therefore various innovative measures have to be taken to reduce the impacts on the environment. For example, adhesive based on hexamine could be used to reduce the impact of formaldehyde. Similarly, environmentally-friendly products such as tannin-hexamine adhesive, and in case of lignin adhesive, adhesives pressed at high speed, in the presence of pre-methylated lignin could be used to reduce the environmental impacts (Yang and Rosentrater 2015). Furthermore, soy-based adhesive has also been effective in increasing the wet bond strength with the use of polyamidoamine–epichlorohydrin (PAE) resin as a co-reactant. That has led to resurgence in soy-based adhesive consumption with minimal environmental impacts (Frihart and Birkeland 2014).
In India, extensive research studies have been carried out since 1980 on extending the soya flour to synthetic resin (Sarkar et al. 1985; Zoolagud et al. 1997). Mamatha et al. (2011) developed phenol-soya adhesive for the manufacture of exterior grade plywood. About 40% substitution of phenol by soya was optimized for making exterior grade plywood having strength properties confirming to relevant standard requirements. The substitution not only helps to minimize the formaldehyde release from the products and disposal of waster for better utilization, but also reduces the air and water pollution along with minimization of production cost of the plywood products due to reduced cost in resin system (Mamatha et al. 2011).
A recently published book “Bio-based Wood Adhesives” by Zhongqi He (2017) provides the synthesis of the fundamental knowledge and latest research on bio-based adhesives from a remarkable range of natural products and byproducts, and identifies need areas and provides directions of future bio-based adhesive research.
Policy measures should be placed on restriction of VOCs to the atmosphere. Likewise, an interesting shift from using less environmentally harmful adhesive in joining wood components for furniture and interior joinery by wood welding technology without the use of adhesive has been also initiated. This could be explained by the polymerization and cross-linking of lignin and of carbohydrate-derived furfural (Gfeller et al. 2003). Many studies have been conducted on wood welding using high speed rotation welding (Pizzi et al. 2004; Belleville et al. 2016) and linear welding (Mansouri et al. 2010; Martins et al. 2013; Belleville et al. 2017). If this technique could be scaled up successfully, it would contribute to reduce the adhesive based emission and environmental hazard involved in the timber productions process.
While choosing the adhesive during the course of timber product manufacturing and production processes, proper attention has to be given to environmentally friendly either bio-based adhesive or techniques without using adhesive as far as possible to reduce the impact both on the environment, and the human health.
Over the past few years, regulation under the Clean Air Act (USA) and consumer demand for low-VOC finishes have led to the creation of a variety of new products. Many penetrating finishes, such as semi-transparent stains, have low solids content (pigment, oils, polymers) and are being reformulated to meet low-VOC regulations. To meet the VOC requirements, these reformulated finishes may contain higher solids content, reactive diluents (dilutants or thinners), new types of solvents and/or co-solvents, or other non-traditional substitutes. These low-VOC requirements favour film-forming formulations over products that penetrate the wood surface, since traditional wood stains were formulated to penetrate the wood, and the new formulations that meet the VOC requirements may not penetrate as well.
Another way to decrease air emissions from wood finishes is to change the formulation to a water-based coating. The new water-based products achieve a dramatic improvement over solvent-based finishes in terms of VOC emissions and human comfort and health. Companies that have successfully switched to water-based coatings have worked closely with their suppliers to determine the best water-based formula for their specific uses.
Wood waste management
Eshun et al. (2012) and EPA (2015) have listed ways to minimize wood waste and wood waste management. Main measures to wood waste management include, among others, good operating practices, technology changes, changes in input materials, waste recycling, and waste reuse/recover practices. Similarly, EPA (2015) has described the waste reduction opportunities via lumber receiving, drying and storage; rough end and gluing; machining and sanding; assembly; finishing; packing, shipping and warehouse; building and equipment maintenance.
It is interesting to note that developed countries such as Australia and Sweden place more emphasis on waste recycling, and waste reuse/recover, whereas other countries such as Taiwan, South Africa, and India have put emphasis on improving almost all processing and manufacturing techniques identified above. This might be due to the fact that developed countries may already have good operating practices and required technology in the timber production sector. A study carried out by Daian and Ozarska (2009) in Australia has highlighted the need for using recovered and waste wood in the mulching and compost sector, bioenergy sector, animal product sector, and engineered wood product sector.
During 2013 and 2014, Italy re-used 95% of the waste wood to produce particleboard, while Germany and United Kingdom shared the account to 34 and 53% respectively (Garcia and Hora 2017).
In Europe, the Waste Framework Directive (2008/98/EC) provides a guideline of basic concepts and procedure related to the waste management. A concept called “end-of-waste criteria” has been introduced that is used as a guideline to determine when a waste ceases to be a waste and becomes a secondary raw material. In this concept, waste hierarchy is maintained from landfill through recovery, recycling, reuse to reduce from the least favoured to most favoured option (Garcia and Hora 2017). The values and ways to wood recovery and recycling, classified into direct and indirect recycling, have been well illustrated by Taylor and Warnken 2008 (Fig. 3). Indirect recycling of wood products results in compost or mulch which will decompose into carbon dioxide aerobically. Similarly, direct recycling and reuse of recovered wood into timber products prolong the service life of the timber and at the same time provides the opportunity of potential recovery at end-of-life. Degradable organic carbon contained in the wood disseminate into methane in the landfill site. Methane has 25 times higher global warming potential, so recovering wood will prevent the greenhouse gases (Taylor and Warnken 2008).
Integrated industrial sites
With due consideration of growing energy demand from the different industrial sectors, an essential strategy would be the development of highly integrated industrial sites. Such sites would serve to lower energy and resource consumption and, at the same time, complement one plant to another. For example, saw mills would supply huge biomass to other pellet plants, pulp and paper plant, and combined heat and power (CHP) plants, and some portion of such biomass would be used to fulfill the internal heat requirements as well (Anderson and Toffolo 2013). Therefore, if these plants were combined it would reduce the energy and resource consumption and help reduce the environmental impacts.
Energy efficient biofuel and improved transportation system
Environmental impacts associated with transportation could be minimized by changing the source of energy and mode of transportation of the timber products. Use of renewable sources of energy such as electricity generated from hydropower, and biofuel, instead of fossil based energy would reduce emission during the transportation. Interestingly, in Sweden it was reported that transporting forest products via railway transport requires less process energy than by using road vehicles. Furthermore, use of biofuel instead of fossil fuel in a lorry could replace about 96% of fossil energy (Lindholm and Berg 2005).
By- or co-product or even wood waste can be a feedstock for second generation biofuel (Cantrell et al. 2008; Havlík et al. 2011; Sklar 2008), or be supplied by dedicated plantations. The latter ones seems more promising and can be established on marginal lands (Tilman et al. 2006; Zomer et al. 2008, Havlík et al. 2011), or enter into direct competition with conventional agricultural production (Field et al. 2008; Gurgel et al. 2007) and other services. Therefore, improved transportation system for timber products with the use of energy efficient biofuel should be promoted.
Environmental impacts related to disposal of wood wastage can be minimized by using a minimum amount of materials required for the production process, and renewable materials, and by avoiding materials that deplete natural resources while prompting recycle and recyclable material and waste by-products. Similarly, those left for disposal should be put into safe disposal landfill sites. Landfill sites represent a major disposal option for wood wastes in many countries. For example, in Australia, it is estimated that approximately 2.3 million tonnes of solid wood products are placed in all Australian landfills each year (Ximenes et al. 2008). There should be reliable landfill side for safe disposal of wood wastage.
Overarching policy and institutional support should be in place in order to realize the improvements with regard to minimizing adverse environmental impacts as a result of the production process of timber products in general and sawmilling in particular. Similarly, it should encourage robust production planning (Zanjani et al. 2010), suitable policy measure of impacts minimization and quality enhancement (Loxton et al. 2013), and further collaboration with other stakeholders.
Apart from aforementioned measures to minimize environmental impact as a result of timber production process, some other social, ecological and economic factors should also be taken into account. For example, in order to obtain sustained supply for raw timber from the forest, the timbers supplied from sustainably managed and certified forest is being encouraged (Päivinen et al. 2012). In addition, timber industry should incentivize and support the endeavors of both government and private sectors on plantation and management of forests, so that it would create harmony among them and help the regular supply of raw materials to the industry. Similarly, societal need, interest, and capacity should also be considered while designing, and operating the sawmill industry. Further, proper coordination and collaboration among different stakeholders are also crucial for the success of the industry.