Biotechnology for Environmental Protection in the Pulp and Paper Industry

The pulp and paper industry comprises a large and growing portion of the world’s economy. Pulp and paper production has increased globally and will continue to increase in the near future. Approximately 155 million tons of wood pulp are produced worldwide and about 260 million are projected for the year 2010.1 However, the industry is very capital-intensive, with small profit margins, which tends to limit experimentation, development, and incorporation of new technologies into the mills. To be able to cope with increasing demand, an increase in productivity and improved environmental performance is needed, as the industry is also under constant pressure to reduce and modify environmental emissions to air and water.

Environmental research on the aquatic impacts of pulp mill effluents has been concentrated in Scandinavia and North America. In the 1970s, the regulatory framework covered conventional parameters such as biochemical oxygen demand, suspended solids, and nutrients. From the 1980s until today, concern has been directed towards chemical toxicity. It has been claimed that the organic materials in the effluents from pulp mills cause a number of biological effects. These suspicions have prompted intensive research to study relationships between exposure and effects, as well as profound process changes within the pulp industry. Chemical toxicity analysis has been performed at various levels of biological organization: community, population, individual, organ, tissue, cell, subcellular, and molecular. Many studies performed on pulp mill effluents have focused upon within-organism effects on physiology and biochemistry.1 Despite the profound process changes in the industry, there is still evidence of physiological changes in organisms exposed to mill effluent. However, the ecological significance of these responses is unknown; in large part they are provoked by lipophilic compounds present in the wood entering the mill (extractives) rather than by materials formed or modified during pulping or bleaching.

At present, pulp is produced from wood either by chemical delignification, mechanical separation of the fibers, or combinations of chemical and mechanical methods. Mechanical pulping methods are being used increasingly because they give much higher yields (>90%) than do chemical methods (40-50%) and require less capital investment. The main disadvantages of mechanical pulping are the high energy requirement, the low strength and the low brightness stability of mechanical pulps (Table 3.1). Addition of chemical pulp is often required to produce papers with adequate strength.

The kraft process accounts for 85% of the total pulp production in the United States. Bleached kraft pulp is a relatively high-value component of the total production of kraft paper. Kraft pulping removes most of the lignin and dissolves and degrades hemicelluloses without severely damaging cellulose. The kraft process results in excellent pulp from a wide variety of wood species. Unfortunately, kraft pulping also generates large quantities of chromophores. Chromophores are composed of residual lignin and carbohydrate degradation products. They are hard to extract because they are physically entrapped in and covalently bound to the carbohydrate moieties in the pulp matrix. Manufacturers use elemental chlorine and chlorine dioxide to bleach the chromophores and then they extract them, along with the residual lignin to make white pulp. Because of consumer resistance and environmental regulation of chlorine in bleaching, pulp makers are turning to oxygen, ozone, and peroxide bleaching, even though these may be more expensive and less effective than chlorine. One new technology has evolved to decrease the use of chlorine for bleaching, and that is the treatment of the pulp with xylanase enzyme. The use of xylanase enzymes to enhance the bleaching of the pulp was first reported in 1986.1 The Finnish forest companies were the first in the world to start millscale trials in 1988.

Lignin is one of the major components in plants. Selective removal of lignin from plant cell walls has significant implications on the pulp and paper industry. Chlorine-based chemicals traditionally have been used to bleach pulps because they selectively and efficiently remove lignin. However, chlorinated organic compounds released in this bleaching process are toxic to the environment. The ever-increasing pressure from environmental protection authorities has forced the pulp and paper industry to seek an environmentally benign bleaching process. Biobleaching is one of the very promising alternatives for eliminating chlorine-based chemicals in the pulp-bleaching process. Since xylanase was found to improve the bleachability of kraft pulp,1 enormous efforts have been made to develop hemicellulase-aided bleaching. The developments in hemicellulase aided bleaching are reviewed in Chapter 4. Xylanase enzymes are now being produced for the pulp industry by several companies around the world. Many mills worldwide have experience in using xylanase prebleaching and a CPPA survey showed that as of early 1995, about 1 million tons/year or 8% of the bleached kraft pulp made in Canada was enzymetreated.5 Xylanase applications are often referred to as prebleaching or bleach boosting because the nature of the effect is to enhance the effect of bleaching chemicals rather than to remove lignin directly. The enzyme does not attack the lignin-based chromophores but rather the xylan network by which the residual lignin particles are surrounded and trapped. A limited hydrolysis of the xylan network is often sufficient to facilitate the subsequent chemical attack on lignin with various bleaching chemicals, without sacrificing yield. Mill-scale experience has demonstrated that xylanase pretreatment usually results in up-to 20-25% savings in bleaching chemicals, with simultaneous reduction of pollutant emissions.

In recent years, the demand for the use of postconsumer paper products in the production of new paper products has risen dramatically. Some of this demand has been driven by politics and some by consumer demand. The use of secondary fibers in producing different grades of paper has increased greatly over the past two decades particularly as a result of development in deinking processes. The consumption rate of waste paper (secondary fiber) in different countries are given in Table 6.1. The worldwide consumption amounts to 37% (110 million tons) in 1995 and likely to increase to 55% by the year 2000.1 Currently, 145 deinking facilities are operating, under construction or announced for construction in the USA.2 Worldwide, deinking capacity is expected to rise to 31 million tons by 2001, with particularly large expansions for newsprint, printing, and tissue grades.³

The forest industry utilizes wood and other lignocellulosic feedstocks as raw materials for the production of paper. The major constituents of wood are cellulose, hemicellulose, lignin, and extractives. Softwoods, hardwoods, and straw have different proportions of chemical components (Table 7.1). The processing of wood in paper mills involves various operations including debarking, pulping, and bleaching that result in the discharge of highly polluted wastewaters. The quantity and types of pollutants in these effluents vary with the type of lignocellulosic feedstock used as raw material, the process conditions applied (pH, temperature, pressure, chemical and mechanical treatments), and the specific water consumption.¹ Chemical additions and, to a lesser extent, high pressures and temperatures result in an increased release of organic matter into the process water and extensive lignin solubilization. Therefore, the pollution loads and the color due to dissolved lignin compounds is very high for chemical as compared to mechanical pulping effluents.^2,3 The COD loads associated with mechanical pulping processes range from 20–50 kg COD/ton of pulp whereas those corresponding to soda pulping processes may be as high as 500–900 kg COD/ton of pulp.^1,4 Nevertheless, the black liquors originating from kraft and soda processes are usually burnt to recover the pulping chemicals and the calorific power from the organic components, diminishing to a great extent, the environmental impact associated with these pulping processes. Conventional recovery processes are not economically viable in small paper mills and in those using nonwoody raw materials with a high silica content.^4,5 Black liquors represent a very important pollution source in several countries where small-scale mills are common.^4-6

The pulp and paper industry ranks third in terms of water consumption and fifth among the major industries in its contribution to water pollution problems in the USA. Pulping, bleaching, and paper-making operations are the three major wastewater sources of the industry.

Most of the pulp and paper mills are located near the major waterways and have access to a large, uninterrupted supply of water. After using the water for pulp and paper production, these mills discharge the used water into the waterways as waste. The paper industry is currently experiencing increasing regulatory pressure to reduce the volume and toxicity of its industrial wastewater. During the past few years, the concept of system closure has been gaining popularity in the forest products industry, mainly because of its potential to drastically reduce or even eliminate liquid discharges and the associated water-quality problems, to separate and recycle valuable resources, and to preserve energy that can be used to reduce cost of production and amortize capital costs. In addition to savings on chemicals and heat, it would also result in saving on capital and operating costs for effluent treatment, reduced use of freshwater, and the possibility of locating new mills independent of water supply or effluent recipient.

A variety of solid wastes are generated by pulp and paper mills that need final disposition. In addition to bark, wood waste, boiler ash, pulping residuals, and mill trash, it also generates wastewater treatment sludges like deinking sludge, primary clarifier sludge, and biological sludge. The quantity of sludges varies from site to site. On average, Canadian pulp and paper mills with activated sludge wastewater treatment system produce primary sludge of 31 kg(od)/ton of pulp while the secondary (biological) sludge generation is 16 kg/ton (Table 10.1).¹ A typical floatation deinking plant produces 80-150 kg of dry sludge/ton of recycled pulp.2 Table 10.2 illustrates how the quantity of sludge generation varies with the type of pulping and paper making or both.³ In the US, an NCASI study indicated that in 1988,50 kg/ton of wastewater treatment sludge was generated averaged across the industry as a whole. Primary sludge accounted for 18%, secondary sludge 7%, and deinking sludge 2% of all the solid waste generation in US pulp and paper industry in 1986.⁴ The US industry generated approximately 4.6 million tons of dry sludge solids in 1989.⁴

People living in or near a kraft pulp mill often complain of the odor nuisance associated with the mill’s operations. These complaints are directly related to the production of odorous compounds during the cooking of wood chips with white liquor and subsequent points of gaseous release to the atmosphere. Even when pure sodium hydroxide is used to treat wood and straw, odors are produced. The cause of these odors is to be found in the residual sulfurcontaining protoplasm which reacts with the alkali to form mercaptans and organic sulfides during the digestion phase. It was found that the mercaptans are formed by the saponification of lignin methoxyl groups by sulfide ions.