The socio-economic importance of paper has its own value to the country's development at it is directly related to the industrial and economic growth of the country. The per capital consumption in India is 2.5 kg. per year, while the Asian average itself is 18 kg. At present in our country, the 380 pulps and paper industries have an aggregate capacity of 3.95 million tones.

Looking into the serious nature of pollution, the pulp and paper industry in India has been brought under the 17 categories of highly polluting industries. The paper industries, under this category having capacity above 30 tonnes per day, are 96 in number. Out of these, 62 units have the required facilities to comply with the effluent/emission standards while 19 units do not have adequate treatment facilities. 15 units have been closed down.

The small units using agro-residue as a raw material are a threat to the environment. In India, the total installed capacity of small pulp and paper industry is about 1.95 million tones per year. These units discharge the black liquor along with unrecovered chemicals, which is difficult to treat since lignin in the black liquor is not easily biodegradable. The presence of caustic in black liquor results in increase in dissolved solids in wastewater streams. The sodium concentration renders sodium toxicity if the effluent is discharged on soil for irrigation. The discharge of untreated black liquor also results in the loss of valuable chemicals. It has been estimated that discharge from a 30 tonnes per day agro-based paper mill is equivalent to the pollution load from a 100 tonnes per day mill with chemical recovery. The major problem is the lack of economically feasible methods of chemical recovery from the black liquor of the small pulp and paper mills.

The presence of silica and small fibers make the task of recovery difficult. Another problem is low concentration of the black liquor from the agricultural residues. The agro-based paper mill should either upgrade the scale of operation for the economic affordability of the process or for the clusters of such mills a mother pulping and common chemical recovery plant should be set up on lines of the common effluent treatment plants. The Hindustan Newsprint Limited, Kerala, has developed the process of desilication, to remove silica, through selective precipitation and carbonization. The black liquor from rice straw may contain 11-14% of silica as dry mass, while that of wheat straw contains 4-8% . The Uttar Pradesh based M/s Amrit Papers Limited, has also developed a chemical recovery technology, best suited for bamboo, bagasse, and straw and reeds-based paper mills. In the process, black liquor is concentrated and then mixed with solid fuel and precipitated sludge. It is burnt to a semi-solid mass, which is leached and as a result green liquor is formed. It is later carbonized to precipitate the silica. This silica is of high purity and may be used for other purposes. This process has been successfully displayed on a pilot scale, but its economy for small-scale agro-based paper mill is still to be assessed.

Direct alkali recovery system has been developed by the Central Pulp & Paper Research Institute (CPPRI). The process involves combustion of semi-concentrated soda black liquor admixed with ferric oxide in a fluidized bed boiler. The sodium ferrite granules are further hydrolyzed for the recovery of sodium hydroxide and ferric oxide. The ferric oxide is used again in the fluidized bed boiler. This process is yet to be tried in small paper mills.

By adopting the cleaner pulping process, the generation of black liquor can be restricted. An attempt in this direction has been made by M/s Pudumjee Pulp & Paper Mills Limited, Pune, by developing and operating clcohol-based pulping. The process is still in the pilot scale. In this process, after pulping of the agro-residues, the solvent is recovered through distillation and pure lignin is recovered as a byproduct. Most agro-based pulp & paper industries belong to small sector, producing as low as 5 tonnes per year of paper. The installation of the chemical recovery system is difficult for them due to high initial cost and a prolonged payback system. The concept of the common chemical recovery can be a solution for such industries situated in a cluster. The black liquor can be suitably collected from each of the units and processed at common chemical recovery plant. There are at least four such sites in the country having number of paper industries in the vicinity.

Traditionally, the bleaching of pulp is performed with the help of elemental chlorine. The bleaching is essential for the improvement in brightness, cleanliness and removing impurities. The advantages of chlorination are that it is most effective delignifying agent and at the same time least expensive of all the bleaching chemicals. It is also excellent for the shive and dirt removal. But, in the recent years, chlorine is viewed as enemy of the environment, as it destroys the stratospheric ozone layer, and produces dioxin and organic chlorides. Its low consistency produces a large volume of acidic effluent, which has to be neutralized before waste treatment. It degrades pulp to some extent and is corrosive in nature.

These days, chlorine is gradually being replaced with several other chemicals, such as chlorine dioxide, oxygen, ozone and hydrogen peroxide. Oxygen was first applied successful for bleaching of dissolved pulp by Russian scientists, V M Nikitin and GL Akin, in the fifties. Since installation of the first oxygen- bleaching plant in South Africa in 1969, this technology found its importance, as reflected by over 12,000 tonne per day of oxygen bleached pulp production per day worldwide. While reducing the pollution load remains the incentive for the first oxygen-bleach plant, later on, other benefits such as lower operating costs and savings of bleach chemicals in subsequent stages, can be realized. Moreover, oxygen is non-toxic and non-corrosive. The only disadvantage with this system is that it needs high temperature and pressure, and increases load to recovery system. In the oxygen bleaching, in addition to delignification, significant brightness is also gained, while in chlorination there is no improvement in pulp brightness.

The use of chloride dioxide as bleaching agent assures very high and stable brightness. It is best for shive and dirt removal, and is highly selective in nature, for which there is little degradation to pulp. It produces less organic chlorine than either chlorine or chlorates. The problems associated with the use of chlorine dioxide is that it is highly explosive, corrosive and toxic. There is possibility to use hypo chlorite for bleaching in the pulp mills, which is less expensive than chlorine dioxide, and also good for shive and dirt removal. It is easy to generate and handle. The disadvantages with hypo chlorite is that if proper pH of the medium is not maintained, it causes severe degradation to pulp, and during the process, chloroform, a toxic gas, is produced. Now a day, hydrogen peroxide and ozone is also used for bleaching. For a better result, many industries use a controlled sequence of chlorination, alkaline extraction, chlorine dioxide, oxygen, hypo chlorite, peroxide and ozone treatment for the pulp bleaching.

Pollution Prevention Opportunities in Pulp Bleaching

Ever-rising energy costs and unrelenting environmental pressures are going to govern not only the bleaching technology but also the course of all other technologies in future. The cost of chlorine and chlorine-based chemicals, the main bleaching chemicals, depends more on the energy cost as compared to oxygen. Also, the chlorine-based chemicals are mainly responsible for the formation of pollutants, in particular of toxic compounds. These two factors tilt the balance in favour of oxygen and its derivatives as the bleaching chemicals of tomorrow. Mixing of oxygen into the extraction stage of existing bleach plants (Eostage) is already practiced by many European and North Americal pulp mills. It is claimed that use of an Eostage can reduce the stages in a bleach plant by one, without sacrificing the pulp quality and with significant savings in operating costs. This approach represents a low-cost alternative to significantly reduce the discharge of the pollutants. Besides oxygen and hydrogen peroxide bleaching, which are well-established techniques, ozone delignification and bleaching are also in the developmental stage.

The paper machines usually employed in the small-scale industries are getting outdated. Productivity-linked approach is being adopted by improving the efficiency of the existing machines and by installing machines of higher capacity with more automation. The new efficient machines save energy and reduce the overall pollution load. The effluent from paper mill section is rich in chemicals and fibers, which can be used in the process. Small-scale paper units either do not have fiber-recovery system or have a low efficiency one. The use of floatation-based units, which clarify the effluent in a better way, recover the suspended fibers more efficiently while simultaneously showing an efficiency range in excess of 90 per cent has been proved to be beneficial. The clarified water can be used for the process, for dilution and for cleaning.

Improved house-keeping, adoption of better pollution abatement technology, development of recycling system and recovery of useful products from waste, and proper training of concerned persons, will certainly help the pulp and paper industry to maintain a cleaner environment.