Water is intricately associated with all the three stages of paper production namely, pulp making, pulp processing, and paper/paper board manufacturing, and their associated activities of cooking, bleaching, and washing. About 85 percent of the water consumed in the pulp and paper industry is used only for processing, thus, leading to the generation of large volumes of contaminated wastewater.
Water Use TrendsWhile screening, cleaning, and paper making operations are the primary sources of effluents, cooling water used in various machineries and stormwater runoff also contribute significantly. Typical water pollution concerns for the wastewater from paper mills are effluent solids, toxicity, color, and biological oxygen demand (BOD) (See table for details). Apart from removing these impurities, the primary and secondary water treatment plants, which are normally a part of any standard paper-manufacturing unit, also treat the wastewater for adsorbable organic halides (AOX) and chemical oxygen demand (COD).
Even though the pulp and paper industry is still highly dependent on fresh water, there have been some remarkable achievements with regard to the water-use patterns. For instance:
1. Europe has cut water use by 50 percent, while the production has doubled in more than a decade
2. In the U.S, since 1950, water use has reduced between 50 and 90 percent for every ton of paper produced
3. During the last decade, the amount of fresh water required per ton of paper has dropped by two-thirds in Japan
Despite these developments, the amount of water recycled in the paper industry, at 11.8 percent is still the lowest amongst water-intensive industries. Will closed-loop manufacturing process help further water recycling?
The principle of closed-loop manufacturing aims to eliminate discharges to the external environment, by recycling and reusing most of the solid and liquid wastes, while reducing air exhausts to bare minimum, both in quantity and toxicity. While reuse of solid wastes was found to be easier (by burning them for energy production), recycling of water within the system had inherent problems. Closed water circuits created a favorable environment for the growth of micro-organisms, which lead to increased build-up of slime deposits that caused severe operational problems. Moreover, the accumulation of contaminants from the paper additives deteriorated the quality of the end product over a period. Hence, the closed system needed to be reopened and cleaned at regular intervals.
Sensing the need to overcome this bottleneck, the European Union (EU) has funded a three-year ‘Paper Kidney Project’ to explore the possibility of using technologies such as membrane separation, aerobic biotechnological treatment, and polishing to achieve trouble-free water recycling. The group, comprising representatives from the paper industry, water treatment companies, and research institutes, has a target of attaining zero liquid discharge.
Apart from the EU project, industry and research institutions across both sides of the Atlantic are working on numerous technologies for achieving cost-effective and most efficient wastewater treatment. A few of them are:
1. Advanced treatment of paper-mill effluent using tertiary bio-filters (submerged aerated up-flow film reactors)
2. Use of ozonation and wet oxidation for the treatment of thermo-mechanical pulp (TMP) circulation waters
3. Advanced pulp and paper industry effluent treatment with a combination of ozonation and fixed bed bio-film processes
4. Nitrogen fixation in the treatment of pulp and paper industry wastewaters
The pulp and paper manufacturers themselves are taking the lead and implementing several practices that are becoming trendsetters for water use and treatment.
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