Enzymes for environment

textan Chemicals Private Limited of Chennai recently launched Texzyme J, an enzyme specifically designed for the pulp and paper industry. It can reduce by at least 15 per cent the consumption of chemicals used for bleaching, says Susheela, manager, Quality Control, Textan Chemicals.

Bleaching is necessary because the pulp obtained from the raw material (wood or agro-residue) is dark brown. This is due to the presence of lignin, a natural substance that binds wood/agro-residue fibres together. Elemental chlorine and chlorine dioxide are commonly used to remove lignin and produce brighter pulp. But chlorine bleaching leads to formation of organo-chlorines -- recalcitrant chemicals that are toxic to the environment and are measured as Adsorbable Organic Halides, or aox, in the effluent.

Finnish scientists were the first to show -- in the 1980s -- that enzyme treatment led to reduction in chemicals needed to bleach pulp. In 1995, the United States Environment Protection Agency listed enzyme pre-bleaching as a pollution prevention opportunity in its sector notebook for pulp and paper.

Says D P Chandrana, senior general manager -- Technical at Century Pulp and Paper, Lalkuan (Uttaranchal), "Using enzymes has led to 25 per cent reduction in chlorine consumption and has enabled Century to lower its aox level in effluent to less than 1 kg per tonne of paper as required under the Corporate Responsibility for Environmental Protection (a set of standards set by the industry)."

The enzyme most commonly used in the paper industry is Xylanase, which acts on xylan (a substance that binds cellulose and lignin). There are two theories to explain how Xylanase acts. The first theory suggests it increases access of bleaching chemicals to pulp fibres by removing xylan. The uncoated fibres then become more susceptible to bleaching chemicals and lignin extraction. Says Pramod Bajpai, Coodinator, Research & Competency Division in Pulp and Paper, Thapar Centre for Industrial Research & Development, Patiala, "Enzyme treatment makes the pulp more porous and thus it requires less chemicals."

The second theory suggests the enzyme acts by breaking down xylan (and thereby the bonds between hemicellulose and lignin). Says B C Banerjee, from Central Paper Mills (cpm) in Surat, "Xylanase attacks xylan which leads to loosening of the lignin. Thus less quantity of bleaching chemical is required to remove lignin during bleaching." The enzyme is added to the washed pulp in what is called the high-density (hd) tower, where it takes 1-4 hours to act on the pulp.

Enzymes have been commercially available for almost ten years. The leading enzyme suppliers include Novozyme, Denmark, Iogen Corporation in Canada and a few local suppliers such as Biocon Limited in Banglore besides Textan Chemicals.

But most manufacturers and research institutions prefer domestic enzymes because they are up to 30 per cent cheaper. Moreover, most imported enzymes are developed for softwood while the Indian paper mills use hardwood such as eucalyptus. For imported enzymes, the process of receiving the product usually takes several months.

Enzyme pre-bleaching is not expensive -- all it needs is pumps for mixing the enzyme with the pulp and a hd tower. cpm spent Rs 2-3 lakh to install suitable infrastructure for the technology, according to Banerjee.

However, enzymes themselves are expensive -- costing Rs 200-250 per tonne of bleached pulp produced. cpm, for instance, spends Rs 84-105 lakk on enzyme for producing 42,000 tonnes of paper annually. Is using the enzyme feasible? The savings on chlorine and alkali would not cover the cost of enzyme, says Bajpai. But according to Chandrana, "Although enzymes are expensive products, they have more than compensated in terms of the environmental gains."

Unfortunately, only a handful of Indian paper mills are using this technology. This could be mainly because most mills are small with limited infrastructure and economic constraints. Says Chandrana, "The initial high cost of enzyme and long period for process stabilisation are some reasons why many mills do not want to go for enzyme pre-bleaching. However, they should be patient. It took us several months to stabilise the process."

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