REDUCTION IN ENVIRONMENTAL LOADS BY USING ENZYME DURING BLEACHING OF CHEMICAL PULPS

Abstract

The Indian paper industries are using bleaching sequences where chlorine is a predominant bleaching chemical. As a consequence the bleach plants have become a major source o f environmental pollution contributing to high chloroganics release with high BOD, COD, AOX, color loads, various organic & inorganic compounds. There have been continuous efforts to minimize their generation, during pulp bleaching. The enzyme pre-bleaching is one of such option, which is expected to decrease the bleach chemical demand and the chloroorganics load in the pulp and paper m ill effluent. The xylanase is also considered as bleach boosting enzyme. It can be successfully used as a pre-bleaching step with existing conventional, ECF and TCF sequences. The aim of the present work is to look at enzyme pre-bleaching of pulps followed by bleaching. Though several studies using various xylanases have been carried out for pre-bleaching hardwood and softwood pulps, the data for non wood pulps is quite scanty. It is not apparent how xylanase enhances the bleaching of pulp, as xylan-degrading enzyme is not expected to act directly on residual lignin. It is not clear how the enzymatic process works due to variable response o f different pulping and bleaching parameters. Xylanase may act by hydrolyzing xylan present on the pulp in different forms. Moreover, the mechanical. strength properties o f enzyme pretreated hardwood & softwood pulps also give contradicting, results for different enzymes. Very little data is available on agricultural residue pulps, which are important in Indian context due to decreasing wood resources. Very few researchers worked on wild species o f sugar cane which can be very useful to conserve wood resource. Specific analysis for the above parameters is necessary and the results have to be carefully quantified to give more flexibility in xylanase pretreatment technology o f the pulp. Both the pulps, non-oxygen mixed hardwood and bagasse, were bleached to a target brightness o f 85% ISO using different bleaching sequences i.e. DED, D50/C50ED and ODED. Results o f mixed hardwood pulp indicate that total bleach chemical consumption for different sequences followed the order DED>D5o/CsoED >ODED. BOD and COD also follow the same order but the AOX is lowest for ODED sequence and the highest for D50/C50ED sequence. The substitution o f chlorine by chlorine dioxide reduces the AOX values of the effluent, the BOD and the COD values increase and the strength properties are comparable for all the three sequence, though the D50/C50ED sequence shows the lowest strength properties. The minimum drop in viscosity was found in DED while maximum was observed in D50/C50ED mixed hardwood pulp. Bagasse pulp also shows similar behavior with regard to bleach chemical consumption, BOD, COD, AOX and strength properties as observed with mixed hardwood pulp. Bleached mixed hardwood pulp is stronger than bleached bagasse pulp o f same brightness when bleached with same bleach chemical sequence. The pollution load o f bagasse pulp bleached with different sequences was more compared to pollution load of mixed hardwood pulp from similar sequence. The mixed hardwood pulp is more responsive to oxygen delignification than bagasse pulp. The oxygen delignified DED bleached mixed hardwood pulp of 85% brightness (medium brightness) required 48.45% less bleach chemicals compared to the non-oxygen pulp. The BOD, COD and the AOX values of the combined effluents were also substantially lower. The mechanical strength properties showed no change at the same level of beating. The oxygen delignified bagasse pulp showed similar behavior as oxygen delignified mixed hardwood pulp at medium brightness levels.. Bleaching o f oxygen, delignified bleached, pulps using ,OD5o/C5oEpD, sequence yields pulp, which is weaker in comparison to oxygen delignified DED bleached pulp for both the raw materials. The BOD, COD and the AO X o f the combined effluents were higher for D5o/C5oEpD sequence. The higher values o f environmental parameters and higher pulp degradation observed with OD5o/C5oEpD bleached pulps is due to longer bleaching sequences required to reach 90% brightness than, 85% observed with DED sequence. Oxygen*delignified bleached mixed hard pulp>is a stronger pulp, than \ bleached.bagasse . pulp o f same brightness when bleached w ith sim ilar bleaching sequence for both medium and high brightness levels. The range o f drop in viscosity of bagasse pulp 7.7 -17.30% was observed. The minimum drop was observed in enzyme pre-treated DED sequence and maximum was in OD50/C50EPD sequence. The pollution load o f bagasse pulps bleached with different sequences was more compared to pollution load o f the mixed hardwood pulp from similar sequences. The enzyme pre-bleaching is possible fo r both mixed hardwood and bagasse pulps with and without oxygen delignification. The optimized conditions o f enzymes pre-bleaching are different for different pulps. Oxygen delignified pulps required lower enzyme dose compared to their respective non-oxygen pulps. There is a substantial decrease in bleach chemical consumption to obtain 85% brightness pulp (medium) brightness, the order o f effectiveness o f enzyme pretreatment on mixed hardwood pulp is DED>ODED>D5o/CsoED. The bleaching behavior o f enzymatic prebleached bagasse pulp is quite different from enzymatic pre-bleached mixed hardwood pulp. AOX value o f combined effluent for both the pulps, show a decrease of 23 - 42% for different bleaching sequences. The BOD, COD and BOD/COD o f combined effluents are higher for enzyme pretreated pulps compared to non treated pulps. The enzymepretreated bleached pulps show marginal increase in tensile, tear and burst index at same target brightness. The impact o f enzymatic pretreatment on bleach chemical consumption o f oxygen delignified pulps of medium brightness is more compared to the respective non oxygen pulps but the impact is lower for high brightness pulp. The BOD, COD and AOX values follow the same trend as non oxygen pulps i.e. decrease in AOX, increase in BOD, COD and BOD/COD ratio with minor change in mechanical strength at the same level o f beating .The results are comparable with the results reported in the literature. The AOX load in XDso/CsoEpD bleached bagasse pulp is 30% reduced in comparison to control pulp. The above studies clearly indicate the effectiveness o f xylanase pre-bleaching^of mixed hardwood pulp and bagasse pulps not only as a means for improving environmental perfqYni/nance of. bleach plants but also have significant reduction in bleach chemical consumption with marginal increase in strength properties. Many researchers have worked on xylanase and reported the potential and benefits o f '■''t xylanase aided bleaching. Experiments were planned to systematically evaluate howixylanase acts in pre-bleaching to elucidate the mechanism o f xylanase aided bleaching o f non .,o|ygen and oxygen delignified mixed hardwood and baggase pulps. ‘ , It is already reported in literature that xylanase tends to hydrolyze surface xylan and remove hexeneuronic acid more due to its higher accessibility. The effectiveness of xylanase aided bleaching is relatively higher in oxygen-delignified pulp where part o f xylan and lignin could have been redistributed to the outer fiber surface. This is also confirmed by reduced pentosan, content that showed that xylanase hydrolyze the xylan. As reported in literature, it.has been observed that after enzyme pretreatment, color o f effluent shows the xylanase attack on chromophoric groups. It is further confirmed by a minor decrease in kappa number, presence o f lignin in enzyme pretreated effluents (infrared spectroscopy) and increase in UV absorbing materials detected (measuring absorbance at 280° A). The increase of color o f the effluent with decrease in kappa number correlates well with stronger lignin peaks detected in infrared spectrum, more UV absorbing material and higher reducing sugars in enzyme pretreated effluents. No change or marginal change in tensile index in enzyme pretreated pulps suggests that enzyme does not cause a change in fiber bonding. This is further confirmed by retaining o f peaks associated with cellulose in enzyme pretreated pulps by infrared spectroscopy. The water retention value, which is a measure o f fiber swelling, shows a sharp increase in enzyme pretreated bleached (beaten) pulp suggesting that the enzyme causes fiber swelling. The infrared and UV spectroscopy studies also indicate xylanase attack on LCC or ligninhemi- cellulose bond. The substitution D50/C50ED bleaching o f chemical mixed hardwood and bagasse pulps show that the environmental loads particularly with regard to AOX are high. The modified bleaching sequences (Dso/CsoEpD) in some cases meet the AOX stipulations but they are found to be economically at a disadvantage. The enzyme pre-bleaching using xylanase is effective in reducing bleach chemical demand in all cases with reduced AOX loads and increased BOD and COD loads. The environmental and economical evaluation shows that enzyme pre-bleaching with sequences like D50/C50ED can be adopted. The response o f enzymatic pre-bleaching to oxygen delignified mixed hardwood and bagasse, pulps were found , to be positive with reduction in bleach chemical demand and associated reduction in AOX loads. However adoption o f these options w ill require addition o f costly oxygen delignification step. The cost advantages in such cases may. be obtained if pulp is bleached to, higher brightness levels (90% ISO). Bleaching o f oxygen delignified enzyme, pre-bleached pulps show that bleaching sequence like D50/C50ED was effective for both mixed hardwood and bagasse pulps though the magnitude o f brightness gain becomes smaller with high pulp brightness, there remain a potential for chemical savings (10 %). The experiments on mechanism o f xylanase pre-bleaching clearly show that surface xylan is not the sole, target substrate for xylanase. Enzyme seems to be attacking the lignin carbohydrate complex (LCC) or is hydrolyzing the bond between lignin and hemicellulose and does not cause a major change in fiber