ISSN ………… - International Network for Bamboo and Rattan

pressures usually between 4 and 8 bar in alkaline medium, performed at high consistencies of 20-30
per cent and temperatures of 90-140 0 C, depending on the alkali used. But as it is not a selective
lignin- degrading chemical, pulps cannot be bleached to high brightness exclusively with oxygen
without considerable attack on the polysaccharides, resulting in rather poor strength properties. Thus
the common practice in mill-scale bleaching today is to remove about one–half of the residual lignin
in unbleached pulps by oxygen, and to finish with conventional multi stage bleaching sequences such
as C-(EO)-H-D, D-C-(EO)-D, O-C-D-E, O-C-E-D-E-D or O-D-E-D, etc. The conventional bleach
plants employing sequences such as C-E-H-H will have to do only marginal changes in their industry
to incorporate the D-C-(EO)-D process so that pulps of high brightness can be produced at
considerably and relatively low cost. The Kamyr, Sund, Beloit Rauma, etc. have equipments for the
needful modifications (Goel et al. 1989) 5 . More than 16 plants throughout the world are employing
this process, the first having been founded in South Africa in 1970. The main practical advantage of
oxygen bleaching is the fact that the effluents from the oxygen step can be processed within the
normal kraft recovery system. In the context of increasing air and water pollution from the traditional
bleaching systems, oxygen bleaching has proven its potential as the more environmentally friendly
system.
4.3.2. Colour reduction in the effluent
Some of the technological changes as suggested by Mall et al. (1989) 52 , for reducing toxicity and
colour of the effluents are: elimination or minimum use of sulphur containing pulping processes and
chlorine containing bleaching processes; extended delignification; modification of bleaching
sequences which includes elimination of caustic extraction stage which is the major contributor of
colour; use of oxygen bleaching, chlorine dioxide bleaching; use of anthraquinone and lower sulphur
high-yield pulping processes; total recycle concept, and reduction in bleached pulp brightness level.
4.3.3. Bio-bleaching
Microorganisms or enzymes can cause modification of the lignin in the pulp rendering it more
accessible to the bleaching agents, thus the application of biotechnology in bleaching can reduce the
consumption of bleaching chemicals which can further help to reduce the effluent pollution and
associated toxicity problems. After the alkaline extraction stage, unbleached kraft hardwood pulps
have been treated with xylanases of Escherichia coli clone to yield pulps which are bright and have a
kappa number 54 per cent lower than the corresponding control samples (Jurasck et al. 1987) 493a . In
addition, the viscosity of pulps was also higher compared to the control. Begasse CTMP and CMP
pulps on treatment with a crude enzyme extract resulted in 2-6 unit improvement in brightness
(Ramaswami and Ramanathan 1989) 66 . There is still scope to improve its performance by
purification of the enzyme and identification of the optimal treatment conditions like temperature and
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