Page 131 - Membranes for Industrial Wastewater Recovery and Re-Use

106 Membranes for Industrial Wastewater Recovery and Re-use

mills. In a non-integrated paper mill, the purchased pulp is the major fibre source
and is delivered at 90% solids. In an integrated paper mill, pulp from the pulp mill
is stored at about 12-16 wt% solids. In both cases the web leaves the press
section at about 40-46% solids content. Consequently, there is a net demand for
mill water in a non-integrated paper mill and a net surplus of water in an
integrated mill. However, exchange of water between the pulp and paper
operations in an integrated mill can change the situation, even though such
exchange is nowadays minimised. The production of mechanical pulp always
exerts the greatest water demand.

3.2.3 Effluent volumes and quality
Over the years production techniques have been modified and made more
efficient and cost effective. Thus the process itself, the paper grade and the
technical age of the mill are all key factors when the effluent volumes and quality
are considered. They also naturally depend very strongly on the pulping process
and on the type of raw materials as well as on the paper grade to be produced
(Table 3.8).

Pulping
In debarking significant amounts of bark, fibres and dissolved organic
substances are released into the water. The debarking effluent is also toxic, since
some of the compounds found in bark are water soluble and heavily toxic. Thus
dry debarking or separate treatment is commonly practised, when possible.
In the Kraft chemical pulping process (Fig. 3.18) dissolved organic substances,
mainly polysaccharides and lignin, are released in the cooking process. Organic
lignin compounds with spent white liquor (the main components of which are
Na2S and NaOH, the pH level being around 14) form the black liquor that is
removed from the washing stage. In the bleaching stage, the first alkaline
extraction, El, extracts organic lignin compounds along with polysaccharides
from the pulp. When chlorine-based bleaching agents, such as elementary

Table 3.8 Examples of major raw materials used for manufacturing of different types of
paper (EIPPCB, 2001)
Paper grade Major raw materials useda
Newsprint No added new fillers: few added chemicals: sometimes pigments added:
very little colour: uses mechanical pulp or RCF
Kraft wrapping No added fillers: coloured: uses mainly unbleached kraft pulp
Kraftliner or testliner No added fillers: uses unbleached kraft pulp and RCF or only RCF
Board All fibre types including RCF
Writing and printing All grades of fibre. mainly bleached: fillers: sizes: colours: brighteners:
may he coated
Tissue No added fillers: wet strength additives: uses chemical pulp + R(3F in
different mixtures
Speciality papers The pulp used can vary considerably in type and quality
a RCF, recovered fibre.