Page 133 - Membranes for Industrial Wastewater Recovery and Re-Use

108 Membranes for Industrial Wastewater Recovery and Re-use

Table 3.9 Eftluents from a modern softwood kraft pulp mill (Myreen, 1993)
Source Flow COD AOX P Colour
(m3/ADt) (kg/ADt) (kg/ADt) (kg/ADt) (%)

Wood room (debarking) 2 5 9 10
Fibre line I 5 20
Bleach plant 30 25 1 .o 36 40
Recovery system 2 5
Spills etc. 5 8 30
From mill 40 48 1 .o 45 100
After AS treatment 40 22 0.5 20 100
ADt, air dried tonne.

paper mill, with a production of over 1 million tonnes p.a., shown in Table 3.13.
represents an example of modern paper making technology with low effluent
pollutant levels and low water consumption. The range of emissions can be
extremely broad, as shown in the case of the tissue mill also shown in Table 3.1 3.
Both of these paper grades use bleached chemical pulp as their major component.
Chemical pulp is much cleaner than mechanical pulp as a raw material due to
the many washing stages the pulp undergoes before entering the paper machine
water system. Mechanical pulp transports, amongst other things, significant
amounts of pollutants, mainly so-called “anionic trash”, to the paper machine
water circuit. The anionic trash is detrimental, causing operational problems in
the paper machine and reducing the efficiency of additives such as retention aids
(reagents added to aid the retention of constituents in the paper). In the case of
peroxide-bleached Norway spruce (Picea abies) TME’, most of the anionic trash
comprises anionic galacturonic acid-rich hemicelluloses, i.e. polygalacturonic
acids (Thorntonetal., 1993).
In mechanical pulp production the effluents result mainly from various
thickening processes (e.g. thickener and pressing filtrates) indicated as water
sources in Fig. 3.19. These water fractions are usually collected to form pulp mill
white water (circulation water). This water typically consists of wood-
originating substances (such as resin and fatty acids and other lipophilic
extractives), lignin, sugars, polysaccharides, simple organic acids and salts. The
concentrations are usually lower in the white water than in the different
filtrates. Some matrix characteristics are shown in Table 3.14, which indicates
the effect of peroxide bleaching, most often used for mechanical pulp, on the
levels of dissolved and colloidal substances (DCS).
In a modern integrated (mechanical) pulp and paper mill (Fig. 3.19) the
freshwater is taken in as wire section, this being the flat belt of metal or plastic
mesh on which the paper web is dewatered (point B in Fig. 3-19), shower water.
The water removed from the web in the paper machine is collected in the wire pit
(point C in Fig. 3.19), and forms the white water from the paper machine. In
most of the modern paper mills the white water is treated with a disc filter.
In older mills, and especially in Central Europe, white water is treated by
microflotation. The aim of the treatment is to recover fibre and to remove