Page 141 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 11 5

chemicals can interfere with the sensitive chemical equilibrium in the wet
processing.
Suspended solids-free process water or effluent is not always sufficiently clean
for recycling purposes. When the freshwater consumption is reduced the water
circulation system becomes enriched in dissolved and colloidal substances (DCS),
which causes several problems within the process operation and/or in the paper
product. High levels of dissolved organics result in enhanced bacterial growth in
the system, possibly producing odour problems in the paper product and
increasing BOD and colour levels in the effluent streams. Enrichment of
multivalent ions also produces problems of scaling (carbonates and silicates) and
corrosion (sulphates, chlorides, Fe2+ and A13+) which tend to arise after
extended periods of operation under conditions of low freshwater use. Also,
brightness reversion can be caused by dissolved inorganic materials such as
Fe2+. Problems of enrichment of the DCS in the white water system can be
substantially ameliorated through advanced purification methods, such as
ultrafiltration (UF), as will be shown later.
The stress imposed on water resources has forced the mills to seek ways to
efficiently treat their effluents to freshwater quality levels of purity. For example,
the total mill effluents treated by biological and membrane processes
(Bentley,1999; Webb, 1999) or evaporation (Stevenson, 1992) have been
successfully used to supplement freshwater supplies. However, coating colour
effluents have proved problematic to treat because they are not degraded by
biological processes and, as a result of this, chemical precipitation has been
widely employed. This produces a solid waste that must be landfilled. During the
1990s concentration of coating colour effluents by UF became more common,
mainly for economic reasons: UF costs have decreased whereas landfill disposal
costs have increased. In the IPPC directive UF treatment of the coating colour
effluents is now stated as a BAT technology.

3.2.5 Membranes in the pulp and paper industry
There has been increased interest in membrane filtration applications in the pulp
and paper industry over the last 15-20 years, for reasons already stated above as
well as in Section 1. Membrane processes offer a high level of purification coupled
with a low footprint and relatively low energy consumption, especially when
compared with the competing desalination technology of evaporation.

Existing full-scale membrane plants
The first full-scale reverse osmosis plant was installed in the white water system
of a board machine of Green Bay Packaging Inc. in the USA in 19 74 (Macleod,
1974). Later when the water balance of the board machine changed the RO
plant became redundant. Since the 1980s tubular module ultrafilters have been
successfully adopted for such purposes as concentration and fractionation of
spent sulphite liquor (Anon., 1982; PCIMembrane Systems, 1988), deresination
(Paterson Candy, 1987) and bleaching effluent treatment (Haagensen, 1982:
Okamoto et al., 1985; Jonsson, 1987; Wickstrom, 1997). An early example of