Membrane filtration in the paper industry
Membrane filtration can be used in the pulp and paper industry to recover material as well as prevent waste by-products from entering the water course. In addition, depending on the application, membrane systems can be used to remove impurities or to concentrate and purify, either as an aid to recovering valuable raw material or to improve product yields.
With increasingly strict legislation on effluent discharge, and the need to
remove pollutants from the waste stream, effective filtration is becoming
more important to a number of process industries. Pulp and paper
manufacturing is no exception. The high cost of conventional effluent
treatment plants, either on site or at a municipal water treatment works,
makes membrane filtration a viable approach for many paper mills, where
large volumes of water are used and where recovery of water or by products
is possible. Indeed, the world’s largest tubular ultrafiltration membrane
effluent treatment plant, designed and manufactured by PCI Membrane Systems,
is installed at a Swedish pulp mill.
Membrane filtration can be divided into four broad groups, each determined
by the size of particle which can be retained by the membrane material.
These range from reverse osmosis which provides the finest level of
filtration, through nanofiltration and ultrafiltration to microfiltration
which uses the coarsest of membranes. Ultrafiltration, which can separate
particles up to a few tenths of a micron in diameter of different molecular
weights, is widely used in the forest products industry. Membrane
filtration offers a number of benefits to the pulp and paper industry.
Firstly, a small footprint can be achieved because of the high packing
density of membrane plants. This saves space on a mill production line.
Secondly, a wide range of membrane materials and geometries have been
developed by companies, including PCI. This means that a plant can be built
to match the process flow of a paper or pulp mill exactly.
for example are particularly robust. They are generally able to withstand
aggressive chemicals and solvent cleaners and can handle suspended solids
without blocking. This means that high throughput and performance can be
Over the past 15 years, PCI has been supplying membrane filtration plants to
solve a variety of problems in the pulp and paper industry. These range
from the removal of resins in bleach effluent on a pulp line, to the
recovery and reuse of lignosulphate fractions from calcium bisulphite spent
“Initially, the paper industry turned to membrane filtration as a means of
reducing effluent discharge,” explained Steve Morris, PCI’s forest products
specialist. “Colour, COD, BOD and toxicity can all be reduced by passing
waste water through a suitable filtration system prior to final discharge.
However, more recently the benefits of chemical recovery using membranes has
begun to be exploited by mills. Valuable paper coating materials, for
example, were being literally poured down the drain because there was no
cost effective method available for recovery. Membrane filtration can now
be used to significantly reduce chemical wastage.”
Membrane technology is being adopted increasingly where manufacturers are
closing their mill production process. As different pulp bleaching methods
have developed over recent years, membranes have proved to be a viable
technology for treating both TCF (total chlorine free) and ECF (elemental
chlorine free) bleaching streams. For TCF bleaching, membranes also provide
an effective method of closing the chelating stages and allow the filtrates
to be used as bleach plant wash liquors. In many countries, particularly the
USA and parts of South East Asia, raw water use represents a significant
cost. Paper making machinery generates large quantities of white water,
which can be recycled, thereby achieving major savings on water usage.
Water for recycling must be of a consistent quality. Any impurities in the
recycling stream could have a disastrous effect on product quality.
Membrane filtration offers a physical barrier to impurities and combined
with the wide range of membrane types allows efficient recycling of water.
Stora Nymolla is one of the world¹s largest manufacturers of TCF bleached
magnephite pulp. The Swedish mill is currently producing 300,000 tonnes of
pulp a year, of which two thirds is then used for its own paper making
process. With 300 tonnes of effluent being produced every hour, 50 per cent
reductions in the total mass of COD emitted from the mill were sought to
ensure that the plant discharge was to a high environmental standard. Not
only was the effluent to be cleaned up, it was also desirable to concentrate
the retentate sufficiently that it could be disposed of by incineration.
PCI developed a process plant after extensive pilot studies, which showed
that a polyethersulphone membrane would be most suitable to treat the
effluent from oxygen delignification. Two effluent streams had to be
treated separately, from hardwood and softwood processes, to ensure that the
retentate could be further processed. Separation is achieved using cross
flow filtration. Here, liquid flow occurs tangentially to the membrane
surface. This inhibits the deposition of material on the membrane surface,
which in turn improves the filtration efficiency. At Nymolla, PCI
experimented with low cross flow velocities, designed to keep energy
consumption to a minimum. This worked well for the softwood stream, but
during scale up it became apparent that fouling on the hardwood stream, even
at high cross flow velocities, meant that a different membrane material
would be required. PCI then set about developing a completely new membrane,
since there was no commercially available material with suitable
characteristics. The mill now operates a two line, thirteen stage
recirculation plant consisting of a total of over 1784 (4600m2) tubular
membrane modules. COD has been reduced to such a level that Nymolla were
awarded the prestigious Swan mark for their products.
The application of membranes at any particular mill is very site specific
and feasibility depends on a number of factors such as the products being
manufactured, the location of the mill, available options for effluent
disposal and availability of fresh water as a resource. Hence the
recommendation of experts is to seek advice from colleagues in the paper and
pulp industry, who have already had experience in implementing filtration
processes, as well as discussing your plant requirements at an early stage
with a company like PCI, who will then undertake feasibility studies and
pilot studies on the process stream.
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