Membrane filtration in the paper industry

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.

Tubular membranes

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

achieved.

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

liquors.

“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.