World-class pulp mill goes green with membrane ultra-filtration, Solutions!, Online Exclusives, February 2004

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Management at Stora Nymölla knew that changes were in order to maintain compliance with increasing environmental legislation. In a joint project between PCI Membranes and MoDo Chemetics, they installed an ultrafiltration plant to reduce COD from the oxygen stage of the bleach plant by 50 percent.

The installation at Nymölla is a two-stage ultrafiltration plant fitted with three full size (3.66 meter) modules per stage, giving a total of 15.6 sq.m membrane area. The plant features seven stages for softwood and six for hardwood. Each softwood stage contains 152 modules and each hardwood stage 120 modules.

StoraEnso Fine Paper’s Nymölla is the world’s largest manufacturer of bleached magnefite pulp. Due to greater awareness of environmental concerns, Nymölla had to make changes if it was to retain market confidence and maintain compliance with increasingly severe legislation. In a joint project between PCI Membranes and Aker Kvaerner, management decided to install an ultrafiltration plant to reduce COD (chemical oxygen demand) from the oxygen stage of the bleach plant by 50 percent. After 1,000 hours of trials, the development of a custom membrane, and different operating regimes tailored to different feed types, the plant has achieved its goals with both cleaning chemical consumption and power consumption within the limits guaranteed. Membranes demonstrate here that they can perform under extremes of pH and temperature, and have helped this mill to meet the conditions required for issue of the valued Swan mark.

Production of pulp started in 1962 at Nymölla, and actual paper manufacture began in 1972. At present, the mill’s capacity is 300,000 metric tons per year, of which 200,000 metric tons is used in the integrated paper mill for the manufacture of high quality printing paper. The remaining pulp is sold to other paper mills. Two types are produced: a short fiber pulp made from beech and birch, and a long fiber pulp made from pine and spruce.

With increasing global pressure for a greater awareness of environmental concerns, Nymölla found itself having to change its operations in order to retain market confidence and to maintain compliance with increasingly severe environmental legislation.

Wood milling, pulp- and papermaking facilities are notoriously difficult to operate along “green” lines. From just the oxygen bleach stage of the pulping process at Nymölla , 300 metric tons of effluent is produced per hour (made up of 135 metric tons per hour from hardwoods and 165 metric tons per hour from softwoods). The average chemical oxygen demand (COD) of this effluent is approximately 10g/liter, meaning a total of about 3 metric tons of COD is produced every hour.

Setting targets
Marketplace demands to strengthen its “green” credentials led Nymölla to set two principle goals to reduce COD emissions: first, to achieve a 50 percent reduction in the total mass of COD emitted from the mill from the oxygen bleach stage; and second, to highly concentrate the remainder, generating a 50 times volumetric concentration of the effluent stream.

Management decided that ultrafiltration was the best way to achieve these goals, and PCI Membranes, of Hampshire, UK, was invited by its Swedish agent Aker Kvaerner to carry out trials at Nymölla . In May 1993 trials got under way, using a multi stage recycle (MSR) pilot plant. These trials indicated that tubular membranes would provide a solution. To resolve these, in October 1993 a second unit (UF15 pilot plant) was installed, and this was far better suited to the operating conditions.

This installation was a two-stage ultrafiltration plant fitted with three full size (3.66 meter) modules per stage, giving a total of 15.6 sq.m membrane area. Earlier trials had indicated the need for a 4000 Dalton (4kD) cut-off membrane, and technicians fitted PCI’s polyethersulphone membrane. The plant could operate in batch or continuous mode, at a stable concentration or VCF (volumetric concentration factor) within each stage. The new plant was well instrumented and proved itself ideally suited for obtaining data for a full scale design, although containing a relatively small membrane area.

The two streams of effluent (hardwood and softwood) needed to be treated separately, allowing each stream to be processed by a membrane designed to manage the different chemical characteristics.

Trial results lead to new membrane
The trials were configured to produce the following data:

  • pressure drop as a function of concentration
  • retention as a function of concentration
  • flux as a function of concentration
  • effectiveness of cleaning
  • power consumption
  • degree of fouling
  • quality of product
  • probable membrane life.

Three months of continuous trials, operating 24 hours a day, were carried out on the softwood effluent using the polyethersulphone membranes. During this period the mill tested a number of different cleaning regimes, and project leaders finally decided to use a variable cleaning frequency. As is common with ultrafiltration plants, front end fouling was evident, meaning that earlier stages were in need of more frequent cleaning. The final design resulted in early stages being cleaned daily, and final stages being cleaned approximately every four days.

During the early part of the trials, the hardwood effluent stream used the same membranes as the softwood effluent stream. However, fouling was far greater than expected, even when a high cross-flow was used. To address this, a smaller pilot plant was taken to site, and other membranes trialled using the new plant, while the main plant continued to produce data on the softwood effluent stream.

It became clear that no standard membrane was suitable, so a new membrane had to be developed. The target was to produce a much more hydrophilic membrane that showed minimal fouling. A new membrane was developed in less than two months, and trials were able to continue.

Once all the necessary data had been collected on the softwood effluent, the new hardwood membranes were tested with the UF15 pilot plant. They worked exactly as anticipated. After fine tuning the cleaning regime it was determined that all hardwood stages of the full scale plant could be operated for two days between cleans.

Problems of continuous running
But the trials, while largely successful, did not go entirely smoothly. Because of the need for long runs of 4 or 5 days non-stop, the pilot plant was required to run overnight and unattended. When operating at low concentrations, this was not a problem, but while carrying out an overnight run at the target goal of 50 VCF, and when the retentate flow was very low, the set conditions drifted resulting in over-concentration. This eventually caused the plant to stop, and when they had cooled, the membranes were blocked solid. Even this had a benefit: it demonstrated the strength and resilience of the membranes, which could be unblocked without damage, and the trials continued.

Because of the high flows involved one of the targets was to design a plant that could operate at low cross flow velocities in order to keep energy consumption down. For the softwood stream, trialled first, it did prove feasible to operate at a substantially lower cross-flow than would normally be used for this type of membrane. However, it was not possible to operate the hardwood line at the same low cross-flows.

Different requirements
The higher cross-flow employed on the hardwood line, and subsequent higher pressure drop, resulted in the plant having to incorporate twin entry endcaps in order to shorten the path length through the module. This led to the addition of recycle pumps on the hardwood line of 432 m3/hr as opposed to the 164 m3/hr pumps used on the softwood line, despite the fewer number of modules in each hardwood stage.

Once Aker Kvaerner had secured the order from Nymölla, the plant was designed and built jointly by Aker Kvaerner and PCI Membrane. Installation and commissioning was also carried out jointly. The final plant design was for a total of 13 stages (seven for softwood and six for hardwood). Each softwood stage contains 152 modules and each hardwood stage 120 modules, and the time schedule for the process of testing and installation covered more than two years.

The plants are designed for continuous running. A “carousel” cleaning system has been employed, so that any stage can be taken off line to enable it to be cleaned without interrupting the effluent processing. This was a critical requirement, as the build up of effluent during the down time to clean the entire plant would have been unacceptable.

Flexibility required
A high degree of flexibility was also required, as there is a significant variation in both the feed volume and concentration. The plant has had to cope with CODs in the range of 6 - 14 g/l, and flows of up to 50 percent either side of the mean.

As well as guaranteeing the capacity of the plant and the separation performance of the membranes (in terms of COD retention), Nymölla also required PCI/Aker Kvaerner to guarantee the following:

  • membrane lifetime – the membranes in both the hardwood and the softwood plants were guaranteed to perform at their specified level for a minimum of fifteen months. A number of factors affect lifetime, principally temperature, pH, resistance to bacteriological attack,chemical resistance, and operating pressure. Since the plants were commissioned in January 1995, the guarantee has been met, with some membranes lasting more than eighteen months.
  • cleaning chemical consumption.
  • power consumption - the (estimated) absorbed power figures for the hardwood and softwood plants are 300kW and 700kW respectively. This corresponds to a kW per m3 permeate produced ratio of approximately 3.9 and 1.5 for the hardwood and softwood respectively - based on a pump working efficiency of 65 per cent.

Fine tuning changes
After some early problems with the softwood line capacity, the team made some changes. The plant was allowed to operate at higher temperature (up to 82°C), eliminating the need for a heat exchanger before the UF plant. Technicians determined that the rapid cooling in the heat exchanger was causing some precipitation of extractives, leading to front end fouling of the UF plant. The pH of the softwood effluent was also controlled to keep it above 11, again reducing fouling of the membranes.

Once the plant had been in operation for a period of time, there was an opportunity to look at further modifications to membranes in order to develop further improvements for the hardwood line. Changes in membrane chemistry produced a replacement which, although slightly more open than the original (6kD cut-off as opposed to 4kD cut-off), offered significantly higher fluxes. This potential increase in capacity of the hardwood plant led Nymölla to investigate possible uses for the “spare” membrane capacity. Operational for more than eight years, the plant now comfortably processes approximately 400m3/hr of effluent, producing a concentrated stream of 6m3/hr, which is a concentration factor of approximately 60 times.

There were several other applications within the pulp and paper mill where membranes could be used, including:

  • nanofiltration of the ultrafiltration permeate to further reduce the COD loading
  • nanofiltration of chelating (Q) stage effluent in order to recover EDTA and manganese. (This is a relatively straightforward application for membranes due to the much lower pH and relatively low COD.)
  • ultrafiltration of peroxide (P) stage effluent. This has a similar COD loading to the oxygen stage, but also contains a high level of hydrogen peroxide.
  • ultrafiltration of the fiber line wash waters.
  • nanofiltration of the evaporate condensates for reuse in chemicals makeup (an area Nymölla are presently conducting trials into using two pilot plants).

System success
In all, the team carried out close to 1,000 hours of trial work, developed a new membrane in only two months, and tailored two different cleaning regimes to the two feed types. The plant at Nymölla has now been in operation for several years and continues to achieve the necessary reduction of COD.

Membranes have shown they can perform under extreme conditions of pH and temperature, the type of conditions usually found in this environment. They have already helped one mill to meet the conditions required for issue of the valuable Swan mark.

World-class pulp mill goes green with membrane ultra-filtrat
World-class pulp mill goes green with membrane ultra-filtration, Solutions!, Online Exclusives, February 2004

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