Metsä Fibre mills use water sparingly, even though in northern Europe they are surrounded by some of the world’s most abundant freshwater reserves. The company has systematically reduced process water consumption at all of its mills by investing in new technology and by enhancing and optimising processes, applying proactive environmental management strategies, environmental risk management tools, and other techniques.
Efficient water recycling has been the most effective way of reducing water consumption, with filtrates from the bleaching process recycled into earlier stages of the manufacturing process. Consumption of process water at the bioproduct mill in Äänekoski continues to fall substantially.
“The amount of process water used in making a tonne of pulp will drop from the present level of about 25 cubic metres to about 10 cubic metres as the bioproduct mill deploys a largely closed water cycle of a kind not seen elsewhere on this scale,” explains Äänekoski Project Manager Johanna Harjula.
STATE-OF-THE-ART CLEANING TECHNOLOGY
The highly enclosed water cycle of the bioproduct mill will generate less wastewater than ever, and its state-of-the-art wastewater treatment plant will also apply more treatment stages. This technology relies on multi-stepped biological purification and subsequent chemical processing. Some of the bioproduct mill wastewater will also be diverted to tertiary processing by chemical precipitation to reduce the non-biodegradable soluble COD content when necessary.
“Even though our current mills already deliver world-class performance in this respect, we shall be able to cut the volume of wastewater at the new facility by nearly half,” says Metsä Fibre Development Manager Ari Poukka.
The organic sludge extracted from wastewater will also be used in a new way by converting it into fuel at the EcoEnergy SF Oy biogas plant, a unique facility that will form an integral part of the bioproduct mill ecosystem.
“One new feature of the wastewater treatment plant is a division of the aeration basin into various stages with a view to improving the sludge. The first stage removes readily degradable COD content, with an adjustment of microbe strains for tackling more resilient organic materials at the next stage. This multi-stage process delivers better purification performance and makes the plant less susceptible to malfunctions,” Harjula explains.
MINIMISING WATERCOURSE IMPACTS
Even though the manufacturing capacity of the bioproduct mill will be nearly three times that of the previous facility, it will remain comfortably within the emission limits of the present environmental permit and European Union values for best available techniques (BAT). Effluent from the mill will have no impact on local watercourse standards, or on the use of these waters.
“Our environment load model from the bioproduct mill wastewater treatment plant for the environmental permit application and environmental impact assessment indicate no deterioration in the ecological state of the watercourse, even at maximum discharge levels,” Harjula explains.
Poukka suggests that technological progress will achieve even lower wastewater volumes in future, even though a completely closed water cycle is not yet possible with current knowledge.
“We are still some way from a process that would enable us to use all of the naturally occurring inorganic material in wood as part of the final product, for example. Trees lift various metals and other inorganic material from the soil as they grow, and this material must be removed from the manufacturing process to prevent it from accumulating as deposits in the machinery or contaminants in the end-product,” Poukka explains.
WORKING FOR SUSTAINABLE DEVELOPMENT
The Effluent Purification and Reuse (EPR) research theme at Aalto University Department of Bioproducts and Biosystems focuses on purifying industrialwastewater, improving purification processes, recovering nutrients, and recycling process water. Metsä Fibre has conducted joint research work with the university, studying such aspects as water recycling and the behaviour of chemicals. The most recent initiative involves investigating ways of using bioproduct mill side streams.
Departmental professor Olli Dahl explains that this research has helped the Finnish forest industry to radically reduce its water consumption.
“For example, we are now using only a tenth of the fresh water that was required for processes in the 1980s. We have learned, in particular, how to recycle water back into the process at points where there is no need to use fresh water,” Dahl explains.
Professor Dahl believes that pulp manufacturing has already achieved a comprehensively optimal standard of wastewater treatment using biological processes. The bioproduct mill in Äänekoski will be one of the first pulp industry installations to introduce a tertiary stage that further enhances wastewater processing, and it is now essential for future research to consider solutions that are environmentally sustainable in general.
“We are always looking at ways of further reducing water consumption, for example by returning even more wastewater to the process. There are nevertheless limits to this, insofar as water treatment involves using energy that in turn creates emissions to the atmosphere. Increased quantities of sludge must also be reprocessed. Even though all wastewater could in principle be purified into drinking water, the local pollution that this could cause in terms of massive energy consumption could have regional or even global repercussions,” Dahl observes.
He nevertheless concedes that the carbon-neutral energy surplus generated by a pulp mill will have numerous potential applications in future, and the Äänekoski bioproduct mill is setting a new standard in this field.
Department professor, Aalto University