THE RIGHT WATER IN THE RIGHT PLACE
Process water quality directly impacts pulp quality, for example in terms of its brightness and purity. Before being introduced into the process, raw water is screened through a rough sieve. The screening prevents fish or other objects from ending up in the plant’s water circulation. The screened water is mechanically purified by filtering it through a layer of sand. Mechanically purified water can be used by the debarking department. The mechanically purified water is chemically purified by adding a flocculation substance to clump the organic matter together. Chemically purified water is used mainly as process water in, for example, pulp washing. Boiler water is water that has been treated in a desalination process. It is used in the recovery boiler and bark boiler, which have the strictest water quality requirements. The absence of salt reduces the risk of boiler corrosion.
Metsä Fibre’s water use is below the lower limit of the Best Available Technique (BAT).
Water usage is being optimised at every mill
Towards wastewater-free pulp production
Metsä Fibre aims to reduce water consumption at all its mills. The Rauma pulp mill uses the filtrates from bleaching in consistency controls instead of fresh water. Additionally, the fresh water has been replaced with circulated water at the debarking process. These measures have reduced wastewater by 3–4 cubic metres per tonne of produced pulp. At the Äänekoski bioproduct mill, secondary conden- sate is used instead of raw water in wood processing, and clean hot water in bleaching has been replaced with secondary condensate and recycled water from the drying department. Wastewater has been reduced by 3–4 cubic metres per tonne of produced pulp. At the Joutseno pulp mill, the use of raw water has been reduced by closing water circulations in, for example, bleaching, screening, and debarking. This has decreased the amount of wastewater by 6–8 cubic metres per tonne of produced pulp. The Kemi bioproduct mill has been designed to be particularly efficient with respect to water usage. The ini- tial months have focused on achieving the established operational targets. In terms of water consumption, the Kemi bioproduct mill is the lowest among Metsä Fibre’s mills.
In 2021, Metsä Fibre started a development project with the aim of creating a mill that generates as little wastewater as possible. The project has reviewed the quality requirements for water in the different phases of the process, looked for new ways to utilise wastewater and created a comprehensive vision for emission-free production. “We used the Äänekoski and Kemi bioproduct mills as the basis for our planning. In addition, we examined solutions in the mining industry, as the two industries create similar water fractions,” says Toivonen. Among other things, the project has looked for ways to reduce the salt discharge from production. The recovery boiler creates fly ash that contains salt, in this case sodium sulphate. Traditionally, fly ash is dissolved in water or condensate water and the resulting solution is transferred to the wastewater purification plant. “We have carried out laboratory experiments and pilot- scale tests to figure out how to reduce the amount of salt load that ends up in the wastewater,” explains Toivonen. According to Toivonen, a wastewater-free pulp mill is an ambitious goal. And the closer to the goal you get, the more challenging it is to achieve. “It is interesting to concentrate on a task that at first seems impossible but may end up being achievable after all.” •
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