Water in the Häränvirta river flows towards Metsä Group’s Äänekoski mill site at the point where Lake Keitele and Lake Kuhnamo meet. The surface seems calm, but the nearly horizontal spar buoys reveal the strength of the current. Wood processing began at the Äänekoski mill site in the 1800s. Since then, operations have expanded to the extent that the area is now home to an entire bioeconomy hub, unique even on a global scale. In Finland, forest industry sites have traditionally been located along waterways. This is partly because proximity to waterways facilitated the transport of wood raw material. Even though waterways are relatively insignificant in terms of wood logistics today, water is still an important resource for the mills. In the middle of the Äänekoski mill site is Metsä Fibre’s bioproduct mill, which produces 1.3 million tonnes of bleached softwood and hardwood pulp and a significant amount of other bioproducts annually. The mill, which takes its raw water from Lake Keitele, uses water in every production phase. The debarking department uses water to defrost wood, for washing and to rinse the equipment. At the pulp mill, water is needed to wash the pulp, for sorting and as a medium to facilitate the transfer of pulp and chemicals. In addition, all departments need cooling water to handle the heat load created by the process.
“Our aim is closed water circulation. The more water we can recycle, the closer we are to this aim,” says Matti Toivonen , VP, Production Technology at Metsä Fibre. Most of the pulp mill’s washing water comes from the drying department or evaporation plant. The secondary concentrate and the filtrates from pulp washing are reused multiple times. The pulp mill’s washers operate on a countercurrent principle, which reduces the need for fresh water even further. This works by using the dirtiest water, which has already gone through several washing phases, in the first washing phase and the cleanest water in the last phase. “This is smart for the process as well. Recycling the washing water prepares the pulp for the conditions of the next process phase and reduces the need to use chemicals,” says Toivonen.
Read more about the ways we reduce our water usage.
Metsä Fibre aims for resource efficiency in all its operations. This is why it is important to find a balance between water and energy use and the quality of the end product.
High utilisation reduces the need for water
Increasing the efficiency of water use requires a systematic review of the processes. In addition, phases where recyclable water can escape must be identified. “For example, the clean sealing water used in the pulp pumping used to end up down the drain. We are now using these water fractions more efficiently. Sometimes, water fractions are created in a different location in the mill to where they could be utilised. In these situations, we have to try to find the most reasonable solution in terms of the big picture,” says Ala-Kaila.
Modern mills use far less water
Pulp production began in Äänekoski in 1937. The bio- product mill, which started in 2017, is the fourth pulp- producing mill in the integrated mill area. The mill’s production capacity is many times greater than that of its predecessors. However, it uses significantly less water relative to production volume. “Until the 1970s, pulp mills consumed over 100 cubic metres of water per tonne of pulp produced. The design value for the Äänekoski bioproduct mill is 15 cubic metres and for the Kemi bioproduct mill just 10 cubic metres of raw water per tonne of pulp produced,” says Kari Ala-Kaila , Director, Technology at Metsä Fibre. The design value refers to the total amount of wastewater created when the mill produces the planned amount of bioproducts. Metsä Fibre’s water use is below the lower limit of the Best Available Technique (BAT), and the aim is to reduce it even further. One of the company’s sustainability goals is to reduce the amount of process water used per tonne of pulp produced by 35 per cent by 2030, compared to the 2018 level.
Steady running of production plants is important when you want to use water more efficiently. Incorrect equipment settings or potential hardware failures lead to losses, so both the settings and the condition of equipment are monitored regularly. Planned preventive maintenance and shutdowns support good water utilisation. Each Metsä Fibre pulp and bioproduct mill has a concrete goal for reducing water use that takes into account the mill’s age and other unique characteristics. Mill-specific solutions and good practices are compared and shared between production sites.
“The operators decide how much water is used in each phase of the process, so it’s important that they understand how their choices impact the overall outcome,” says Ala- Kaila.
Risks must also be identified
Metsä Fibre aims for resource efficiency in all its operations. This is why it is important to find a balance between water and energy use and the quality of the end product. “Reducing water and energy consumption go hand in hand. The process water must be between 60 and 80 degrees Celsius. If we always replaced process water with raw water, it would consume a disproportionate amount of energy to heat it,” explains Toivonen. However, recycling water in closed circulation may lead to the accumulation of harmful substances in the process. For example, inorganic salts might form sediments that clog up the washers or cause boiler corrosion. Recycling water can also lead to quality issues with the end product, such as unwanted residues. “While we aim to reduce the use of water at our mills, we must also make sure that the quality of the end product remains high,” says Toivonen.
New solutions enhance process water recycling
RESPONSIBLE WATER USE
The modern technology at Metsä Fibre’s production plants enables water to be purified and recycled more efficiently. For example, water consumption in bleaching has been reduced thanks to more efficient washers. An entirely new kind of technology has been implemented to recycle certain water fractions. A chlorine processing plant, built in connection with the recovery cycle, makes it possible to recycle the alkaline filtrates created in bleaching. Reducing water use also requires training personnel and establishing correct operating methods for the mills.
All Metsä Fibre’s mills are located in water-rich areas and only use surface water from local waterways. The activities do not impair the availability of water to other parties, such as residents or agriculture. All the process water discharged is thoroughly purified before being returned to nature.
Process water is recycled several times
The raw surface water from Lake Keitele is purified before being introduced to the process. It is then recycled and reused. This reduces the need for fresh water, lowering the total water consumption.
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