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Limestone filtration reduces the environmental footprint of water treatment in the Helsinki metropolitan area

Tiina Roine (info a nordkalk.com), 24 February 2021

HSY (Helsinki Region Environmental Services) delivers drinking water for more than a million people in the Helsinki metropolitan area. Before high-quality drinking water flows out of the taps of a home in Helsinki, it has undergone a multi-stage treatment process, in which lime-based products are necessary. Thanks to limestone filtration, the environmental impact of water treatment has been reduced and the water quality has been improved during recent years.

The raw water of the Capital Region is surface water obtained from Lake Päijänne in Central Finland. Water is conducted through the 120 km long continuous rock tunnel to two water treatment plants in Helsinki.

During the treatment process, water’s pH, alkalinity and hardness is adjusted using lime products, which affect both its quality and corrosivity or aggressiveness,” said Heli Härkki, Unit Manager at Water Treatment Division of HSY.

HSY uses both crushed limestone and lime water in its water treatment process. Lime water is made from quicklime by slaking or adding water into it at the water treatment plant. Limestone alkalisation is carried out in the filter which contains 2/3 sand and 1/3 crushed limestone during the initial phase of the process. It is a chemical process in which carbon dioxide is bound in water to get bicarbonate and the pH is raised to 7. At the end of the treatment process, fine adjustment of pH to 8.4 is performed using lime water.

The limestone alkalisation has reduced the need for environmentally burdening quicklime because now lime water is used only once. The amount of carbon dioxide added during the process has also been reduced. In addition, we get water which has a more homogeneous quality,” Härkki said.

Positive environmental impact from limestone alkalisation was expected but the transition to this method became possible only in connection to the major improvement made to Vanhakaupunki plant in 2016.

We searched alternatives to lime water, and when limestone alkalisation produced good results in groundwater treatment plants, we wanted to try it also for surface water. After pilot tests, plant tests were performed, and new equipment for limestone filtration was designed based on  the test results. It automatically adds limestone to the filter in order to keep the pH of the filtered water constant,” Härkki said.

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Heli Härkki, Unit Manager at Water Treatment Division of HSY, at Pitkäkoski Water Treatment Plant. Its process will be improved by adding and automating the use of limestone-based products in the coming years.  

Population growth in the Helsinki metropolitan area increases water consumption and challenges us to optimise our processes. Climate change also affects the water treatment: organic matter content in Lake Päijänne has increased which, in turn, has increased consumption of chemicals and costs in recent years.

Life Cycle Assessment reveals environmental impact of processes

The operation of HSY is guided by a vision about the Helsinki metropolitan area as the world’s most sustainable urban region. To thoroughly understand the environmental impact of the operation, HSY has implemented Life Cycle Assessment (LCA).

“In Life Cycle Assessment, the entire life cycle of a product from sourcing materials to disposal is taken into account. The assessment shows where and when the largest environmental impacts of the process arise and which are the most important and most immediate areas for improvement,” project engineer Panu Laurell said. Environmental impacts are assessed in several categories: biodiversity, climate change, depletion of fossil fuels, etc.

Laurell has been involved in drafting common criteria for life cycle models for the water services of HSY. Such models were made for both water treatment plants and sewage treatment plant. The first life cycle assessment of water treatment was carried out as a master’s thesis for Vanhakaupunki Water Treatment Plant in 2017. Laurell was the supervisor of the thesis, which was written at Aalto University. At that time, a major improvement had been made at the plant and limestone alkalisation implemented.

“The assessment showed that environmental impacts of water treatment are caused especially by usage of activated carbon and disinfectant chemicals. Among chemicals used to adjust pH, the largest burden was caused by quicklime, the lifecycle of which begins at the time it is quarried and involves energy-intensive lime burning. A positive observation of the assessment was a remarkable 30% reduction in the environmental impact of pH adjustment based on limestone filtration,” Laurell said.

The use of limestone filtration will be more efficient

Thanks to the good usage experience, limestone alkalisation was also increased at another HSY water treatment plant in Pitkäkoski. However, so far, crushed limestone is dosed manually there and lime water is needed after filtration. The planning of the reconstruction of the Pitkäkoski plant is underway, including the installation of equipment for processing and dosing of crushed limestone. So the environmental footprint caused by preparation of drinking water for the inhabitants of the Helsinki metropolitan area continues to decrease.

Our operation involves constant development and innovation in cooperation with stakeholders. I am feeling very positive about the future. It is gratifying, for instance, that some areas for improvement are identified in the lime industry, such as use of renewable fuels and electricity instead of fossil fuels,” Laurell stated. 


Text: Anne Foley

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The next phases of the treatment process are ozonisation to destroy possible microbes and carbon dioxide supply to add alkalinity. Remaining organic matter is removed during activated carbon filtration. Then the water is disinfected using UV light, and bound chlorine or chloramine is added into the water to restrict microbe growth. Finally, “fine tuning” is performed in which the pH of water is adjusted with lime water and its alkalinity with carbon dioxide.
The next phases of the treatment process are ozonisation to destroy possible microbes and carbon dioxide supply to add alkalinity. Remaining organic matter is removed during activated carbon filtration. Then the water is disinfected using UV light, and bound chlorine or chloramine is added into the water to restrict microbe growth. Finally, “fine tuning” is performed in which the pH of water is adjusted with lime water and its alkalinity with carbon dioxide. 

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