The raw water from Päijänne is treated into domestic water in the Pitkäkoski and Vanhakaupunki water treatment plants. The water treatment process is similar in both plants.
First, ferrous sulphate is added to the raw water. It precipitates the organic matter in the water at low pH levels. The precipitation is then stirred so that the small sediment particles bump into each other, growing in size. After this, the precipitation is separated from the water in a horizontal sedimentation tank and sand filters.
Before disinfection, the water’s pH is increased with lime water. The possible microbes in the water are destroyed with ozone, which also improves the water’s odour and taste. Next, carbon dioxide is fed into the water. It increases the water’s alkalinity, thus decreasing corrosion.
Remaining organic matter is removed through a two-phase activated carbon filtering process, after which the water is disinfected with UV light. Finally, bound chlorine, i.e. chloramine, is added to the water in order to limit the growth of microbes in the supply network. The water’s pH is adjusted with lime water and its alkalinity with carbon dioxide.
The picture upper is from Pitkäkoski water treatment plant, the lower picture is from Vanhakaupunki water treatment plant.