The water treatment process in brief

Lake water from Päijänne is purified into supply water at the Pitkäkoski and Vanhakaupunki surface water treatment plants. The water treatment process is similar at both water treatment plants.

First, a chemical coagulant is added to raw water, which precipitates the organic matter in the water. The resulting precipitate is stirred, causing the precipitate flakes to collide with each other and increase in size. The precipitate flakes settle to the bottom of the basin and the rest of the smaller precipitate remains in the sand-limestone filters. 

Any microbes in the water are destroyed with ozone, which also improves the smell and taste of the water. After this, carbon dioxide is supplied into the water, which increases the alkalinity of the water and thus reduces corrosion. 

The remaining organic matter is further removed by activated carbon filtration, after which the water is disinfected with the help of UV light. Finally, bound chlorine or chloramine is added to the water to limit microbial growth in the water distribution network. The pH of the water is adjusted with lime water and the alkalinity with carbon dioxide.

HSY has one groundwater intake plant, which is Kuninkaanlähde in Tuusula. The water from Kuninkaanlähde goes to a small amount of residents in northeast Vantaa.

At the Kuninkaanlähde groundwater intake plant, carbon dioxide typical of groundwater is removed by aerating it. To improve the technical quality of the water, the water passes through limestone filtration. Disinfecting water with ultraviolet light minimizes the use of disinfectant chemicals. Before leading supply water into the network, a small amount of a bound chlorinated compound called chloramine is added to the water, so that the water remains of good microbiological quality in the network.