HSY operates two domestic water production plants in the Helsinki area, the Vanhakaupunki water treatment plant and the Pitkäkoski water treatment plant. Domestic water is made from surface water that is sourced from Lake Päijänne. Both plants use the same water treatment process, and the plants also supply water to Espoo, Kauniainen and Vantaa.
The microbiological quality requirements for domestic water were met in all but one control study sample, in which the concentration of intestinal enterococci was one colony-forming unit per 100 millilitres of water. The same sample also had a higher than usual heterotrophic colony count, which is used to measure the total number of bacteria, among other things, and is one indicator of the microbiological quality of domestic water. Enterococci of intestinal origin were not detected in the re-sample. The chemical quality requirements were met in all samples.
The iron quality target value was exceeded in six samples, but these do not have adverse health effects. The likely reasons for the high iron quality target values were low flow rates and precipitation in the pipelines. Microbiological quality thresholds were exceeded in one sample, in which three coliform bacteria per 100 millilitres of water were detected. Coliform bacteria were not detected in the repeat samples taken. Samples taken from taps in properties do not always demonstrate the quality of the domestic water supply.
The quality of the domestic water supplied by HSY is monitored on a weekly basis by determining microbiological and chemical parameters from the samples. In 2024, a total of 349 samples were taken from the domestic water delivered to Helsinki, 11 of which were also examined for heavy metals, among other things. In addition, 12 samples were taken from the water leaving the treatment plants, which were tested for nitrite content. 18 samples were taken from the raw water, which were tested for arsenic and uranium, among other things. In addition, 11 repeat samples were taken to verify the quality of drinking water.
In disturbances, we cooperated with the treatment plants and the situations were appropriately handled by HSY. In disturbance situations, the quality of the water was ensured with additional sampling, when necessary.
In accordance with the Decree on Domestic Water, a control study programme was drawn up in 2024 for HSY to monitor during 2025–2030, which includes, among other things, a sampling plan. The control study programme was been prepared in cooperation with the health protection authorities of Helsinki, Espoo and Vantaa and HSY.
