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Wastewater treated with UV disinfection meets the threshold values set for bathing water.

Wastewater treatment plants

Wastewater treatment began in Lahti in 1910, and wastewater treatment is subject to an environmental permit. Our area of operations has four wastewater treatment plants: Kariniemi, Ali-Juhakkala, Nastola and Hämeenkoski.

We treat approximately 90% of the wastewater in our area of operations at the two largest wastewater treatment plants, Kariniemi and Ali-Juhakkala.

The wastewater treatment process is continuous and goes on non-stop every day. The wastewater circulates at the treatment plant for an average of one day before it enters the Nikula equalisation tanks. There it stays for about two days, after which it is further disinfected with ultraviolet lights before it enters the river Porvoonjoki.

The first phase of the wastewater treatment process

The cleaning process starts with the mechanical phase, in which coarse solids are removed from the wastewater by screening. Sand, gravel, fat and other mixed waste are then removed from the wastewater. The treatment plant also receives wastewater from sparsely populated areas, i.e. septic tank sludge, to go through the mechanical phase.

The second phase of the wastewater treatment process

Phosphorus is chemically removed from the wastewater after the mechanical treatment process. Iron salt, i.e. ferrous sulphate, is added to the wastewater, which precipitates phosphorus from the water in separable form.

The pH value of the wastewater is adjusted if necessary, and this is done using soda or caustic soda in our treatment plants.

The third phase of the wastewater treatment process

The third phase, the biological treatment of the wastewater, involves using the bacteria contained in the wastewater. The biological treatment takes place in aeration tanks in wastewater treatment plants, where the aim is to create optimal conditions for bacteria to grow and reproduce. Bacteria form activated sludge, which consumes organic matter from the wastewater. More than 70% of the nitrogen in the wastewater is removed as a result of the bacterial growth and activity.

The final phase of the wastewater treatment process

As a final step in the process, the effluent is disinfected efficiently to a level that meets the requirements of the Bathing Water Directive. This is done by disinfecting the outgoing water with ultraviolet light. The UV unit is on during the bathing season.

Wastewater treatment plants treat approximately 12.3 million m3
of wastewater every year!

It’s about 40 bucketfuls every second.


25,000 m3

of septic tank sludge is received every year.

More than 98% of organic matter is removed from the wastewater in the biological treatment.

The Ali-Juhakkala wastewater treatment plant has a total of about 24,000 m3 of process tanks, equivalent to about

ten Olympic-size swimming pools.