Finland. City of Mikkeli

Date of agreement:02 Dec 2015
Customer:City of Mikkeli
Amount in EUR:EUR 20 million
Maturity:25 years
NACE sector / loan type:Collection, purification and distribution of water

This loan has a positive effect on maritime environments: 100%

Financing from NIB Environmental Bond proceeds

NEB-eligible share: 100%

NEB category: Water management and protection

Amount disbursed: EUR 20 million

Note: For loans in other currencies than EUR, the equivalent in EUR is based on the exchange rate effective for the disbursement. Read more about the NIB Environmental Bonds


The loan has been provided for constructing a new underground wastewater plant in the City of Mikkeli, Finland.

The public Mikkeli Waterworks division of the City of Mikkeli will construct the new Metsä-Sairila wastewater treatment plant (WWTP), which will replace the Kenkäveronniemi WWTP when it is completed in 2020. The Metsä-Sairila plant will be constructed in underground caverns.

The new facility is the first water treatment facility in Finland applying membrane bioreactor (MBR) technology. MBR technology is proven to increase the efficiency and quality of the treatment process.

Mikkeli Waterworks will also build a 4.5-kilometre transmission sewer from Kenkäveronniemi to Metsä-Sairila, a 4.1-kilometre discharge pipeline and a 0.7-kilometre emergency discharge pipeline from Metsä-Sairila to the Lake, Saimaa.

Mikkeli Waterworks is fully-owned by the City of Mikkeli and responsible for producing drinking water and treating wastewater for about 55,000 inhabitants in the city and its surrounding areas.

Fulfilment of NIB's mandate


Replacing the water treatment facility with new technology is expected to increase the daily wastewater treatment capacity by 50% to 30,000 cubic metres.

The new facility is the first water treatment facility in Finland to apply membrane bioreactor technology. The project is expected to have some demonstration value for other municipalities in Finland.


The Mikkeli WWTP project is estimated to reduce the load of nutrients, organic matter and suspended solids into Lake Saimaa, which is within the Baltic Sea catchment area.

The wastewater treatment units will be built in underground caverns. This is expected to reduce the odour and noise impact on the surroundings.

The WWTP is planned to be self-sufficient in terms of heat energy through heat recovery from wastewater.

Sustainability summary

The key environmental issues of the project include noise and vibration, impact on flying squirrels during construction, as well as effluent discharge into Lake Saimaa and odour emissions during operation.

The noisiest phase of the project will be the drilling and blasting excavation work, which is estimated to last for 1.5 years. The closest residential buildings and holiday homes are located at a distance of 500–700 m. The authorities have defined threshold values for vibration and noise, and aboveground excavation will therefore not be performed during the summer when people visit their holiday homes. 

Six likely breeding and resting places of flying squirrels have also been identified within the project area. The WWTP has therefore been situated and planned in a way that protects these areas.

Due to the new technology, the impact on the water quality of Lake Saimaa is assessed to decrease. The odour is also expected to be reduced due to the plant being built underground.

Press release

NIB finances wastewater treatment plant in eastern Finland

Press release

NIB finansierar reningsverk i S:t Michel i Finland