Citizens and businesses of the Helsinki Metropolitan Area produce around 100 million cubic metres of wastewater annually. This wastewater is lead through the sewer network to be treated at the treatment plants before they are released back into natural waters. The HSY’s two wastewater treatment plants, in Viikinmäki, Helsinki, and Suomenoja, Espoo are in charge of treating the wastewaters of the Helsinki Metropolitan Area. Construction of Blominmäki wastewater treatment plant is underway.
A biological and chemical method is used to treat the wastewaters. Its by-products are sludge and biogas. The sludge is refined into soil and the gas is utilised in energy production. The treated wastewaters are led to open sea through a tunnel.
In addition to domestic wastewaters, also industrial wastewater and other different forms of wastewater are conducted to the sewers. Leading these waters into the sewer requires a permit and an industrial wastewater contract.
Wastewater treatment has an important role in the protection of coastal waters and the Baltic Sea, as the wastewater to be treated includes large amounts of phosphorus and nitrogen nutrients, which would cause eutrophication if they were directly conducted to the sea.
Both EU directives and national legislation have limitations for wastewater treatment. In addition to this, wastewater treatment plants are obligated to fulfil the plant-specific environmental permits set for them. Their execution is supervised by the Environment Centre.
Removing solid waste from wastewater
In mechanical cleaning, solid material is removed from wastewater. Among others, this includes sand, gravel, grease and other mixed waste. This mixed waste is referred to as screening by the water services.
Removing screening and sand in the beginning of the process is important, so that the matter does not hinder the water treatment process and damage the devices. Annually, over one million kilos of foreign matter is flushed into the sewers in the Helsinki Metropolitan Area alone.
The mechanical cleaning process also includes a primary sedimentation, during which finer solid matter is removed and reduced in size and the load for the next, biological treatment is levelled.
Phosphorus is removed through chemical treatment
Wastewater has a large amount of phosphorus. It originates from consumed food and washing detergents, for example.
In chemical cleaning, iron salt, i.e. ferrous sulphate, is added to the wastewater. It precipitates the phosphorus in the water into so-called biophosphorus, which sinks to the bottom of the pool and becomes part of the sludge. The sludge will be collected from the bottom of the pool to be treated further.
Bacteria work efficiently
The biological treatment of wastewater is done by utilising the wastewater’s bacteria. In biological treatment, small bubbles of air are conducted to the water, i.e. it is aerated. In such conditions the bacteria in wastewater start to grow and multiply.
This forms activated sludge. As the bacteria grow, they consume the wastewater’s organic matter. This also releases nitrogen into air as nitrogen gas.
After this, the water is conducted to secondary sedimentation, where the sludge sinks to the bottom of the pool, leaving the clean water on the top. The sludge is collected and most of it is recycled to the beginning of the biological treatment process. This way the bacteria are ready to start working again right away. After the secondary sedimentation, water is clean enough to be conducted into the sea. A small amount of the sludge is continuously removed from the process to be led to sludge processing.
Removing nitrogen is a biological process
As they grow, the bacteria in the activated sludge also consume nitrogen. However, most of the nitrogen is removed as the bacteria change it into nitrogen gas.
Most of the nitrogen removal happens during the activated sludge process. In Viikinmäki, the treatment process is continued further with the help of biological nitrogen removal filters. The plant removes nitrogen much more efficiently than required by its environmental licence.
Anaerobic digestion produces biogas
The organic matter in the sludge is utilised in the wastewater treatment process through anaerobic digestion. Anaerobic digestion is a process taking place in an oxygen-free environment, where part of the organic matter in the sludge continues to decompose further. This process enables both our treatment plants to recover biogas, i.e. methane.
We produce fossil-free i.e. carbon-neutral electricity and heating from biogas for the plant’s own use.
Sludge is turned into biogas and soil.
After the digestion, sludge is composted and processed further into soil products. This enables the recycling of the sludge’s organic matter and its phosphorus and nitrogen.