Wastewater treatment process briefly
HSY's two wastewater treatment plants treat the wastewater in the metropolitan area. The processes mainly correspond to each other.
Chemical treatment removes phosphorus
Wastewater is rich in phosphorus. It comes from the food that people eat and, for example, detergents. In chemical treatment, iron salt (ferrous sulfate) is added to the wastewater. It precipitates the phosphorus in the water into so-called biophosphorus and sinks to the bottom of the sedimentation basin as part of the sludge. At the bottom of the basin, the sludge is collected for further treatment. At the Blominmäki plant, phosphorus removal is further enhanced with disc filtration. Due to the disc filtration, the Blominmäki plant’s phosphorus removal level is even better than in the Viikinmäki plant.
The bacteria are working efficiently
Biological treatment of wastewater happens by utilizing the bacteria in the wastewater. In biological treatment, air is introduced into the water in small bubbles, i.e. it is aerated. This makes the wastewater bacteria grow and multiply. In this way, activated sludge is formed, and as the bacteria grow, they consume organic matter from the wastewater. At the same time, nitrogen is released into the air as nitrogen gas.
After that, the water is directed to the secondary sedimentation stage, where the sludge sinks to the bottom of the basin and the clean water remains at the top of the basin. The sludge is collected and most of it is recycled to the beginning of the biological treatment process. In this way, the bacteria are immediately ready to work again. The water that has passed through the sedimentation stage is clean enough to be released into the sea. A small portion of the sludge is continuously removed from the process and led to sludge treatment.
Nitrogen removal is a biological process
The bacteria of the activated sludge use nitrogen as they grow. However, most of the nitrogen is removed when the bacteria turn it into nitrogen gas. The main part of nitrogen removal is done with an activated sludge process, but the treatment is continued by biological nitrogen removal filters. The plants remove nitrogen significantly more efficiently than their environmental permits require.
Sludge is made into biogas and soil
After the anaerobic digestion, the wet sludge is composted and processed into soil products. This way the organic matter in the sludge and the phosphorus and nitrogen it includes can easily be recycled.