sekaviemäri

A sekaviemäri, also known as a combined sewer, is a type of wastewater collection system that carries both sanitary sewage and stormwater runoff in a single pipe. This means that water from toilets, sinks, and showers, as well as rainwater and snowmelt from streets and roofs, all flow together into the same sewer infrastructure.

Combined sewers were common in many older urban areas, built when the distinction between sewage and stormwater management was less pronounced. The primary purpose of a sekaviemäri is to transport all collected water to a wastewater treatment plant. However, during periods of heavy rainfall or rapid snowmelt, the volume of water can exceed the capacity of the treatment plant and the sewer system itself.

When this overflow occurs, a mixture of untreated sewage and stormwater is discharged directly into local waterways. This phenomenon is called a combined sewer overflow (CSO). CSOs can introduce pollutants such as bacteria, viruses, nutrients, and debris into rivers, lakes, and oceans, negatively impacting water quality, aquatic life, and public health. Many municipalities are undertaking projects to separate their combined sewer systems into separate sanitary and storm sewers or to implement green infrastructure solutions to reduce the volume of CSOs.