Yhdysviemäri

Yhdysviemäri, also known as a combined sewer, is a wastewater system that carries both sewage and stormwater runoff in a single pipe. This system was common in older urban areas, built when the distinction between sanitary sewage and stormwater was less understood or prioritized. In a combined sewer system, all wastewater, including domestic sewage, industrial wastewater, and rainwater from streets and roofs, flows together through the same underground pipes.

During dry weather, the combined sewer system functions as a sanitary sewer, transporting all wastewater to a treatment plant. However, during periods of rainfall or snowmelt, the volume of water entering the system can significantly increase. When the combined flow exceeds the capacity of the treatment plant or the sewer pipes themselves, a phenomenon known as a combined sewer overflow (CSO) occurs. CSOs release untreated or partially treated wastewater directly into nearby rivers, lakes, or oceans.

These CSOs can have significant environmental impacts, introducing pollutants such as bacteria, viruses, nutrients, heavy metals, and debris into receiving waters. This can harm aquatic ecosystems, contaminate drinking water sources, and pose risks to public health. Many municipalities are undertaking projects to separate their combined sewer systems into separate sanitary and storm sewer networks or to implement green infrastructure solutions to manage stormwater and reduce the frequency and volume of CSOs.