yhdysviemäriä

Yhdysviemäri is a Finnish term referring to a combined sewer system. In such a system, sanitary sewage (wastewater from homes and businesses) and stormwater (runoff from rain and snowmelt) are transported in the same underground pipe. This is a common design in older urban areas, particularly those developed before the mid-20th century when separate systems for sewage and stormwater were not yet widely adopted.

The primary advantage of a combined sewer system is its initial simplicity and lower construction cost compared to building two separate networks. However, combined sewers present significant challenges, especially during periods of heavy rainfall. When the volume of stormwater entering the system exceeds its capacity, the mixture of sewage and stormwater can overflow, leading to combined sewer overflows (CSOs). These CSOs discharge untreated or partially treated wastewater directly into rivers, lakes, and other water bodies, causing pollution and posing risks to aquatic life and human health.

To mitigate the environmental impact of CSOs, many municipalities are undertaking projects to separate their sewer systems or implement infrastructure upgrades. These upgrades can include increasing the storage capacity of the sewer system, constructing treatment facilities specifically for CSOs, or implementing green infrastructure solutions like permeable pavements and rain gardens to reduce the amount of stormwater entering the system. The transition from combined to separate sewer systems is a complex and costly endeavor, often spread over many years.