Erillisviemärijärjestelmien

Erillisviemärijärjestelmät, known in English as separate sewer systems, are wastewater management systems where storm water and domestic wastewater are transported through distinct and separate underground pipe networks. This contrasts with combined sewer systems, which carry both types of water in a single pipe. In a separate system, one network is dedicated to collecting and transporting sanitary sewage from homes, businesses, and institutions to a wastewater treatment plant. The other network, often called a storm drain or surface water sewer, collects rainwater runoff from streets, rooftops, and other impermeable surfaces and typically discharges it directly into nearby water bodies without treatment.

The primary advantage of separate sewer systems is the prevention of combined sewer overflows (CSOs). In combined systems, heavy rainfall can overwhelm the pipes and treatment plant capacity, leading to untreated sewage and stormwater being discharged into rivers and lakes. Separate systems avoid this by keeping the flows distinct. This also means that wastewater treatment plants only process domestic sewage, which is generally more concentrated and easier to treat, rather than diluted stormwater. However, the construction and maintenance of two separate, extensive pipe networks can be more costly than a single combined system. Furthermore, if separate systems are not properly designed or maintained, pollutants from surface runoff can still directly impact water quality.