Abstract
Combined sewer overflows (CSOs) pollute receiving waters and have a negative impact on ecosystem services. In urban areas rehabilitation of the sewer system to avoid CSOs is associated with high investment costs. Furthermore, not all CSOs can be closed due to the need for hydraulic reliability of the system. Local treatment of CSO with high rate filtration offers an alternative to rehabilitation of the sewer system that is flexible with respect to design and has lower investment cost than separating sewage and storm water runoff. Results from DESSIN, a 4-year EU demonstration project, are presented. The results showed on average 50% removal of particulate matter during CSO events, with higher removal (80%) in the initial first flush period. Other constituents, for example heavy metals, were removed through their association with particles. Potential impacts on ecosystem services in the catchment and the sustainability of the solution were assessed.
Highlights
State of the art in combined sewer overflow (CSO) technology for large scale sewers consists of sedimentation in the Combined sewer overflows (CSOs) tank, which is not sufficient to ensure retention of pollutants
The instantaneous separation efficiency typically varied between 20% and 80%, The turbidity separation efficiency varied with different feed water quality during the CSO events
The innovative high rate filtration system has been demonstrated for local CSO treatment at Hoffselva in Oslo, Norway
Summary
State of the art in combined sewer overflow (CSO) technology for large scale sewers consists of sedimentation in the CSO tank, which is not sufficient to ensure retention of pollutants. Some studies include further treatment, for example with disinfection of treated effluent (Tondera et al ) or constructed wetlands (Masi et al ) this may require large areas, and in smaller sewer systems CSOs are commonly without a tank. Ochowiak et al ( ) investigated a swirl sedimentation tank, but in most solutions no, or very limited, particle removal is achieved due to lack of a settling volume. The priority in CSO management is to reduce the overflow volumes, which can be done for example by separating wastewater and stormwater pipes. Due to hydraulic reliability, not all CSOs can be. There is a need for solutions that can limit the discharge from CSOs to recipient waters
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