First Flush Characteristics in Separate Sewer Stormwater and Implications for Treatment

The characteristics of stormwater need focus due to climate change. Paved areas and recipients receiving more stormwater have an enhanced need for treatment before discharge to remove suspended solids, nutrients and xenobiotics. To handle and treat stormwater efficiently, knowledge concerning first flush patterns is crucial. Therefore, we have studied 12 entire rain events and correlated water volume, suspended solids, and dissolved and particulate phosphorus to catchment characteristics and precipitation characteristics. We have mainly studied smaller rain events (average 12.6 mm), as nearly 90% of the events in the studied catchment are <10 mm. We revealed first flush tendencies in 50% of the rain events, concerning both suspended solids and phosphorus. We also found significant correlations between catchment size and discharged masses, and most importantly between precipitation patterns and discharged mass per volume. A long dry period and low amount of rain during the previous event, as well as high rain intensity, gives a high chance of first flush. We conclude that stormwater treatment should focus on the initial part of the event, especially in areas with a lack of space and/or economy to handle and treat the entire event.