In the Netherlands, many separate sewer systems have been upgraded to Improved Separate Sewers (ISS) over the last few decades. The storm sewer of an ISS has no direct outflow to surface water. Instead, it has a small pump that pumps any wastewater from illicit connections and a first volume of polluted runoff towards the treatment plant. Only during large storm events, stormwater is directly discharged to surface water via a storm sewer overflow. An ISS has an important drawback: it pumps roughly 70% of the annual stormwater run-off towards the wastewater treatment plant plus large volumes of infiltration and inflow.
To mitigate this problem Partners4UrbanWater, in cooperation with STOWA and Stichting RIONED, has developed a new sewer concept: ISS 2.0. An ISS 2.0 eliminates the effects of illicit connections like a traditional ISS but without causing a large flow of clean water towards the wastewater treatment plant. This reduced flow allows smaller capacities for pumping stations, pressure mains and the treatment plant itself as well as to improved treatment efficiencies. Moreover, the clean stormwater can be used locally for replenishment of surface waters. The concept can be applied on both existing Improved Storm Sewers as well as on normal separate sewers.
The below animation explains the concept of an Improved Storm Sewer 2.0.