1. Introduction
In spite of their name, separate sewer systems comprising sanitary and storm sewers may not be fully separated (Schmidt and Spencer, 1986). Where such systems do not include any stormwater treatment, untreated sanitary sewage may find its way into the receiving waters, impair their quality and increase the risks to public health and aquatic organisms, especially where such waters are used for drinking water supply or recreational purposes. There are also habitat and ecological impacts, whose management is becoming increasingly important under the EU Water Framework Directive (EC, 2000).