Stormwater Intrusion into Separate Sewerage Systems - Overflows, lmpacts and Controls

The relative contributions of stormwater entering separate sewerage systems are examined in regard to the resulting sewage overflows. Monitoring of various locations in Sydney was carried out. A comparison is made with the stormwater and sewage overflow discharges, particularly their pollution load and receiving water impacts. Sewage overflows and stormwater were found to be major contributors of pollution to receiving waters. Overflows were found to occur 20-50 times per year and receiving water impacts lasted some 5 to 10 days after overflows. Having determined the nature of the problem practical and cost effective solutions are discussed. Storage of sewage overflows would substantially reduce overflow numbers and volumes, although there are physical, economic and environmental factors which also require careful consideration. The control of overflows requires integrated strategies involving an understanding of stormwater and sewer system operation related to rainfall, and receiving water impacts.