Manly is at the end of the longest ocean outfall in the world – it is 45 kilometres from where it starts at Blacktown, in Sydney’s Western Suburbs to the end of the ocean outfall, 3 kilometres out to the sea. The North Head sewage catchment is 470 square kilometres, connecting the equivalent of 1.2 million people to one of Sydney’s three largest coastal plants which together discharge about one billion litres of effluent to the ocean each day.
The average dry weather discharge at North Head is about 300 million litres (ML) per day, increasing in wet weather to 1.4 BILLION litres per day with the inclusion of the Northside Storage Tunnel. The Tunnel was completed in 2000 to take the sewer overflows from upstream at Scott’s Creek, Lane Cove, Tunks Park, Quakers Hat and Shelly Beach. To avoid scouring, flows in excess of 600ML bypass the sedimentation tanks. These high flows are caused by stormwater infiltrating sewage pipes many of which are close to 100 years old and were designed and built long before urban consolidation and highrise development.
75% of oil, grease and solids are discharged annually - 5,910,000 kg of oil and grease and 35,010,800 kg of solids through the diffusers of the Long-distance Ocean Outfall onto the rocky reef around North Head. This outfall was commissioned in December 1990 and a four year monitoring program showed that it was having a significant impact on the diversity and abundance of marine line on this rocky reef. Continued impacts on the rocky reef have been of community and scientific concern for some time.
In 1994 the NSW EPA announced that Ocean Outfalls were not sustainable and inserted an objective into Sydney Water’s Licence that wet weather discharge only through the outfall should be the long term aim. This should encourage decentralisation and beneficial re-use upstream for irrigation and industrial use, as well as considerably reduce the drawdown from the Hawkesbury Nepean and Shoalhaven catchments. It has been estimated that the equivalent of 25% of the Australian population is drinking from the Hawkesbury Nepean which in dry weather can be mainly treated sewage.