All cities produce great quantities of waste water which need to be reused or discharged. Sydney's waste water, which is a combination of stormwater and sewage, is discharged into the surrounding ocean or estuaries.
The sewage that is discharged contains more than 99% water. It also contains domestic and industrial wastes including degradable organic material (poo), viruses and bacteria, detergents, grease and oil, salts, nutrients, heavy metals and organic chemicals. Stormwater is contaminated with sediments, organic matter, nutrients, bacteria, pesticides, litter and oil. All of this waste water is discharged via seven outfalls (3 major and 4 minor) and numerous stormwater drains. The three major outfalls discharge into deep ocean water about three kilometres offshore. The others discharge at the shoreline.
Waste water can have negative effects on ecosystems, water quality, fish communities, human health and aesthetics. Before 1990, when the major outfalls still discharged at the shoreline, the effects on beaches, water quality and coastal species were worse than they are today. Also, it was sometimes unsafe to swim because beaches were often polluted with bacteria, viruses, grease and sewage solids (poo).
Since the opening of the deep-water outfalls, shoreline effects have been greatly reduced. Surf beaches are now free of sewage pollution about 95% of the time, with sewage trapped well below the surface of the ocean and diluted before it can reach the shore. But we don't know how deep-water outfalls affect oceanic ecosystems. The worst waste water effects on beaches and harbours occur after heavy rain when polluted stormwater and sewage overflows pour into the sea.
High levels of nutrients in waste water can cause tiny floating plants to bloom which may kill fish and invertebrates. Blooms can also produce toxins which make water unsafe for swimming and seafood dangerous to eat. Blooms caused by sewage nutrients are more likely to occur in harbours and estuaries than at open-ocean sites.
Nutrients and sedimentation are the agents most responsible for causing degradation of estuaries. Not surprisingly, there are increased efforts to develop and implement new technologies for removing nutrients from waste water. Systems of re-using waste water for productive purposes would be even better.
Alan Jones
Marine Ecology
Australian Museum
Copyright © Australian Museum, 2003
This page: http://www.amonline.net.au/factsheets/waste_water.htm
