3. Stormwater and sewage
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Background information
Background information compiled from:"PipeCheck" Program for Private Sewers, A community environmental initiative of the Total Catchment Management movement, April 2000. Beachwatch and Harbourwatch State of the Beaches 1999 and 2000, NSW EPA 1999 and 2000 . |
In a typical urban setting, every time someone flushes a toilet, rinses a plate, takes a shower or washes their clothes, the wastewater goes through a private sewerage pipe to a sewerage main. All wastewater from inside premises should enter the sewerage system to be transported to a sewage treatment plant where it is treated before being discharged into a local waterway or the ocean.
When it rains, the run-off from roofs, streets, gardens and other outdoor areas flows to a separate pipe - the stormwater system. Stormwater flows directly to local waterways and is untreated. (See Figure 3.1)
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Stormwater
In urban areas a large proportion of the land surface has been paved or covered with impervious surfaces (e.g., roads and buildings). This leads to increased water run-off during rainfall. It is this runoff that is called stormwater. As the stormwater runs over the land surface, it picks up pollutants such as leaves, soil and dissolved chemicals and carries them to local waterways.
Although not usually considered pollutants, plant seeds can cause problems when transported to waterways by stormwater, particularly if the stormwater also contains high levels of nutrients. The seeds from many gardens are considered weeds when growing in natural areas such as urban bushland or national parks.
Stormwater systems are connected directly to streams and other waterways and as a result have an immediate impact on the quality of these receiving waters. Minimising the pollution from stormwater requires everyone in a catchment to be careful to ensure that only rainwater enters the stormwater system.
Stormwater pollution is the single biggest threat to the health of our waterways. In high rainfall events, large volumes of water enter rivers and creeks, transporting sediment, nutrients, toxic chemicals, herbicides, pesticides, oils and grease, animal waste and sewage overflows. Stormwater pollution is more significant in urban areas, where large areas of paved surfaces result in increased run-off.
Some heavy metals are now widespread in stormwater, especially lead. Lead originates primarily from motor vehicle emissions onto roads and flakes of lead from older paints. Levels of copper and zinc are also problematic.
Sewage
Although sewage is nearly all water, it also has lots of other things in it that must be treated before the effluent is returned to the environment. Some of the things people put into the sewerage system (such as oil, grease, chemicals, cotton tips and plastics) are difficult to treat.
How is Sewage Treated?
When the sewage leaves your house it flows through sewerage pipes to a sewage treatment plant where it is treated. (Note: sewage is the term used to describe the mixture of water and human waste. Sewerage is the term used for the system of pipes that carries sewage to a sewage treatment plant).
There are many sewage treatment plants in and around Sydney; some are on the coast and discharge into the ocean, and some are inland and discharge into rivers. Sewage treatment plants use many different processes and levels of treatment for cleaning the sewage. The levels are primary, secondary and tertiary treatment.
Primary treatment involves screening for the removal of things such as paper and plastics, grit and grease removal, and the settling and removal of solids.
Secondary treatment includes the primary treatment process together with an additional process where good bacteria (tiny organisms) are used to break down and eat wastes.
Tertiary treatment includes a secondary treatment process with a further filtering and the addition of chlorine to disinfect the effluent.
At the end of the sewage treatment process there are two products left:
Effluent - The liquid water that is left after treatment
Sludge - The solid waste which settles to the bottom of the sedimentation tanks in the sewage treatment process.
Some of this effluent and sludge is now being used beneficially. Sludge is being used in compost and as a soil conditioner. Effluent is being used for watering grass and gardens, and also in industry for cooling.
Sewerage Problems
Problems can occur when stormwater runs into the sewerage system. The integrity of sewers is often compromised by illegal stormwater connections, faulty joints, tree roots cracking pipes, redevelopment of property and age.
Stormwater infiltration into the sewer during and following rainfall events can cause the sewerage system to have too much water and to discharge through designed overflow points in creeks and stormwater drains.
Stormwater can enter the sewerage system in the following ways:
Low Sewage Gullies ~ Gullies are the openings from sewerage drains, usually found under outdoor taps, or outside kitchens and bathrooms. If the opening to the gully is not elevated above ground level, stormwater will enter the sewerage system, especially during periods of heavy rain.
Illegal Connections to Sewer ~ Premises where stormwater pipes have been connected to the sewers are a serious problem. This is illegal. In Sydney, Sydney Water carries out tests to locate these connections. Testing involves pumping smoke into the sewer and looking for signs of smoke emerging from stormwater pipes such as roof gutter downpipes.
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Cracked and Leaking Pipes ~Stormwater can infiltrate into sewers through cracks in pipes or faulty joints. Cracks can form in sewers over time due to small earth movements. Old, cracked and leaky sewer pipes have long-term environmental impacts, contaminating groundwater, rivers and harbours (see Figure 3.2).
Sewer Overflows ~ Sewer overflows are designed points within the sewerage system, where sewage can overflow into creeks and waterways in the event of a blockage or overfilling of the sewerage system. These surcharge points ensure that the sewerage system does not back up and overflow into private residences.
Sewage overflows can be a major source of pollution, especially within estuarine and enclosed waterways. Overflows can be caused by rainwater entering the sewer system and increasing the pressure within the system. Sewage overflows occur at designated access points in the sewer system, when the hydraulic capacity of the system is exceeded. There are an estimated 3,000 designed overflow points in Sydney Water's sewerage system.
Waterways within Sydney such as Middle Harbour, Lane Cove River and Cooks River are heavily impacted by sewer overflows. To compound this problem, these waterways are poorly flushed by tidal action, and it may take days before bacterial contamination falls to acceptable levels. The time taken for these waterways to recover is influenced by the quality of the water in the local tidal region and the proximity of the site to clean ocean waters (Beachwatch and Harbourwatch: State of the Beaches, 2000).
Impacts of Sewage Pollution
Sewage leaking and overflowing into local waterways is a major pollution problem. Most urban waterways contain dangerously high levels of sewage contamination after rain. Sewage contains very high levels of nutrients such as nitrates and phosphates and pathogenic (disease causing) micro-organisms (refer Section 4).
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