Ocean Discharge of Treated Wastewater
Background
On average, people produce about 200 litres of wastewater every day. It comes from our homes, our schools, our places of work, hospitals and other services and is more than 99.7% water.
Most comes from our homes - from showers, baths, sinks and washing machines as well as the toilet. It contains some pollutants, in the form of suspended and dissolved matter, some including oil, greases, detergents, nutrients, and potentially harmful bacteria and viruses.
Wastewater also comes from commercial and industrial premises. Limits are placed on pollutants from discharges from industries so that the wastewater quality is similar to that coming from a home.
Wastewater treatment plants remove much of the pollutant material from the wastewater. The wastewater that has been treated and is suitable for discharge into the environment is called treated wastewater.
What is ocean discharge?
Ocean discharge returns wastewater to the marine environment, normally through an outlet pipe some distance offshore. The design and location of the outlet pipe is determined by the depth of the water, the nature of the currents and the sensitivity of the coastal environment.
Salt and sunlight combine to form a natural and effective process for the reduction of bacteria. Dissolved oxygen levels in the open ocean help to decompose organic material, while currents provide energy for mixing and dispersing the diluted treated wastewater.
Current Situation
How does ocean discharge work?
The Perth metropolitan wastewater system comprises more than 6000 kilometres of sewer pipes which, with more than 450 pumping stations, collect the community’s wastewater. The majority goes to three major wastewater treatment plants located at Beenyup, Subiaco and Woodman Point.
These plants provide primary and secondary treatment (which is a biological process that incorporates nitrogen reduction) before discharge to the ocean.
The treated wastewater reaches the ocean through an outlet and is discharged through a series of small holes (called ports) near the end of the pipe. This section of pipe is called a diffuser. A typical diffuser contains about 50 outlet ports, each 150 millimetres in diameter spread over hundreds of metres.
The treated wastewater mixes with the seawater as it goes. The deeper the water the greater the mixing, and for every metre of water depth it is diluted about ten times by the time it reaches the surface (called the initial dilution). The ocean currents, which run parallel to the shore, further disperse the already diluted treated wastewater.
All Perth’s ocean outlets discharge into water at least 10 metres deep and at least one kilometre offshore, and so achieve at least 100 times initial dilution with seawater.
How safe is ocean discharge?
The longer and deeper the outlets and the higher the level of treatment, the safer the discharge is for public health and the environment.
In Western Australia, ocean outlets are approved by the Environmental Protection Authority (EPA). This approval requires detailed studies of ocean currents and the effects of the discharge on marine life and matters relating to public health.
Perth’s wastewater treatment plants are all licensed by the EPA under the Environmental Protection Act 1986 and discharge of the treated wastewater to the ocean is required to meet the EPA’s Water Quality Criteria.
The Water Corporation collects samples from each treatment plant every day to ensure that all treatment plant processes are running effectively.
The Water Corporation has monitored the environmental effects of discharge into Perth’s coastal waters since the early 1960s. The Corporation has a comprehensive ocean monitoring program called the Perth Long-term Ocean Outlet Monitoring Program (also known as PLOOM). Under this program, surveys throughout the year are conducted near each ocean outlet and analysed for the presence of bacteria, nutrients and heavy metals. Water samples are also collected at shoreline sites on the coast.
Samples of seabed sediments and marine life are collected to detect any build up of contamination. This analysis forms part of more detailed surveys that are undertaken around each ocean outlet every three to five years.
Bathing beaches throughout the Perth metropolitan area are monitored every two weeks in summer and monthly in winter. Monitoring by the Water Corporation and the Department of Health indicates that the existing ocean outlets meet health criteria.
Sustainability considerations
Economic | Social and Cultural | Environmental |
| Water that is recycled rather than discharged may have value to some end users. | Ocean discharge is safe from a public health perspective. | The 2006-2007 summer water quality survey indicated water quality conditions for ecosystem protection and public health criteria were met. |
| Ocean discharge is the least cost to the domestic, commercial and industrial sewerage rate payers. | Water discharged to the ocean is seen to be wasted – recycling should be optimised where feasible. | Ocean discharge uses little to no energy. |
How do we compare?
We compare very well with other major Australian cities and the rest of the world.
Ocean discharge is the most common method around the world of disposing of treated wastewater from larger coastal cities. In Perth, the wastewater is treated to a high standard to protect ocean water quality and is an environmentally acceptable and economical method that can be used all year round.
Of the major Australian cities only Sydney has longer outlets, but they discharge primary treated wastewater. Adelaide has tertiary treatment but only short outlets, while Brisbane discharges tertiary treated wastewater to the river. Melbourne has shoreline discharges of secondary treated wastewater.
Perth compares very favourably with its wastewater treatment plants discharging highly treated tertiary wastewater through long (greater than 1 kilometre) ocean outlets.
The Future
While recycling of treated wastewater will continue to increase, it will always be necessary to have a safe back-up option of discharge through ocean outfall. This is designed into local treatment plants and operating parameters.