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SYDNEY’S DEEPWATER OCEAN OUTFALLS
Long-term environmental performance
2007
Table of contents
Introduction.................................................................................................................................................3
Environmental monitoring...........................................................................................................................4
Independent reviews ................................................................................................................................11
Contributing to clean beaches ..................................................................................................................12
Meeting future challenges ........................................................................................................................13
In summary...............................................................................................................................................14
Introduction
Until the early 1990s, Sydney’s beaches were polluted with primary sewage that was discharged from
cliff-face outfalls at North Head, Bondi and Malabar sewage treatment plants. Environmental
assessments from 1979 recommended deepwater ocean outfalls (that discharge treated effluent far
offshore) as the preferred option for managing wastewater. The assessments predicted that closing the
old cliff-face outfalls would deliver significant environmental improvements close to the coast and
beaches, while opening the outfalls further offshore would have minimal new impact on the environment,
due to the natural high-energy mixing processes of the ocean. Since then, extensive studies have
assessed the environmental performance and public health benefits of the deepwater ocean outfalls.
This paper outlines their performance and benefits.
What are deepwater ocean outfalls and how do they work?
Deepwater ocean outfalls release treated sewage two to four kilometres off the coast where it mixes with
seawater on the continental shelf at depths of 60 to 80 metres. The primary treated effluent is conveyed
through tunnels under the ocean floor, and is released through a series of diffusers. These diffusers
release the effluent in fine jet streams, so it mixes immediately with seawater and disperses into the
strong East Australian Current. Because it is less dense than the salty seawater, the effluent moves
upward and outward into the current as it disperses into an area called the mixing zone. At the same
time, the current continues to move it away from the coastline. Natural processes eventually break down
the effluent components, which are by now very highly diluted.
Sydney Water operates deepwater ocean outfalls at North Head, Bondi and Malabar. These serve
almost 2.9 million Sydneysiders and treat up to 1,040 million litres of wastewater a day. This volume
would fill more than 1,000 Olympic swimming pools.
The deepwater ocean outfalls opened in 1990-91. For most of the preceding century, effluent was
discharged from pipes along Sydney’s shoreline. This degraded the coast and was the main cause of
poor water quality at beaches.
The outfalls represent a long-term investment in protecting Sydney’s coastline. They use much less
energy than shoreline tertiary treatment and disposal, therefore producing less greenhouse gas, and
they use fewer chemicals. They also occupy less land, which keeps the coastline free of large sewage
treatment infrastructure and reduces costs and disruption to the community.
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Environmental monitoring
History
Approval for construction of the deepwater ocean outfalls was granted on the proviso that Sydney Water
develop and implement a detailed environmental monitoring program (EMP).
The EMP ran between 1989 and 1993, and was one of the most comprehensive in Australia for
assessing the impact of sewage discharges. It included pilot studies and extensive environmental
monitoring that spanned before and after the commissioning of the outfalls. The extensive dataset it
produced has been the basis for ongoing assessment of ocean discharges along Sydney’s coast.
The EMP aimed to determine if the outfalls would make beaches safe for swimming and fish safe to eat,
and if they would protect the marine environment.
The program found that the ocean outfalls performed well, meeting or exceeding the predictions of the
environmental impact statements, and mitigated all of the previous environmental problems associated
with the old cliff-face outfalls.
The EMP resulted in a series of peer-reviewed scientific reports and publications on:
ocean and beach water quality
sewage plume dispersal
contamination of fish and molluscs
seabed ecology and sediments
plankton
fish communities
the effects of organic contaminants.
Current monitoring
Sydney Water has an ongoing monitoring program to assess how the deepwater ocean outfalls perform
over the longer term.
Data is collected at 20 ocean monitoring sites (shown in Figure 1), from Terrigal on the NSW Central
Coast to Shoalhaven Bight south of Sydney, to detect any potential impacts on the marine ecology and
sediments.
The monitoring program studies four main areas:
marine ecology
effluent toxicity
marine sediment characteristics
oceanography.
It forms part of the requirement of Sydney Water’s Environmental Protection Licences for coastal
systems as required by the Department of Environment and Climate Change (DECC).
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Figure 1: Location of Sydney’s deepwater ocean outfalls (red bars) and the ocean monitoring site
(yellow diamonds). An additional monitoring site is located in the Shoalhaven Bight.
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Marine ecology
Sydney Water compares the diversity and abundance of seafloor organisms at sites close to the
deepwater ocean outfalls, to those at control sites far from the outfalls. Examples of the organisms
sampled under this program are listed in Table 1 and are shown in Plates 1-4.
If a deepwater ocean outfall were damaging the local marine ecology, the effects would be most evident
close to the outfall, and less evident further away. At the Malabar Outfall, for example, monitoring is
done at intervals along the sea floor. Data is collected at the outfall, and at sites 3 km, 5 km and 7 km
away.
The results are presented in Figure 2(a). A negative impact would show as four separate coloured
clusters for each of the sites (such as the hypothetical case shown in Figure 2(b)). Instead, the sites
cluster together as a mixed group regardless of distance from the outfall. This indicates there is no
obvious difference in the marine ecology close to the outfall and that further away – implying the outfalls
have had no statistically significant effect on the environment even after 16 years of continuous
operation.
Table 1: Animal groups that are monitored in the marine ecology component.
Marine Animal
Groups
Example Organisms
Mollusca
Bivalves (e.g. clams) and snails
Annelida
Marine worms (Polychaetes)
Arthropoda
Crustaceans and isopods such as
water slaters
Poriphera
Sea sponges
Echinodermata
Starfish, sea urchins
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Plates 1-4: Marine organisms sampled from the sea floor in the vicinity of Sydney’s deepwater ocean
outfalls. A comprehensive and ongoing marine ecology monitoring program (conducted over 16 years)
has not detected any adverse impacts of the deepwater ocean outfalls on the local marine life.
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Malabar benthic macrofauna 2002, replicate data, 1L corrected
Dimensions 2 and 3 of 3
Expected scatter for an impacted transect
Stress = 0.15
Actual scatter at the Malabar transect
Expected scatter for an impacted transect
0 km
0 km
3 km
3 km
5 km
5 km
7 km
7 km
Figure 2 (a)
Figure 2(b)
Figure 2(a): Cluster diagram of actual marine ecology results for the Malabar Deepwater Ocean Outfall.
If this deepwater ocean outfall was negatively impacting the marine ecology, we would expect monitoring
sites close to the deepwater ocean outfall to plot as a separate group from monitoring sites located
further away (the expected scatter pattern for an impacted transect is shown in Figure 2(b)). The actual
results shown in Figure 2(a) show that there are no significant differences in the marine ecology at
various distances from the deepwater ocean outfall.
Effluent toxicity testing
Toxicity testing became mandatory in 2004 under Sydney Water’s DECC licences. The tests were
introduced to better measure the impact on the environment of treated effluent.
To test for toxicity, indicator organisms are chosen for their sensitivity. Their reaction when exposed to
effluent is observed to determine how likely the effluent is to affect organisms in receiving waters. If
effluent is not toxic to sensitive indicator species, it is unlikely to be toxic to others.
The sea urchin Heliocidaris tuberculata was identified as an appropriate indicator species for Sydney’s
waters, as it is particularly sensitive to toxins during its fertilisation lifecycle stage. Local Sydney sea
urchins are specially cultured in the laboratory for the toxicity testing. The testing complies with the
requirements of the NSW Animal Ethics Committee.
For each effluent sample, scientists determine the concentration required for a 50 per cent mortality of
fertilised sea urchins. The higher the effluent concentration required to achieve this mortality, the less
toxic the effluent is.
The DECC sets stringent licence targets for toxicity. Results for all three deepwater ocean outfalls
comply with licence targets over a number of years, with no evidence of ecotoxic effects in the effluent
released from the deepwater ocean outfalls.
Results can be obtained from the Interactive sewage treatment plant (STP) map in Sydney Water’s 2007
online Annual Report.
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Marine sediments
Marine sediments at the monitoring sites are surveyed each year for a range of physical and chemical
parameters, shown in Table 2. The sediment sampler is shown in Plate 5.
To date, sediment results have varied between the monitoring sites. However, the variations do not
appear to be associated with the deepwater ocean outfalls. In fact, concentrations of total organic carbon
and metals (chromium, nickel, copper, cadmium, iron and zinc) are highest near the entrances to the
major Sydney waterways – Broken Bay, Port Jackson, Botany Bay and Port Hacking.
This suggests that general urban pollution generated in Sydney is the major source of low-level,
background pollution in the marine sediments. Research is continuing to confirm these patterns.
Table 2: Parameters studied in the Marine Sediments Monitoring Program
Sediment parameters
•
•
•
•
•
•
•
Nutrients: Nitrogen and Phosphorus
Total Organic Carbon
Metals: Aluminium, Iron,
Heavy Metals: Arsenic, Chromium,
Copper, Lead, Nickel, Zinc, Cadmium,
Mercury, Selenium, Silver
Organic Compounds: Organochlorine
Pesticides, PCBs
Polyaromatic Hydrocarbons
Cresols
Oceanography
Sydney’s deepwater ocean outfalls operate in a dynamic natural environment. Wave and current
conditions offshore are highly energetic. The outfalls harness this natural energy to disperse and break
down effluent.
Oceanographic conditions affect how the deepwater ocean outfalls operate and how the marine ecology
and sediments behave. To better understand how the outfalls interact with oceanographic forces,
Sydney Water operates a permanent Ocean Reference Station (ORS) (see Plate 6), three kilometres
offshore from Bondi. The ORS measures and records oceanographic and meteorological variables, as
shown in Table 3.
Data from the station is essential to the long-term oceanographic modelling used by a number of
collaborative research projects. Sydney Water shares the data with agencies including the DECC, the
Bureau of Meteorology, the Defence Science and Technology Organisation, the CSIRO, the Integrated
Marine Observing System, universities and local governments.
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Plate 5: The Smith-McIntyre sampler used to collect sediment from the sea floor for the deepwater
ocean outfall monitoring program. This device is deployed on a line from a boat, and automatically
collects a sediment sample when it reaches the sea floor.
Plate 6: The Ocean Reference Station is housed in a permanently moored marine buoy, approximately
two kilometres off the Sydney coast. This device continually monitors ocean current and wave activity
offshore.
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Table 3: Variables measured at the Ocean Reference Station
Oceanographic variables:
•
•
•
•
Ocean current speed and direction [vertical profile]
Ocean temperature [vertical profile]
Ocean salinity [vertical profile]
Wave height and length
Meteorological variables:
•
Wind speed and direction
Independent reviews
Sydney Water has commissioned several independent, detailed reviews of the environmental
performance of the deepwater ocean outfalls. As a result of some of these reviews, a special edition of
the Marine Pollution Bulletin in 1997 (Volume 33, Number 7-12) featured articles from a range of
scientific experts who had studied Sydney’s deepwater ocean outfalls, including aspects such as
oceanographic and physical processes, water quality and marine ecology. Topic areas studied in the
Marine Pollution Bulletin Special Edition on ocean outfalls are shown in Table 4.
The Marine Pollution Bulletin summarised the scientists’ consensus as follows:
“Generally, the environmental monitoring program has found that the outfalls are performing well, ie that
sewage plume dilutions exceed design specifications, and that they have reduced the degree of beach
pollution in the Sydney area as well as leading to the recovery of degraded areas in inshore waters. The
studies … demonstrate that the outfalls appear to have, to date, created no new problems in ocean
waters near the outfalls or in the adjacent sediments.” (p. 121)
Table 4: Topic areas studied in the Marine Pollution Bulletin Special Edition on ocean outfalls.
Topic area
No. studies
commissioned
Oceanography, Physical Processes & Numerical
Modelling
Water Quality & Potential Contaminants
5
Marine Ecology
6
Beaches
2
Community
1
3
General Environmental Monitoring & Design
4
TOTAL
21
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Contributing to clean beaches
One of the most positive outcomes of the deepwater ocean outfalls has been the marked improvement
in water quality at Sydney beaches.
Under the BeachWatch and HarbourWatch programs, the DECC continuously monitors swimming
conditions. Water quality samples are collected from beaches every six days and tested for
concentrations of two indicator bacteria: faecal coliforms and Enterococci. These bacteria are typically
found in sewage but are also commonly found in stormwater discharges. High concentrations can
increase the risk of infectious illnesses in humans.
Since the deepwater ocean outfalls opened, there has been a significant decrease in concentrations of
these bacteria at Sydney’s beaches. This is shown in the results from Bondi Beach in Figure 3. The
deepwater ocean outfalls have been very effective in safeguarding beaches from bacterial pollution.
Water quality has greatly improved, making it safe to swim except after rain due to urban runoff and
sewer overflows.
Water Quality at Bondi Beach:
Faecal coliforms during the Swimming Season
Opening of deepwater ocean outfalls
14
12
10
8
6
4
2
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
0
1990
No. of samples where faecal coliforms
exceed 100 cfu
16
Year
Figure 3: Water quality at Bondi Beach before and after the opening of the deepwater ocean outfalls.
One indicator for water quality is the concentration of faecal coliforms. Higher concentrations of faecal
coliforms are associated with lower water quality. After the deepwater ocean outfalls opened, the
concentrations of faecal coliforms were markedly reduced across Sydney’s Beaches, improving
swimming conditions.
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Meeting future challenges
Diffuse source pollutants
Pollutants generated by a large city and its surrounding catchment inevitably find their way into rivers
and estuaries, and eventually into the ocean. These diffuse source pollutants are a key source of
contaminants and are generally associated with urban runoff and estuary flushing after wet weather
events. Freshwater flushing of the Hawkesbury Estuary is shown in Plate 7.
Distinguishing between diffuse source and deepwater ocean outfall effects is a major challenge of the
monitoring program.
Plate 7: Freshwater flushing of the Hawkesbury Estuary
Climate change
The prolonged drought and evidence of climate change have reinforced the need for demand
management and other water saving initiatives.
As a result, lower consumption of water and recycling schemes have reduced inflows to sewage
treatment plants. Reduced inflows can mean higher concentrations of pollutants and other substances in
the wastewater.
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In summary
Sydney’s deepwater ocean outfalls have delivered high-quality outcomes for the environment and the
community. Beaches and harbours are cleaner and the marine environment is healthy.
Since the deepwater ocean outfalls opened 16 years ago,
swimming conditions have significantly improved
beach grease has been eliminated
there has been no detectable negative effect on marine ecology or sediments
effluent discharged has consistently been shown to be non-toxic at its diluted state.
Sydney Water will continue to monitor and report on the environmental performance of the deepwater
ocean outfalls. Sydney Water is committed to complying with its licence conditions and to meeting the
expectations of customers and the community.
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