Ocean disposal of treated wastewater
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 | Perth's Coastal Waters: Environmental Values and Objectives - The regulatory framework underpinning Ocean Outlet Environmental Monitoring |
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The Water Corporation of Western Australia (Water Corporation) operates four major wastewater treatment plants (WWTPs) in metropolitan Perth: Beenyup, Subiaco, Woodman Point and Alkimos. A fifth major WWTP servicing the greater Bunbury region is located approximately 7 km south of the Bunbury township. Prior to disposal, domestic and industrial wastewater (accounting for 92-98% and 2-8% of the wastewater respectively) is treated via mechanical and biological processes, all of which are designed to make the wastewater suitable for disposal to the ocean environment.
Disposal of treated wastewater (TWW) is achieved via deep-water (10-20 m) ocean outlets located at Ocean Reef, Swanbourne, Sepia Depression, Alkimos and Bunbury. Ocean disposal of secondary treated wastewater presently represents the most efficient and cost effective means of disposal from a social, economic and environmental perspective.
The ocean disposal process
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Following disposal to the marine environment, TWW undergoes rapid dilution via a number of physical and chemical processes. Because TWW is primarily fresh, it is also lighter and lower in density relative to seawater. Mixing of the TWW occurs as the buoyant solution rises and mixes with the surrounding water column, acting to reduce the concentration of TWW between the bottom and the surface of the water column by a factor of 100+ times. This initial stage of mixing is known as near-field mixing. Factors affecting the efficacy of this process include (a) the depth of the receiving water (greater dilution is achieved in deeper water) (b) the energy of the receiving environment (dilution is increased by windy and/or stormy conditions) and (c) the design of the outlet pipe diffuser. Water Corporation’s outlet diffusers are designed to maximise dilution of treated wastewater through the use of multiple small diameter ports. These act to increase the exit velocity of TWW, a process which enhances the mixing process.
The receiving environment
The seafloor around the outlets is comprised of open sandy habitats interspersed with patches of rocky limestone reef. Limestone reefs, particularly at Alkimos, are dominated by macroalgal (seaweed) communities. Small patches of seagrass may also be present in the vicinity of the outlets. All of these habitats provide refuges for a wider variety of marine fauna, right down to microscopic crustaceans - the smallest of marine creatures - that inhabit the ‘watery’ spaces between sand particles. The open water surrounding the outlets also house an array of fin-fish species, zooplankton communities and microscopic plants known as phytoplankton (or micro-algae).
Over time, the wastewater outlets have also become encrusted with an array of marine life, including ascidians, sponges and complex macro-algae communities. In turn, these communities attract and support an array of marine life comprising demersal fin-fish, molluscs, crustaceans and echinoderms.
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Is ocean disposal sustainable?
Scientific investigations have resolved that the short answer to this question is yes! (if it is done properly).
However, this is a simple answer that masks the number of criteria against which this outcome was (and continues) to be assessed. The extent to which disposal of treated wastewater to Perth's coastal waters is considered sustainable is measured against the Environmental Values and Objectives published by the Environmental Protection Authority (see Regulatory Documents Box). There are four Environmental Values (EVs) and six underlying Environmental Quality Objectives (EQOs) (see Table below):
Environmental Values and Objectives
| Environmental Value | Environmental Quality Objective |
| Ecosystem Health | EQO1 Maintenance of ecosystem integrity |
| Fishing and Aquaculture | EQO2 Maintenance of aquatic life for human consumption |
| Recreation and Aesthetics | EQO3 Maintenance of primary contact recreation values EQO4 Maintenance of secondary contact recreation values EQO5 Maintenance of aesthetic values |
| Industrial Water Supply | EQO6 Maintenance of water suitable for industry use |
Although each of the Environmental Values are assigned equal weighting in the design of the Perth and Bunbury ocean outlet monitoring programmes, the effect of treated wastewater on Ecosystem Integrity is one area that receives special attention. One of the biggest threats to ecosystem integrity is the potential for nutrient inputs to cause accelerated growth of marine algae - growth that would normally be 'limited' under prevailing conditions. Enhanced growth of marine algae has the potential to (a) smother seagrass or seaweed communities or (b) lead to unsightly algae blooms. Although such effects are unlikely in the high energy environment of Perth's coastal waters (as opposed to sheltered embayments, estuaries or riverine systems), a number of sentinel monitoring programmes have been established to determine the likelihood and/or extent of such effects.
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Scientific Evidence
The effects of the long-term disposal of treated wastewater to the Perth coastal environment was the subject of a four year multi-disciplined study conducted between 1991 and 1994 (The Perth Coastal Waters Study), and is presently the focus of two major ongoing ocean monitoring programmes in Western Australia: Perth Long Term Ocean Outlet Monitoring (PLOOM) and the Bunbury Ocean Outlet Monitoring (BOOM) programmes.
One of the key objectives of the Perth Coastal Waters Study (PCWS) was to determine the effects of the nitrogen (contained within the treated wastewater) on Perth's coastal waters. Although treated wastewater contains both nitrogen and phosphorus, nitrogen is the nutrient in limited supply (i.e. the nutrient most likely to be utilised by algae) and is therefore the nutrient most likely to lead to nutrient related environmental impacts.
Results of the PCWS indicated that the continuing disposal of treated wastewater was unlikely to compromise the Environmental Values in the vicinity of the Ocean Reef and Swanbourne treated wastewater outlets. Although there were indications of the effects of treated wastewater in the vicinity of the Sepia Depression, these were considered only minor and not ecologically detrimental.
Results of the PCWS study were based on an extensive data set collected prior to commencement of substantial WWTP upgrades. These upgrades saw the Beenyup and Swanbourne plants upgrade from secondary to advanced secondary level treatment and the Woodman Point plant upgrade from primary to secondary level treatment. Each of the upgrades resulted in a significant reduction in the volume of biological available nitrogen reaching the marine environment. This reduction alone has acted to reduce the level of disturbance to Perth’s coastal environment. Indeed, extensive monitoring in the years subsequent to the PCWS has found no indication of detrimental environmental impacts. This is an outcome that the Water Corporation is enormously proud of, and an outcome that the Water Corporation has resolved to maintain.
Ongoing monitoring
Although scientific investigations have resolved that discharge of treated wastewater has had little detectable impact on the marine environment, ongoing monitoring is undertaken by Oceanica as a means of detecting impacts should they arise in the future. Ongoing monitoring of the marine environment focuses on the following elements:
- Water quality (nutrients, physical-chemical, micro-biological);
- Sediment quality (nutrients, metals, pesticides);
- Phytoplankton (volume and community composition - including the presence of toxic and harmful species);
- Seagrass Communities (shoot density);
- Macroalgal Communities (species assemblages)
- Near-field dilution modelling;
- Ecotoxicological studies (as a flag for potential toxicity issues); and
- Comprehensive treated wastewater analysis (to ensure toxicants are less than the nationally accepted trigger values).
In addition to the normal programme of monitoring, specialist investigative studies are conducted as deemed appropriate. Examples include:
- Marine habitat surveys - including rocky limestone reefs, the spatial extent of seagrass meadows and macroalgae (to look for community composition changes);
- Wastewater plume dynamics using numerical modelling; and
- Remote Autonomous Underwater Vehicle missions to gain a 3 dimensional perspective of the extent and makeup of treated wastewater plumes.
Results and further details of ongoing monitoring programmes in the Perth, Sepia Depression and Bunbury regions of Western Australia can be found in the underlying PLOOM, BOOM SDOOL and ASOOM pages.
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