We started discharging treated wastewater from the Alkimos Wastewater Treatment Plant via ocean outfall in April 2011.

Each year, we discharge approximately 2.5 billion litres of treated wastewater via the Alkimos ocean outfall. This volume is expected to increase as population in the northwest corridor continues to grow.

We have engaged an independent consultant, BMT, to conduct the monitoring program on our behalf.

Regulatory framework and reporting

The Alkimos Wastewater Treatment Plant (WWTP) and associated ocean outfall are regulated under both an operational Licence (L8434/2010/1) and a Ministerial Statement (MS755) issued by the Department of Water and Environment Regulation (DWER) and the Minister for Environment. These regulatory instruments include conditions that require preparation of both an Annual Report and a Compliance Report, which document environmental performance and results of monitoring programmes for the reporting period.

How we monitor our performance

Ministerial Statement MS755 sets the requirement for and objectives of the Alkimos ocean monitoring programme, via a Marine Treated Wastewater Discharge Monitoring and Management Plan (M&MP). The objective of the M&MP is to ensure that discharge of treated wastewater from Alkimos WWTP meets Environmental Quality Objectives as determined by the Environmental Protection Authority.

The key components of the M&MP are summarised below:

Each year, we undertake a comprehensive analysis, incorporating over 100 components, of treated wastewater at Alkimos ocean outfall, to determine the concentration of metals, organics, nutrients and bacteria contained within the wastewater stream.

Further testing for a smaller sub-set of analytes is undertaken seasonally. The concentration of analytes is then compared against the national ANZECC/ARMCANZ (2000) guidelines for contaminants. This provides a comprehensive cross-check of potential contaminants with the national guidelines, which ensures human health, and marine flora and fauna are protected.

Initial dilution modelling is used to determine the extent of treated wastewater dilution around the ocean outlet under calm sea conditions. The initial dilution modelling is undertaken annually during summer. This data is used to ensure all contaminants contained within the treated wastewater meet the national guidelines for protection of marine species following the initial dilution stage—which is typically complete within meters of the diffuser.

Whole of Effluent Toxicity (WET) testing involves exposing organisms to different concentrations of treated wastewater and then measuring the growth and reproductive capability after a specific period of time. All tests, where required, are undertaken with appropriate animal ethics approval.

Results of WET testing to date have indicated that the dilution of treated water is sufficient to reduce contaminant concentrations to levels well below ANZECC/ARMCANZ (2000) guideline trigger levels.

We monitor water quality at a number of locations around Alkimos ocean outfall on a fortnightly basis from December to March. Summer is specifically targeted as this is when the effects of nutrients are more likely to be detectable.

Monitoring the concentration of nutrients (ammonia, nitrite and nitrate, ortho-phosphate) and chlorophyll a enables scientists to clarify the effect of treated wastewater on the biological components of the ecosystem over the summer period.

The monitoring also ensures concentrations of bacteria do not exceed the recommended levels for primary and secondary contact recreation and the guidelines for sea food consumption.

Seaweed health is assessed annually in January and February by a team of marine ecologists from BMT. The scientists use SCUBA diving equipment to measure the composition of seaweed communities and collect samples for chemical analysis. These measures are used to determine if seaweed is assimilating sewage based nitrogen and if it has changed as a result.

We conduct fortnightly testing as part of the water quality compliance monitoring for microbiological contamination. Sedentary, filter-feeding shellfish, such as mussels, process large amounts of water and have the tendency to accumulate contaminants, and are often used as biomonitors.

We will report to the Department of Health if sentintel mussel tissue exceeds relevant environmental and health guidelines.

Sediment surveys are used to determine the spatial variability in concentrations of potential wastewater contaminants in sediment sites in the vicinity of the Alkimos ocean outlet.

Results are compared against ANCECC/ARMCANZ (2000) guidelines to provide an indication of trigger levels for contaminants in sediments that may have the potential to cause ecological effects in marine communities.

Sediments will be collected every five years and analysed for:

  • Grain-size distribution (laser diffraction and sieving)
  • Organic matter content (loss on ignition at 550ºC)
  • Carbonate content (loss on ignition at 1000ºC)
  • Pesticides and herbicides
  • Nutrients (TKN, TKP)
  • Metals (aluminium [Al], arsenic [As], cadmium [Cd], cobalt [Co], copper [Cu], lead [Pb], mercury [Hg], nickel [Ni], selenium [Se], silver [Ag], zinc [Zn])

Contact details

The Alkimos ocean outfall monitoring program team comprises a multi-disciplinary group of scientists, responsible for carrying out monitoring and analytical tasks specific to their area of expertise.

For more information about the program please contact Heather Fergusson, Technical Advisor – Environment on (08) 9371 4058 or email Heather.Fergusson@watercorporation.com.au