P Tate, C Marvell
Publication Date (Web): 16 March 2016

Sydney Water’s three deep ocean outfalls (servicing the wastewater treatment plants at North Head, Bondi and Malabar) were commissioned in the early 1990s and turn 25 years of age in 2015/16. Combined, the outfalls service the wastewater disposal needs of about 80% of Sydney’s population.

This paper focuses on the process leading to the decision to build the deep ocean outfalls, an overview of how the outfalls work and the environmental performance based on data collected and analysed over the last 25 years.

The paper concludes by assessing whether the continued operation of the deep ocean outfalls can be maintained without causing detrimental effects to the marine environment and without compromising public health.

An increasing population, greater beach use and a better environmental awareness all contributed to an understanding that the cliff face outfalls would not be part of a long-term solution for Sydney’s wastewater disposal.

The deep ocean outfalls were built in response to this growing population and the need to dispose of wastewater “safely, efficiently and economically … without detriment to the environment”. Adoption of the deep ocean outfalls option had three main objectives: (1) to make the beaches suitable and safe for swimming; (2) to ensure fish and shellfish were safe to eat; and (3) to protect the marine environment. They were massive engineering feats, a world first, that continue to operate as designed.

A number of problems had been encountered with overseas outfalls. Some of these problems are discussed in this paper and it is shown how these problems were resolved via the unique design of the Sydney deep ocean outfalls. An assessment is made comparing the original design criteria with modelled performance over the last decade. Results show that the design criteria have been met each year since commissioning, and that the outfalls continue to operate as designed.

An Environmental Monitoring Program (EMP) was undertaken to assess whether changes to the marine environment resulted from the operation of the deep ocean outfalls. This was a comprehensive five-year monitoring program managed by the New South Wales Government’s Environmental Protection Authority. Results from their monitoring program concluded that there was “… little detrimental environmental effect of the new mode of sewage disposal”. They further noted that previously degraded areas of the inshore waters recovered after the cessation of the shoreline discharges.

However, the EPA did recognise that the duration of the EMP was short term compared with the anticipated life of the outfalls. This led to the implementation of an ongoing monitoring program to determine whether long-term impacts associated with the discharges are likely to occur. The long-term monitoring program integrates outfall hydraulics, wastewater toxicity and quality, oceanography and numerical modelling, marine benthic ecology, sediment characteristics and microbial indicators on the beaches. This integrated ‘weight of evidence’ approach to assessment is used to demonstrate the sustainability and success of these outfalls. Results from this monitoring indicate that no statistically significant impacts have been detected.

Figure 1. The highly visible plume from the Malabar old cliff-face outfall (left) and examples of beach grease (right).

Click here to read the full paper

Not a member? You can purchase the paper in our Online Document Library.