CONTENTS

Introduction
Sewers seaward
Toothless watchdog
Toxic fish
Sewer-side surfing
Public relations battle
Events of 1989
Beyond Sydney
Conclusion
Bibliography

Consequences of a submerged field

Caldwell Connell argued that when the sewage field was submerged it would be carried southwards by the current. The reliance of the Board and Caldwell Connell on the East Australian Current is not even supported by their own re- search. In a 1976 Caldwell Connell study, onshore currents at Bondi were observed a significant percentage of the time throughout the year and at North Head and Malabar they were observed in all seasons except spring. For summer, when the submerged held is supposed to be working best, Caldwell Connell show 35 per cent of deep ocean currents going towards the shore at North Head, 50 per cent at Bondi and 50 per cent at Malabar.

In a later Caldwell Connell study in 1980 it was found that the frequency of deeper onshore currents throughout the year was 40 per cent for North Head, 30 per cent for Bondi and 42 per cent for Malabar. Current meters had been installed in the vicinity of the proposed outfall diffusers at 30 metres’ depth. However, it seems that Caldwell Connell was determined these results would not interfere with their prediction of the held being carried away by a southerly current. The conclusion of their principal investigations engineer was that long-term discharges were carried south! 

In the report tendered by the Water Board to the SPCC as part of its application for approval or the Malabar extended ocean outfall in 1983 the following table was given. Table 4.2 shows that the probability of sewage reaching shore because of a surface field as 2 per cent in summer and 40 per cent in winter. But the table does not include the probability of sewage reaching shore when it is submerged. If submerged fields, which are subject to deeper onshore currents, reach the shore then the table completely underestimates the probability of sewage reaching shore.

Table 4.2 Seasonal behaviour of the sewage field

Season

Probability of Surface Field

Probability of Onshore Surface Current

Probability of Reaching Shore

Summer

4%

50%

2%

Winter

80%

50%

40%

Source: Clean Waters Advisory Committee Meeting, Business Papers, 8th September 1983, p. 18

However, Caldwell Connell assumed that submerged fields which travelled towards the shore would remain submerged and turn parallel to the shore before coming in. This assumption is not substantiated. No good reason is given why the submerged sewage field would turn away rather than come on into shallower waters, where it would be mixed with the surf zone and Caldwell Connell did not try to find out what would happen to a submerged field as it came close to shore.

Submerged fields are not necessarily preferable to surface fields if sewage can still reach shore. Brain contends that submerged fields would hang around for a long time, the bacteria in them protected from the sunlight in the deeper water, and occasionally remixing with beach waters. The Department of Mineral Resources suggested that if a submerged field was maintained then this might lead to an increase in the tainting of fish by detergents. It would also keep detergents under the surface ‘hence ensuring a wider and more dilute distribution of the grease released during degradation of the detergents.’

If the submerged field is of such dubious benefit in terms of keeping sewage off the beaches and even hindering its decomposition, why have Caldwell Connell tried so hard to achieve it? The answer seems to be that a submerged field would not be visible. Caldwell Connell say it is essential that the sewage discharges do not cause ‘aesthetic nuisances’, and that this can be achieved firstly by dilution and secondly by maintaining a submerged field for as much of the year as possible.

The SPCC has been particularly concerned about the visibility of surface sewage fields, as it has emphasised in meetings with the Board. It wanted to know at what dilution would the surface field cease to be visible to a 1ay person from the shore, a boat and an aircraft and under what circumstances would surface slicks of floatable material become visible. It was assured by Caldwell Connell that the sewage held would generally not be visible. It would only be on a rare occasion, such as a malfunction, illegal discharge, or particular combinations of t wind and wave conditions that a person with keen eyesight would be able to see it.

When the new outfalls are commissioned, the toxic waste will still be discharged, the viruses will still be discharged. The extended outfalls do not treat the sewage, they transport it and if they work as designed they will deliver the sewage into the sea so that you won’t be able to see it. But it will still be there!