Services CASE STUDIES

Keeping Salt Water in the Harbour

Package 3B was bundled to include both investigation and rehabilitation of sewer lines around the harbour foreshores with the aim of reducing the ingress of salt water into the system.

Typically around Sydney Harbour, wastewater flow is collected from harbour side suburbs and taken to carriers along the foreshore which deliver it to sewage pumping stations situated at the lowest point of the system. From here the flow is pumped to a treatment plant.

The foreshore pipelines are subject to ingress of salt water which leads to corrosion of pumps and pipelines as well as the additional costs of pumping and treatment.

Package 3B, awarded to Interflow Pty Limited, required much more than rehabilitation of these deteriorated pipelines. It required Interflow to conduct source detection then propose a program of rehabilitation that offered the greatest reductions in salt water ingress up to the fixed value allocated to the Contract.

Thirty-two “problem” waterfront catchments were nominated and given a priority order for investigation.

The steps in the contract were:

• Source detection


• CCTV and cleaning in areas where source detection indicated problems


• Preparation of a catchment report and a rehabilitation program to reduce ingress


• Obtaining Sydney Water approval for the proposed rehabilitation work


• Undertake rehabilitation work to reduce ingress


• Testing the effectiveness of the solution

Sea water ingress was thought to be typically due to a point source or section of damaged sewer line or maintenance hole. To most efficiency use the available funds, source detection was specified to narrow down problem areas before carrying out rehabilitation work.

The first stage of source detection was to conduct salinity testing, starting at maintenance holes near the pumping stations.

Salinity testing by measuring conductivity is a quick and easy method to identify and quantify any seawater intrusion. Sewage normally has a salinity of less than 0.1%, while seawater salinity is 3.5%.

Interflow commissioned an independent laboratory to undertake salinity testing on samples taken from maintenance holes along the foreshore. From these readings it could be broadly seen where the problem areas lay. CCTV testing and maintenance hole inspection was then conducted in these areas to identify where rehabilitation work should take place.

Once this data was gathered a plan of action was proposed to achieve the intent of the project – i.e. maximum reduction of seawater ingress up to the specified value of the contract.

Samples for salinity testing were taken during a time period from one hour before high tide to two hours after.

One of the first catchments to be actioned under this Package was at Connells Point in south eastern Sydney. Salinity testing at the worst of the locations at high tide showed some 90% of the flow in the 225mm pipeline was seawater. At other locations testing showed that no rehabilitation was required.

The program proposed by Interflow involved rehabilitation of both maintenance holes and pipelines. Liners were subsequently installed in 55 lengths of sewer pipelines.

Testing after the completion of the rehabilitation program showed the aims of the project had been achieved. Flow in the pipeline at high tide at the same time of day was less than 10% of the amount found before rehabilitation. The salinity of the flow was the same as that for normal sewage, indicating that seawater infiltration was now negligible.

To achieve these aims, expenditure at Connells Point was in the following proportions:

• Salinity testing 1%


• Source detection: 19%


• Maintenance hole rehabilitation: 15%


• Pipeline rehabilitation 65%

While the program is on-going, similar reductions in infiltration are being found in other catchments once rehabilitation is completed. Investigations have shown that varying amounts and proportions of work are required to achieve these results. It has also been shown that rehabilitation of the entire catchment is not necessarily required to meet the project’s desired outcomes.

For Interflow the project has seen the need to modify some work practices to take into account its unique requirements. Some of these modifications have been due to:

Heavy internal encrustation in cast iron pipelines subject to salt water infiltration. Interflow has needed to develop new procedures to remove this.

Maintenance holes in remote locations and some actually in the water. New procedures have been developed to safely set up equipment in these situations.

For Sydney Water the continuity of process from source detection to rehabilitation has meant less administration and tendering cost. The form of the contract allows Sydney Water to maintain control but operation remains the responsibility of Contractor. Having the Contractor in tune with the intent of the project, rather than just carrying out rehabilitation, has meant a learning curve for Interflow and the need to further develop their capabilities. This will lead to continued benefits on future contracts of this nature.

The results obtained so far indicate that the objectives of the program are being met.

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