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, particularly at high tide, which leads to corrosion of pumps and pipelines as well as the additional costs of pumping and treatment.

Sydney Water’s 2005 Sewer Rehabilitation Program introduced another innovation in the way part of the contract was constructed.

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

Package 3B, awarded to Interflow, 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, * Rehabilitation work to reduce ingress, and, * 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 efficiently 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 per cent, while seawater salinity is 3.5 per cent.

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. rehabilitation up to the fixed value of the contract to achieve maximum reduction in salt water ingress.

Samples for salinity testing were taken during a time period from one hour before high tide to two hours after. Testing showed that in some of the lines, over 90 per cent of the flow was from seawater. On other lines there was no significant infiltration.

The results of the salinity testing allowed identification of areas where more precise source detection was needed.

Investigation showed that the rehabilitation needs varied between catchments. For example, extensive lining of the pipelines was required at Connells Point, a residential area on the Georges River in southeastern Sydney. In contrast, investigation of the Circular Quay catchment in central Sydney showed that a higher proportion of expenditure was required on maintenance hole rehabilitation.

Results of the program have exceeded expectations. Flow reductions of over 90 per cent at high tide have been regularly achieved. The salinity of the flow shows that salt water ingress has been all but eliminated.

The program has 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 simply carrying out rehabilitation, has challenged Interflow and provided opportunity to further develop its capabilities. This will lead to continued benefits on future contracts of this nature.

The results obtained indicate that the objectives of the program have been met.