This saltwater ingress causes sewer asset deterioration (corrosion) and unnecessary operating cost in transporting, treating, and disposal of the effluent.

As part of its Sewer Rehabilitation Program 2006-09 Sydney Water Corporation (SWC) separated out what was called Package 4: Tidal Ingress Sewer Gauging. After careful consideration the two year term contract for Package 4 was awarded to Kembla Watertech in early January 2008.

The tidal ingress gauged data will be used for:

* Determining system ingress flows and salinity, * Assessing and predicting flows, infiltration-inflow, exfiltration and capacities * Assessing effectiveness of sewer renewal programs, and * Reporting.

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Scope of Works

The scope of works for SRP 2006-09 Package 4 is to supply accurate, uninterrupted, quality-coded sewer level, velocity, flow and salinity data at the nominated locations for a minimum period of two weeks.

The tender specification requires that flow/salinity readings be taken in maintenance holes upstream of each Sewer Pumping Station (SPS) catchment where the likelihood of flow/salinity is highest. Physical closeness to tidal waters, topography, sewer access etc needs to be considered in determining the sites to be sampled. The flow/salinity sampling and CCTV surveys are to be used to narrow down the source of the tidal ingress. Rehabilitation of areas affected by tidal ingress will include design, manufacture, supply, installation and testing of structural liners. Activities will also include inspection and sealing of maintenance holes.

An allocation of $7 million has been provided by Sydney Water for the two year contract period.

The table below was provided by Sydney Water as indicative of typical spot conductivity readings and the corresponding classification of water salinity:

Rather obviously, salinity is the measure of ‘salt content’ in water. Seawater contains a higher level of sodium and chloride ions than potable water or domestic sewage and consequently a much higher conductivity as well.

Sewage normally has a salinity of less than 0.1 per cent (conductivity between 40 and 200 milliSiemans per metre or mS/m) while seawater has a salinity of 3.5 per cent (conductivity in the order of 5,000 mS/m). Because of this large difference, conductivity can be used to measure salinity in sewage.

This conductivity reading (mS/m) can be converted to a nominal reading of parts per million (ppm) by multiplying by 5.5.

Works Execution

One of Kembla’s experienced civil engineers has been appointed as Project Manager to head up a specialised team for this critical project.

Initially, Kembla Watertech is to propose a flow and salinity gauging strategy that will significantly split up the catchment so as to allow investigation and assessment of manageable sections of catchment. It is anticipated that the flow/salinity gauging will take readings every five minutes.

Kembla is to analyse all the flow and salinity data obtained including the use of information such as tide charts and graphs.

Kembla should then be able to effectively eliminate particular sections of the catchment that are not contributing to tidal ingress. Other sections of catchment that cannot be eliminated will require additional field investigation.

The process used for assessing the data will be clearly defined and each decision will be documented as part of the proposed Rehabilitation Plan.

This plan is to have two clearly defined sections, firstly a section detailing the gauging data investigation and analysis (the justifications behind decisions made), and a section detailing the proposed rehabilitation works.