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Salinity modeling for the Altona sewerage strategy

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Courtesy of Innovyze

City West Water’s Altona sewerage system is possibly one of the most studied sewerage networks in Melbourne, Australia. Currently the tertiary sewage effluent from the Altona treatment plant is discharged to Port Philip Bay with an average salinity of approximately 4500 mg/litre. 

This study involved the development of a detailed model that accurately reflected the sources and dilution of salinity as it was conveyed through the sewerage system. The model was then used to assess the probable reduction in salinity entering the treatment plant from a range of rehabilitation and system augmentation scenarios. The ultimate goal of the project being to assess the extent of works necessary to reduce the salinity of the effluent from the treatment plant to approximately 600 mg/l to make it suitable for recycling for irrigation and industrial uses.

Parsons Brinckerhoff were engaged as the primary consultant for the overall system strategy, with Neil Moody from Urban Water Solutions acting as the specialist modeling sub-consultant undertaking the modeling and analysis work using Infoworks CS V8.05.

Network characteristics
The sewerage network has around 190km of pipe servicing 16,000 properties in a primarily residential area with minor commercial and industrial premises. The network has one deep pumping station at the Altona treatment plant, serving the deep gravity branch sewers. Approximately 45km of the sewer network is located under the groundwater table.

Average dry weather flow to the treatment plant is approximately 12.6MLD, around a third of which is groundwater infiltration (GWI). The daily salt load to the treatment plant is some 57 tonnes, with less than 5 tonnes of this derived from sanitary flows. Modeling was undertaken to understand the origins and possible ways of solving the salinity issues.

The network was modeled as a high-detail, all pipe model, with individual properties modeled as sub-catchments and an additional set of sub-catchments developed for assigning the saline GWI flows.

Commercial and industrial dry weather flows were included as ‘additional flows’, with the trade waste flows and trade waste profiles were reserved for GWI.

To undertake flow and salinity modeling, 23 temporary flow monitors were installed in the system to complement the magnetic flow meter installed at the pumping station. Twenty three salinity probes were installed in the system, generally on a manhole upstream of the flow monitors so that they didn’t interfere with the flow results, plus one in the wet well. Seven hydrogeological boreholes were drilled and monitored as part of the 6 month study running from late November 2006 to late May 2007, and recorded variations in groundwater salinity from 6,000 to 35,000 mg/l. Using the data from these boreholes in conjunction with the geology of the area a hydrogeologist developed a groundwater salinity contour for the catchment.

A review of the recorded salinity data indicated a number of challenges. The data range revealed salinity levels from 400 to 800mg/l at sites with no saline water intrusion and from 5000 to 30,000mg/l at other sites. In addition, flow data drop out occurred at some sites during low flows and equipment clogged, which dampened the recorded peaks and troughs.

Salinity data obtained from one site downstream of a purely residential catchment provided a particularly good example of the diurnal variation in residential salinity load. Data from this site was used to develop weekday and weekend average salinity pollutographs that were subsequently applied to all residential flows.

The average dry weather flow per property in this catchment was determined to be approximately 360 l/d/d (litres/dwelling/day), compared with the historically accepted ‘normal’ discharge of approximately 600l/d/d. The dramatic reduction in daily sewage discharge attributed to increased grey water re-use and wide scale implementation of domestic water saving devices following nearly a decade of water restrictions in Melbourne

The average flow weighted salinity recorded from this catchment was around 580mg/l compared with historically accepted salinity values of around 350mg/l. While these recorded values appeared dramatically different from the historical norm, the resultant daily salt loads per property was very similar at around 210 g/d/d. 

Model Calibration
Calibrating the model for both dry weather flow and salinity over the six month period was an interesting challenge. To achieve a suitable fit for the diurnal variation in salinity at each site, it was necessary to correctly attribute the sanitary and groundwater flow components as well as the ground water salinity from each catchment. The fact that the salinity peaks occur during the minimum early morning flows, and the salinity troughs at the maximum morning flows was an interesting inverse paradigm to contend with.

While there were issues with the quality of the salinity data at a number of the upstream sites the data quality and quantity improved in the larger downstream monitoring sites.

The figure below illustrates the comparison of the modeled (brown) against the recorded (blue) salinity at the influent pumping station to the Altona Treatment Plant.

Application of the Model
The calibrated model proved to be a useful analysis platform to test the effectiveness of various system augmentation scenarios to reduce flows and salinity to the treatment plant, including relining, interception, low pressure and vacuum alternatives.

Key Lessons
For future projects of this nature Mr Moody recommended a shortened monitoring period with more frequent maintenance visits to ensure that salinity probes are regularly cleaned and data checked. Sites with minimal groundwater infiltration, and hence minimal low flows, tend to be the most problematic sites to monitor and should be excluded from the program following preliminary site visits.

It also became clear from the modeling that the water reduction strategies implemented by the Melbourne water authorities have resulted in a significantly stronger concentration of sewage being conveyed by the sewerage system and discharged into the treatment works. These changes in customer behavior may have long term ramifications for the system and treatment plant operators.

Outcomes
A key outcome was that City West Water was able to demonstrate to the Environment Protection Agency that it understood the source, and feasible augmentation options to mitigate the effects of salinity. This will help City West Water to meet its long term objective to close the outfall at the Altona Treatment Plant Outfall.

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