An accurate InfoWorks model of the Altona Treatment Plant (ATP) is playing a key part in determining how the system operates currently and may operate as future developments come on stream. City West Water has demonstrated that InfoWorks has the flexibility to model the full operation of the plant, which includes features such as intermittent decanting, flow separation devices, sludge and low lift pumps and overflow mechanisms. The plant also operates on a four hour cycle.
Advanced treatments
The original ATP plant was built in the 1960s to a traditional design that featured among other things, trickling filters, grit channels and lagoons. City West Water upgraded it in 2005/06 to an Intermittent Decant Extended Aeration (IDEA) plant, keeping little of the original plant except for the inlet pump station and lagoon structures. In the IDEA plant, activated sludge is aerated over a number of pre-determined cycles. Treated effluent is decanted every four hours from just below the liquid surface, with separation of the solids and liquid taking place in between the decanting operations. A further stage of treatment is also being proposed at Altona, with the provision of a Micro-Filtration Reverse Osmosis (MFRO) plant.
The sewerage network upstream of the plant had already been extensively studied, with InfoWorks CS used to produce a detailed model that accurately reflects the sources and dilution of salinity.* The modeling has enabled City West Water to consider approaches to reduce the salinity of sewage entering the plant with a view to producing effluent that is suitable for irrigation and industrial uses.
Developing an InfoWorks model
City West Water identified several reasons for modeling the plant itself in InfoWorks CS. In particular, the model provides the opportunity for scenario modeling of the hydraulic loads through the plant. These include loss of power to the plant and the effect of taking key components offline for maintenance. The InfoWorks CS model can also be used to develop contingency plans, including how to pass storm flows through the plant while continuing to meet environmental requirements. It helps establish whether the plant has sufficient hydraulic capacity for particular storm scenarios and how any excess flows should be dealt with.
The InfoWorks model has also been developed to provide assistance with the MFRO design and investigating the optimization of its operating regime.
Standalone modeling
The new InfoWorks model has been developed as a standalone system that covers the entire network downstream of the pump station, including the planned MFRO. The plant model has been kept separate from that of Altona’s catchment. This allows the plant itself to be calibrated accurately using the existing metering. In time, the two models may be connected.
The plant’s inlet pump station brings flows to the surface from the entry point which is approximately 19m (62’) deep. Flows gravitate through the IDEA plant into the balance tank, passing through digesters for where sludge is removed from the process. The water from sludge dewatering is passed back through the loop via a return pump station. The effluent leaving the balance tank then passes through a cloth filter, and a UV disinfectant treatment prior to discharge through an outfall into the bay.
The treatment plant’s inlet flow meter is used to generate the inflow file for use in InfoWorks. The model also takes account of the complex behavior of the return pump station, which is used as part of an internal recycling regime that supplies various processes within the plant.
Use of eight further calibration points ensures that the model’s results give accurate representation of the plant’s behavior. The plant is modern and has many Supervisory control and data acquisition (SCADA) points, which give information on aspects such as depths, flows and pump timings. There was no need to incorporate every aspect of the SCADA data; bulk hydraulic points provide sufficient information for accurate modeling.
Calibration of the model is done to both depth and flow at various points of the plant. Depth readings are calibrated in the three IDEA reactors as well as the balance tank. Flows through sludge pumps and the plant’s outflow meter provide calibration points for flows exiting the model.
There is a flow control device at the inlet works that split the flow up between the reactors. Each of the reactors was initially modeled as a storage node but the approach was changed to model them as long flat open channels as this gave greatly improved results.
Modeling of the plant’s inlet works currently takes a simplified form that does not include the headlosses through the screens or grit channels. These losses as well as other monitoring done at this stage, could be added in the future if required.
Issues with Modeling the Plant
The most difficult feature of the plant to model has been the intermittent decanter. An arm swings down into the reactor tank every four hours and decants the top of the flow down to a constant level, regardless of the starting level. The decanted flow is deposited into the balance tank. City West Water devised an effective way of replicating this in InfoWorks by treating the decanter as a variable sluice gate with an invert below the minimum tank level, and applied real-time control (RTC). This provided a reasonably accurate method of replicating the drawdown affect in the reactors, whilst also controlling flow into the balance tank.
Another aspect that was difficult to model was the outlet flow regime, in particular the low lift pumps. The pumping operations are variable and are at the day to day discretion of the plant operators. Currently the regime is adjusted to keep the balance tank level as low as possible.
Modeling the balance tank depths depends on a combination of the decanting operation as well as the low lift pump operation. It too has already achieved good results, with scope for further enhancement by adjusting pump values.
As mentioned earlier the plant’s operation varies depending on the incoming flows, operator preferences and the biological, chemical and hydraulic loads. The complexity of the internal recycling process also added to the difficulty of calibration.
One of the calibration exercises involved a period of wet weather, when the flows rose from about 180l/s (48gps) to 300l/s (79gps) over a few days. Operational changes were made during the wet weather, including stopping the pumps for a period and changing the flow through the low lift pumps. InfoWorks was able to take account of these changes through adjustments to the RTC for the pump operations.
Planning for MFRO
A key development of the InfoWorks model has been the inclusion of the planned membrane brackish-water treatment MFRO plant. The final design is yet to be determined, but its core components have already been created in InfoWorks to build an initial model of its process. The hydraulics will be refined at a later stage.
There will be constant flow into and out of the MFRO, and the initial modeling replicates the flow paths. The MFRO will provide varying degrees of treatment to suit different customers and end uses. This type of detail will be applied to the InfoWorks model once the plant’s final specification has been established. The model will then be able to explore a wide range of scenarios for the plant’s operation, including establishing the expected reliability and the distribution of the recycled water.
Another useful application of the InfoWorks model has been in modeling the outfall into the bay to investigate the possible effects of the MFRO process. InfoWorks allows City West Water to investigate a number of regimes and scenarios in respect to daily and seasonal tide levels as well as long term sea levels. Modeling the outfall in InfoWorks CS will help determine what infrastructure is required for efficient operation as well as protection of the plant.
Looking ahead
Future developments will expand the detail within the InfoWorks model considering that it already covers all the major hydraulic structures. There are plans to include additional aspects such as the screens and grit chambers in the inlet works and the head losses in the cloth filters and the UV disinfectant process. There is also further work to be done in developing the model to provide more detail about the flows that are recycled internally to build an ever-more comprehensive picture of the Altona plant’s operation.
This article by Matt White of City West Water is based on a presentation given to the 2008 Wallingford Software user conference in Melbourne, Australia.
* Click here to read about this paper and other papers presented at Wallingford Software’s 2008 Australian user conferences