Dalby is a town of about 10,000 people in the middle of the Darling Downs region of southern Queensland, Australia.  About 160 km (100 miles) from the coast, it is surrounded by flat, fertile plains.  The nearby Condamine River is the most significant watercourse for many miles.  Dalby's economy focuses largely on agriculture, agricultural services and the manufacture of agricultural machinery, with significant areas of crops, such as cotton, grown under irrigation.

Since the early 1960s, the town had enjoyed a good, reliable water supply from a scheme based on groundwater, with surface water from Loudoun Weir¹ on the Condamine River incorporated for peak-demand supply and emergency backup.  Traditionally, the groundwater has come from a relatively large number of shallow alluvial bores adjacent to the river.  Apart from chlorination, this water required no further treatment for potable use, whereas the river water must be clarified and filtered before chlorination.

Output from the bores has been decreasing for many years.  Dalby Town Council countered this reduction by sinking more bores to maintain or increase capacity; however, in the mid-1990s it was recognised that the high-quality aquifers were being depleted faster than they were being recharged.  Although the Loudoun Weir capacity was doubled and an increased allocation was obtained, the weir's reliability remained at only 50 percent of this allocation annually.  The surface-water treatment plant was used increasingly to meet baseload demands, but the weir's low reliability meant that the town often had to reduce bore supply and impose water restrictions.  Borewater quality was declining at a rate similar to its output. 

Our Team Recommends Desalination

In 1999, Dalby Town Council contracted PB to help it find new ways of meeting the town's future water needs.  This was the beginning of an ongoing relationship that was to yield great benefits for both parties.  We began our work by conducting a water supply options study.  Sixteen options were identified, including stormwater recycling, wastewater recycling, a new weir, off-stream storage, desalination, and a regional pipeline.  Of these, we concluded that a desalination plant that would treat brackish groundwater from bores was the most reliable and cost-effective option.  This conclusion was supported by follow-up groundwater and feasibility studies.  Ultimately, Council accepted a package of options that included demand management and supply-side infrastructure. 

We recommended a desalination plant of approximately 20 L/sec (315 gpm) capacity.  The product water would be blended at the water treatment plant with groundwater and treated water from the Condamine River.  

Once Council decided to proceed with the plant's construction, PB was responsible for preparing design-build tender documents, and preparing and administering the contract.  Given the project's scale, three membrane filtration technologies that remove ions from solution were under consideration:

• Reverse osmosis, by which a pressure is applied that exceeds the osmotic pressure of the salt solution against a semipermeable membrane, thereby forcing pure water through and leaving salts behind.
• Nanofiltration, which is a variation of reverse osmosis but allows a higher passage of monovalent salts, thereby reducing the energy required to achieve adequate softening of moderately saline water
• Electrodialysis reversal (EDR), whereby the polarity of the electrodes is reversed from electrodialysis, in which ions are transferred through a membrane from a less concentrated to a more concentrated solution as a result of the passage of direct electric current.  The result is the direction of ion movement in a membrane stack is reversed.

The tender process was used to finalise the method of desalination used at the plant.  No tenders were received for nanofiltration or EDR, however, the final decision was to build a reverse osmosis membrane desalination plant.  The schematic is shown in Figure 1. 

Review of Raw Water Quality

An important factor in selecting a desalination process and designing a desalination plant is the quality of the inflow stream, so we collected samples regularly from selected feed-water bores and sent them for comprehensive testing (Figure 2) that included elements associated with fouling, such as calcium, iron, manganese and silica (soluble and colloidal).  The water quality data collected from Bore 10, which was to provide the largest portion of water to be treated, indicated:

• The raw water was low in turbidity and colour.
• The raw water was highly mineralised and well above the recommended maximum total dissolved solids (TDS)
• Total hardness and chloride concentration were above the acceptable limits for potable water.
• Iron and manganese were levels were not significant.
• pH was generally between 7.2 and 7.7.
• Silica concentration was approximately 30 mg/L.

Two additional parameters were determined to be useful in the design of desalination plants-hydrogen sulfide (H₂S) and silt density index (SDI).

Processing borewater that is high in H₂S through desalination can increase the H₂S concentrations and convert this substance to a gas; however, field tests at Bores 10, 11 and 12 showed H₂S levels to be well below acceptable limits.

SDI provides an indication of the likelihood of the feed water to cause fouling of a membrane.  SDI is not tested by general laboratories in Australia, so Dalby Town Council obtained a methodology and performed the tests on site by exposing a 0.45 micron filter to the feed water under pressure and then calculating filtration rates.  An SDI of less than 5 is considered acceptable for the reverse osmosis systems, although there are exceptions when an SDI of less than 3 is desirable due to the nature of the suspended solids in the feed water.

The SDI tests for Bores 10 and 11 indicated that the feed water was highly likely to cause fouling of the membranes (Table 1).  In Bore 10, increasing the length of time that the pumps ran prior to sampling appeared to result in a decreasing SDI.  Therefore, the continuous running of pumps to supply the desalination plant was seen as the probable way to reduce the tendency for membranes to become fouled.  Bore 12 was constructed the same month much of this testing was performed, so only very limited water quality data was available.

We also noted that Mn and Fe bacteria were identified in the raw groundwater from bores in the region, including nearby bores at Dalby, and that these should be taken into account in the plant design.  We recommended pretreatment, which was likely to be as important as the desalination process itself,  as the correct pretreatment process train would:

• Maximise the recovery ratio of the desalination process
• Optimise the product water quality
• Protect the desalination components, including the membranes in a reverse osmosis process.

Pilot Plant Trials

After the tenders were opened but before the contract was awarded, the client purchased and operated a pilot plant for about 6 months to reduce the level of uncertainty related to the levels of silica, barium sulfate and silt in the water samples and their potential to either:

• Cause scaling and fouling of the RO membranes
• Limit the possible recovery to an unacceptably low figure.

The process train used was considered to have a significant chance of success, but avoided high reliance on chemicals for pre-treatment pH adjustment and complex/ expensive systems.  The plant had an output of 5.5 L/min (1.5 gpm) and operated at around 80 percent recovery.  Pre-treatment was provided by a multimedia filter, followed by 5 and 1 micron cartridge filters.  Flocon 260 anti-scalant was dosed at around 6 mg/L. 

The reverse osmosis unit was designed to simulate the last two membranes in a full-scale plant because these are the membranes most likely to be subjected to scaling.  This condition was simulated by using only two membranes, but recycling at a high rate the concentrate stream from the last membrane back to the first.  Daily monitoring and regular trend analysis indicated no apparent scaling problems.

The higher SDIs that were detected in initial testing reduced to acceptable levels once the bore had stabilised after a few hours of operation.  This phenomenon had been noted during laboratory testing and continuous operation of the pilot plant, and Bore 12 proved that this was the case.

The multimedia filter proved invaluable for removing colloidal matter and precipitated iron. Backwash frequency based on round-the-clock operation was every 2 to 3 days. Five-micron cartridge change out based on a maximum pressure differential of about 100 kpa was between 600 hours and 700 hours of operating time. Membranes needed to be cleaned only about every three months. Estimated costs for the final plant are shown in Figure 3.

Conclusions

We believe that the Dalby Desalination Project is the forerunner of other desalination plants  as existing water sources come under increasing pressure in the future.  Experience gained on this project will be of value to other communities considering desalination as a means of improving the yield of their water supply systems.

The partnership developed by Dalby Town Council and PB on this project has been based on an open sharing of information and "best for project" decisions reached after technical discussions on a wide range of issues.  The plant opened in September 2004.  It is capable of providing 25 percent of the Dalby's water requirements, and has drought-proofed the town for the current prolonged drought that Australia is enduring.