Perth’s next major water supply project is taking shape beneath the ocean, where engineering precision is making an ambitious infrastructure development possible.
This article features Perth’s Alkimos Desalination Plant, highlighting three engineering achievements and what they could mean for the future of large-scale water infrastructure and sustainable water supply.
One of the Alkimos Desalination Plant’s major engineering achievements is the construction of more than 6 km of tunnels beneath the seabed without excavating open trenches across the ocean floor. According to Water Corporation, the project used two massive tunnel boring machines (TBMs) to construct the marine tunnels that connected the onshore treatment facility to the ocean.
Maintaining accurate alignment beneath the seabed was a critical engineering challenge because even small deviations from the planned tunnel routes could affect the positioning of offshore connections and require difficult corrections during excavation.
The TBMs used VMT’s laser-based guidance system, a navigation technology that uses laser measurements to monitor a tunnel boring machine’s position and alignment relative to its planned route in real time, to allow operators to detect deviations and adjust their course. As excavation progressed, precast concrete segments were installed to form continuous tunnel linings, providing structural support and to help limit water seepage.
Combining these technologies allows the project to establish critical marine connections while minimizing direct disturbance to the seabed and surrounding marine environment. For future large-scale water infrastructure projects, this approach could enable the construction of underwater intake and discharge connections in environmentally sensitive coastal areas while reducing seabed disturbance and managing construction risks.
Another engineering achievement at the Alkimos Desalination Plant is the installation of specialized offshore structures, including seawater intake structures and an outfall diffuser, designed to manage seawater extraction and concentrated brine discharge. According to Water Corporation, the project includes two 12 m-wide seawater intake structures approximately 1.6 km offshore and a 130 m-long outfall diffuser approximately 3 km offshore.
The intake structures allow seawater to enter the treatment system, where suspended particles and impurities are removed before high-pressure pumps force the water through reverse osmosis, a treatment process that uses semipermeable membranes to separate freshwater from dissolved salts.
This process also produces concentrated brine, which is transported through the outfall tunnel to the offshore diffuser. The diffuser then distributes the discharge through multiple outlets to promote mixing with surrounding seawater. This helped limit localized increases in salinity that could affect marine ecosystems.
The challenge was to manage the large volumes of seawater required for drinking water production while controlling the environmental effects of brine discharge.
This required careful consideration of intake flow rates, discharge velocity, and surrounding ocean conditions such as tidal movements, water depth, and background salinity. For future large-scale desalination projects, integrating these hydraulic and environmental requirements into offshore infrastructure design could help maintain reliable water production while at the same time reducing potential impacts on marine ecosystems.
Beyond its construction and marine systems, the Alkimos Desalination Plant also demonstrated how infrastructure can be designed to accommodate future water demand.
According to Water Corporation, the plant’s first stage is expected to produce 50 billion liters of drinking water annually when operational in 2028. Its marine tunnels have also been designed to accommodate future expansion beyond this initial production capacity.
This forward planning is important because increasing desalination capacity involves more than adding treatment equipment. Higher production volumes would require greater seawater intake and brine discharge capacity. If the marine tunnels were designed only for the plant’s initial requirements of 50 billion liters of drinking water annually, future expansion could involve constructing additional offshore connections and continuous upgrades of existing infrastructure. By sizing the tunnels to accommodate increased water flows from the outset, the project could reduce the need for further marine construction as production capacity expands.
For future large-scale water infrastructure projects, incorporating additional capacity into the initial design could help reduce long-term expansion costs and minimize disruption to marine environments. It could also allow infrastructure operators to respond to increasing water demand by expanding treatment facilities while retaining existing marine connections, provided the remaining systems can support the additional capacity.
The Alkimos project demonstrates how advances in construction methods, engineering design, and water treatment technology can address growing water demands while supporting the development of more sustainable and resilient water infrastructure.
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This article was published November 10th, 2026 and the content is current as at the date of publication.
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