The availability of clean water for drinking and other uses has been a hot-button issue in Australia for at least two decades. The Millennium Drought, which ravaged Australia between 1996 and 2010, made water supply front-page news across the country and led to the construction of Australia’s first large-scale municipal desalination plant, in Perth, followed by similar plants on the Gold Coast and in Sydney, Melbourne and Adelaide. At the same time, Brisbane was constructing the largest recycled water scheme in Australia, consisting of three advanced water treatment plants that together can produce 180 megalitres of purified recycled water per day.
The technology that makes all this possible is reverse osmosis. Modern reverse osmosis technology consists of semipermeable membrane sheets spiral-wound around a central tube that collects and transports the desalinated water. To separate salt water or brackish water into a purified stream and a concentrated stream, sufficient pressure must be applied, both to overcome the osmotic pressure of the feed water and to push the water through the cylindrical membrane element. For seawater, pressures of about 6000 kilopascals are typical, while for water recycling, where the salt concentration in the feed is much lower, pressures are 1000–1500 kilopascals. While this gives desalination in particular a reputation for being an energy-intensive technology, when considered at a household level, the energy usage required to provide desalinated water to a family of four is comparable to the energy used to power a family-sized refrigerator. Many water authorities also choose to power their desalination plants with renewable energy.
Reverse osmosis technology for drinking water production originated in the mid-20th century and provided an alternative to thermal desalination (distillation), which was already in use in the Middle East at that time. Since its inception, reverse osmosis has spread around the world, becoming the dominant desalination technology in Australia, the US, Mediterranean Europe and North Africa. The Middle East has also embraced the technology alongside the continuing use of thermal techniques, which remain in play due to the availability of cheap thermal energy in that region.
Reverse osmosis technology is now well understood, but it is not without its challenges. A large desalination plant houses tens of thousands of individual membrane elements collectively worth tens of millions of dollars. If poorly managed, contamination with hydrocarbons or uncontrolled introduction of oxidising agents such as chlorine can cause millions of dollars’ worth of damage, while biological fouling can lead to significant increases in energy usage.
Reverse osmosis also produces a concentrated waste stream called brine that needs disposal. For seawater desalination plants, this brine is typically discharged back into the ocean through diffusers, which are designed to ensure that the salinity of the surrounding ocean returns to background levels quickly, minimising the impact to the receiving environment. For inland reverse osmosis plants without the advantage of an ocean disposal pathway, waste disposal is not so straightforward, and can involve complex treatment processes such as brine concentrators and crystallisers for a zero liquid discharge solution. Beneficial reuse of desalination wastes is often discussed in the industry but is yet to become common because of a combination of technical and economic considerations.
It is clear to all who work in the water industry that reverse osmosis technology will be a key part of our national water infrastructure in the future. Sixteen years since the first major desalination plant in Australia was officially opened in 2007, new or expanded desalination plants are now on the cards in five states with the combined potential to add hundreds of millions of litres of water per day to the nation’s supply.
In addition to this expansion in desalination, water recycling is still a viable option for drinking water augmentation that has by no means been discarded by planners. The Western Corridor Recycled Water Scheme in Queensland has to date been used only for industrial customers, but it was designed to meet the strict regulatory requirements for use as drinking water. The purified recycled water it produces is able to be discharged into Wivenhoe Dam, from which point it could be traditionally treated as a part of Brisbane’s drinking water supply. In New South Wales, Sydney Water is currently building a purified recycled water demonstration plant and visitor centre at Quakers Hill in the Sydney’s north-west. The plant is for education purposes only and the water produced will not be added to the city’s drinking water supply, but the technology used is robust and able to meet the strict requirements for drinking water use.
Although some Australians may still feel reluctant to drink purified recycled water, there are parts of the world where this is a well-established reality with a proven track record of safety and reliability. In Orange County, California, purified recycled water has been used since the mid-1970s to replenish the groundwater basin from which the region draws its drinking water. Closer to home, Singapore’s NEWater recycling scheme purifies water through membrane filtration (including reverse osmosis) and ultraviolet disinfection before discharging it to reservoirs where it mixes with rainwater before being further treated and added to the city’s water supply. NEWater will celebrate its 20th anniversary of operation this year.
Water recycling plants typically use a combination of technologies, but reverse osmosis is usually a core element due to its ability to remove pathogens, salts and other contaminants from a wastewater matrix. Reverse osmosis technology was designed to remove salts from seawater, and as a consequence has a pore size of about 1 nanometre or less. Common pathogens (bacteria, viruses and protozoa) are at least one, and often many, orders of magnitude larger, which makes reverse osmosis a highly effective physical barrier to disease-causing microorganisms. The percentage removal of chemical contaminants by reverse osmosis is lower, but still substantial, with factors such as the size, charge and hydrophobicity of the molecule influencing how readily it is removed by reverse osmosis membranes. If reverse osmosis alone is insufficient to reduce chemical contaminants to the required level, it is often accompanied by other technologies that use oxidation, adsorption or other mechanisms to further reduce trace chemicals.
On Australia’s east coast, rainfall patterns in recent decades have oscillated between famine and feast. A severe drought between 2017 and 2019 was followed by the arrival of a La Niña weather pattern in 2020 that has brought floods to Australia’s eastern states with alarming frequency in the years since. Desalination and water recycling are primarily considered to be drought mitigation measures, but recent floods have again brought them to the fore in ways that might surprise some. Flooding has affected the quality of water catchments by bringing silt, ash and organic matter into rivers and dams. This degraded raw water quality has been challenging for conventional water treatment plants in many regions. Desalination plants, because of both their technology and their feed water source, are less affected by these challenges and have been used to take the pressure off stretched traditional treatment plants.
Desalination is being considered as part of a strategy to manage flood risks in some cities. Supplementing water supplies with desalination may allow planners to operate dams at lower levels, giving communities additional flood protection through the freeboard created between the operational water level and the height of the dam.
Taken together, the benefits of reverse osmosis make it a highly valuable technology in any strategy for a reliable, climate-resilient water supply for the future.