DuPont combines subsea technology with RO

Communities and ecosystems depend on water for health, hygiene and sanitation. Desalination is the fastest growing source of fresh water around the world and one Norwegian company is using subsea technology combined with reverse osmosis to make desalination more affordable and sustainable.

The subsea system avoids detrimental effects on coastal marine life by eliminating the discharge of concentrated brine into coastal waters. (Image: Waterise)
The subsea system avoids detrimental effects on coastal marine life by eliminating the discharge of concentrated brine into coastal waters. (Image: Waterise)

According to a World Bank report from 2019, over half the world’s population experiences some form of water scarcity each year. This affects populous areas where supply is constrained and demand from water-intensive economic activity is high. On a global level, one in four major cities is already facing water stress. With increasing population, urbanisation and economic growth, water scarcity is projected to worsen. In 2016, the United Nations predicted that by 2030 the world could face a 40% shortfall in water supply if no changes are made to how water is managed.

Subsea desalination
Today, more than 150 countries are already using desalination in one form or another to meet different areas of demand, supplying over 300 million people with potable water. One Norwegian company, Waterise is developing subsea technology which addresses the sustainability challenges of traditional desalination (energy consumption, brine generation and greenhouse gas production) but its system is still able to provide affordable fresh water to communities that need it.

The Waterise subsea desalination system combines existing reverse osmosis technology with established subsea technology and off-the-shelf equipment from leading Norwegian subsea suppliers. The solution consists of standardised subsea modules with a capacity of 50,000 m3 per day. The company has partnered with DuPont Water Solutions, which is providing its reverse osmosis expertise and its durable FilmTec seawater membranes to work with the subsea technology.

There are many environmental benefits to this type of desalination. Firstly, subsea operations dramatically reduce the costal land required compared to a traditional land-based desalination plan. Secondly, Waterise’s systems are placed on the seabed, at about 400 m below sea level, and deep-sea water also has much lower organic content and more stable operational properties throughout seasons than surface water. As a result, feedwater to a subsea plant requires significantly lower levels of pre-treatment than a land-based facility.

Reverse osmosis
Terrestrial reverse osmosis employs mechanical pressure to reverse the natural osmotic pressure and semi-permeable membranes to separate saline water into two streams: a high quality permeate stream, which is the fresh water, and a concentrated reject stream, which is the brine or discharge.  The seawater intake and the brine outfall are both located in coastal waters, where marine life thrives.

Seawater reverse osmosis (SWRO) is used globally to produce close to 25 million m3 of fresh water per day. However, traditional land based SWRO is often criticised for its energy intensity. This stems from the need for pre-membrane pumping pressure to overcome the osmotic pressure. It is also criticised for the potential impact on marine life around the intake and discharge areas.The Waterise system uses the subsea hydrostatic pressure to facilitate reverse osmosis and, as a result, energy consumption is 40% lower than for commonly used desalination technologies. The combination of lower energy requirements and the savings on chemicals for pre-treatment and the disposal of organic material results in a reduced CO2 footprint of more than 50%. Finally, because the system is placed on the seabed, land use is reduced by at least 80%.

The subsea system also avoids detrimental effects on coastal marine life by eliminating the discharge of concentrated brine into coastal waters. Chemical-free effluents are released subsea, where they are dispersed and diluted by natural sea currents.

Productive partnership
As Waterise’s desalination system is placed on the seabed, Waterise was looking for the most durable, longest-lasting reverse osmosis element on the market and finally decided on DuPont Water Solutions’s FilmTec seawater membranes.

DuPont Water Solutions (DWS) specialises in sustainable water purification and separation technologies, including ultrafiltration, reverse osmosis (RO) membranes and ion exchange resins. DuPont has been producing solutions in support of desalination for more than 40 years and particularly reverse osmosis products that pursue sustainability, from lower energy membranes to the recent launch of dry-tested seawater reverse osmosis (SWRO).

In addition to exclusively using DuPont’s FilmTec seawater membranes for all systems, DuPont and Waterise will share knowledge and expertise, as well as collaborate on research and development to advance the subsea desalination operations and performance. HP Nanda, global vice president and general manager, DuPont Water Solutions, said: “We are excited about Waterise’s new approach to desalination that not only reduces water scarcity, but also minimises energy consumption and environmental impact.”

Waterise is currently performing tests using a demonstration rig which can be transported to customer sites to demonstrate the concept and desalinate water at ~ 400m depth, in their own waters (at their specific salinity and temperature). The company has worked closely with Dupont during testing and has achieved very high quality permeate water, with conductivity readings of 260 µS/cm (equivalent to 130 ppm TDS).  

Future plans
Waterise is working with potential clients to secure the first commercial reference. Once proven at commercial scale, the solution will undoubtedly become rapidly attractive to desalination customers with near-shore deep waters and environmental concerns about traditional desalination plants.