Water, water everywhere…
The United Nations will observe World Day to Combat Desertification and Drought on June 17th, explaining that ‘desertification’ does not refer to the expansion of existing deserts but to the degradation of land in arid, semi-arid and dry sub-humid areas, caused primarily by human activities and climatic variations.
Dryland ecosystems cover over one third of the world’s land area and are extremely vulnerable to overexploitation and inappropriate land use. Poverty, political instability, deforestation, overgrazing and bad irrigation practices can all undermine the productivity of the land.
Water scarcity – which is greatly exacerbated by desertification – has been declared the biggest risk to the global population in the coming decade, since in many parts of the world people already have no running water and are often forced to drink directly from contaminated sources.
More than two-thirds of the earth’s surface, however, is covered by oceans, inland waters and rivers – even if fresh water makes up only a relatively small amount of the 1.4 billion cubic kilometres currently available. The oceans themselves represent 97.2% of the planet’s total water resources.
The oceans are drought-proof and practically limitless and consequently, seawater desalination provides a logical solution for the sustainable, long-term management of growing water demand.
Desalination technology, which has been available for decades, is already at work in many arid areas of the world, and according to a 2019 report by Elsevier, there are some 15,906 operational plants producing around 95 million cubic metres of desalinated water for human use each day, with around 50% of them currently in the Middle East and North Africa
Meanwhile, the Carlsbad Desalination Plant in San Diego, which opened in 2015, is currently the largest of its kind in the Western Hemisphere and supplies 400,000 Californian residents with 50 million gallons (227,300 cubic metres) of water a day.
The Carlsbad plant and many others employ the reverse osmosis process in which water is pushed under high pressure through a semi-permeable membrane to separate salts and other solids from the water molecules. The membranes act like microscopic strainers that only allow the water molecules to pass thorough.
So where do nonwoven fabrics come in?
Membrane-based separation represents an extreme limit of separation that nonwoven filter media cannot yet achieve – although with nanofibre-containing products such as Ahlstrom-Munksjö’s Disruptor and DuPont’s HMT (hybrid membrane technology) the gap is narrowing all the time.
Polymer-based membranes, however, can be delicate and lack rigidity and as a consequence are often laminated to nonwovens which act as the reinforcement and support structures. Leading INDEX™20 exhibitors such as Berry Global and Freudenberg supply specially-engineered nonwovens for this application.
In addition, membranes are much more expensive than nonwovens, and often it is economical to use nonwovens as prefilters to filter out a large volume of contaminants and extend the life of the membranes employed at the final filter stage.
Both depth cartridge and pleated cartridge filters made from nonwovens are employed in water treatment plants. The filters are installed inline and must be very rigid to withstand high hydraulic pressures. Depth cartridge filters exploit the entire volume of the filter media to trap suspended contaminant particles. The media in these disposable filters are designed with a density gradient so that various-sized particles are captured at different locations within the volume of the media.
Pleated cartridges work by a surface filtration mechanism. In this type of filter, nonwovens are pleated to increase the filtration surface area. The advantage of pleated filters is their potential integration with downstream membrane-based separation.
These are examples of the frequently invisible supporting roles played by nonwovens across a wide range of industries and visitors to INDEX™20 in Geneva will discover many, many more.
Water filtration membranes are becoming increasingly contaminant resistant and reliable and the energy costs of operating membrane systems has also been significantly reduced. Higher volume sales have also cut module costs to make processes like reverse osmosis much more cost effective in recent years.
Energy is still one of the largest costs associated with seawater desalination, but advances in technology and equipment have resulted in a reduction of 80% of the energy used for water production over the last 20 years.
According to the International Water Association, advances in desalination technology have experienced similar dynamics to the computer. Like computers, reverse osmosis membranes today are many times smaller, more productive and cheaper than the initial working prototypes. Conventional technologies, such as sedimentation and filtration, have seen modest advances since their initial use for water treatment several centuries ago, but new, more efficient desalination membranes, hybrid technologies and equipment improvements are released every few years.
The steady reduction of production costs, coupled with the increasing costs of water treatment driven by more stringent regulatory requirements, are expected to accelerate the current trend of increased reliance on the ocean as a water source. This will further establish ocean water desalination as a reliable, drought-proof alternative in many areas around the world.