Nano Filtration and Reverse Osmosis are the same process adapted to two different situations via membrane texture and operating pressure to get potable de-mineralized water.

Desalination
Desalination is the removal of salts and ionic species from water. Historically desalination was simply the conversion of seawater to drinking or potable water. In some coastal areas the over abstraction of water from the aquifers has resulted in the previously fresh water becoming brackish and this now requires desalination to render it drinkable. The same process is used to treat water to produce water of higher quality than drinking water. Therefore Desalination is now seen as a broad term covering the removal of salts from water irrespective of the feed concentration. There have been many advances to desalination over the past 20 years and costs are now comparable with conventional water treatment processes.

Seawater Desalination
The region of greatest activity in seawater desalination has been the oil rich Middle East. The process that is most commonly used is a distillation process known as Multi-Stage Flash (MSF) and these large “desalters” are built as an integral part of an oil fired power station. The low-grade steam wasted from the power generation turbines is used to heat the seawater in the desalination process. The vapour flashes off as the pressure is sharply reduced in a series of chambers. The vapour is condensed in a heat exchanger. This process is used as the cost of oil is low and power stations need to be built irrespective of whether the desalination plant has to be built.

Over the past 10-15 years many other areas of low rainfall such as Spain, countries on the Mediterranean rim and the Caribbean are also turning to desalination. However, the costs of energy are too high for MSF to be considered. Ames Crosta Babcock, later to be known as Biwater Treatment, built one of the first seawater reverse osmosis plants in the Canary Islands in 1986.

Since these early days there have been many developments to the membranes and to the pumping systems that have reduced both the capital and operating costs. The operating costs of a sea water reverse osmosis plant have reduced threefold since 1980. Reverse Osmosis is now the preferred process around the world, in particular where energy costs are high. Reverse osmosis is the most energy efficient desalination process available.

Very often the drinking water source is located far away from a coastal development where that water is required. There will not only be considerable investment in the construction of the infrastructure to supply water but there will also be significant pumping costs to deliver the water. A sea water reverse osmosis plant can be built where the water is required and as the plants are modular they can easily be expanded in the future as the water demand grows.

Brackish water desalination
The energy required for a reverse osmosis plant reduces as the feedwater salinity decreases, whereas the energy required for thermal or distillation processes is independent of feedwater salinity. Consequently thermal processes become even less competitive for brackish water.

Whilst major brackish water desalination plants (BWRO) have been installed for many years in the Middle East and the USA, particularly Florida, it is relatively recently that major schemes are considered in other parts of the world. When Ames Crosta Babcock built the 59,000 m³/d plant at Buwayb, Kingdom of Saudi Arabia, in 1978 the plant represented 10% of the world’s reverse osmosis capacity.

Since then Biwater have continued to build many brackish water reverse osmosis plants for different applications from the production of drinking water to boiler feed water.

Nanofiltration
Nanofiltration is another membrane process with many similarities to brackish water reverse osmosis. For example, the removal of pesticides. Unlike reverse osmosis, which typically removes 99% of all ionic species from the water, nanofiltration tends to soften the water but not significantly reduce the overall salinity. Consequently the water produced is not as corrosive as water treated with reverse osmosis so the re-hardening requirements are reduced. The main application of nanofiltration is in the production of drinking water. Biwater AEWT is the largest nanofiltration system supplier in the World and one of the largest RO system suppliers.

In Nano Filtration and Reverse Osmosis, the system configuration, operation, and process calculation equations used are the same where the difference is in the membrane texture and the operating pressure.