Reverse osmosis (RO) is a water purification process where a membrane under pressure is used to separate relatively pure water from less pure water. In the normal osmosis process, the natural movement of a solvent through a membrane is from an area of low concentration to an area of high concentration. RO is the application of external pressure to reverse this natural process: the pure water passes through the membrane while the impure solute is retained on the other side of the membrane.
RO has been widely used to purify water for medical and industrial purposes since the 1970s. It is also used by the military to generate potable water from nearly any water source. RO is also becoming more available for use in domestic water purification.
However, treatment of water by reverse osmosis is imperfect and complicated. Much research is still needed, particularly in the areas of new composites/pollutants and membrane weaknesses and failures.
One of the primary benefits of RO is the production of minimal waste water. Traditional filtration processes produce large quantities of wastewater which are often just disposed of. RO systems produce less wastewater than other methods, but still do not purify all of the feed water. Industrial RO units purify between 75% and 90% of feed water.
This number is much higher than any other kind of filtration, but it only applies to industrial units. To achieve this level of efficiency in water purification, it must be possible to generate very high pressure. In domestic and other non-industrial environments it is not usually possible to generate sufficiently high pressure to achieve optimal pure water output. Thus smaller units only recover between 5% and 15% of feed water, generating high quantities of wastewater: sometimes up to 50 litres of wastewater per litre of pure water generated.
RO is extremely effective at purifying salt water, a process that is necessary in particularly arid parts of the world where fresh water is rare or unavailable. Salt water is flowed over a semi permeable membrane at high pressure to remove the undesired minerals. However, the pressure required to perform this task can exceed 70 times atmospheric pressure. Much equipment is needed to accomplish this, specifically a series of large pumps. This all takes immense amounts of energy to operate, particularly in the quantities required to produce water for whole regions.
RO is also less efficient at purifying high saline water like seawater.
RO systems are widely implemented due to their efficiency in comparison with other desalination methods. For the quantities of water that need to be desalinated, reverse osmosis is by far the most energy efficient process. But it is hugely energy intensive and has
As increasing amounts of water are required to meet the needs of an expanding population, efficient desalination of seawater is becoming a more pressing issue.
Despite many recent refinements in the field of desalination, the energy requirements are still tremendous. 7 to 30 kW-h of energy are required per 1000 gallons of desalinated water. (Veerapaneni) Energy requirements vary based on salt content in the feed water and the type of desalination used.
So while Reverse Osmosis systems are the most efficient of the available desalination systems, they are overall hugely energy intensive. “responsible development requires attention to the most energy-efficient methods of purifying water.” (Parise)
A major operation problem for RO systems is membrane fouling: the buildup of waste on the feed side of the membrane. This is an inevitable result of the RO process and must be closely monitored in all RO systems. Fouling occurs over different lengths of time depending on the type of membrane in use.
This can be monitored to ensure the water produced by RO systems maintains the desired purity. However, regular testing and maintenance are required to maintain optimal operating standards. Plus the testing required to ensure constant productivity is energy intensive and expensive.
Membrane surface damage
Over time membrane surfaces can become damaged. Industrial membranes typically last between 2 and 3 years. There are many reasons why a membrane can become damaged, such as pressure increases, scaling damage, oxidation processes and degradation over time.
Careful monitoring and maintenance can ensure that membranes stay at peak operational levels.
However, membrane surface damage can often be obscured by membrane fouling (Pena Garcia). Plants using industrial units evaluate membrane damage through performance failure and reduction in water quality. Thus they commonly miss minor membrane damage and damage obscured by fouling. “...in many cases it is not possible to detect membrane damages on site after fouling is removed” (Pena Garcia)
Feed Water Types
RO systems are quite sensitive in terms of the content of the feed water they can process. Feed water must be adjusted to the correct pH and prefiltered before it can be used in RO systems. The amount of materials within the filtered feed water must fit the system’s limitations or it will produce poor results and damage the RO technology. Feed water must be extensively tested before and during use. This is both time consuming and energy intensive.
The quality of the available source water can increase or decrease the amount of pre-testing required, and systems must account for a large number of variables. “The quality of seawater impacts the design and operation of the downstream pretreatment processes of the SWRO plant and the ability to operate the primary membrane system in a viable and efficient manner.” (Missimer)
All RO systems contain working parts that must be regularly maintained and replaced. This means that consumers need to keep parts in stock or be able to order them on demand. Damaged or broken parts require replacement which can take time and result in periods of potable water being unavailable. “Equipment and personnel changes also impact performance, requiring constant vigil and preparedness for continuous improvement of the [RO] plant.” (Ning)
Additionally, a large number of cleaning solutions is required to address issues of fouling within different parts of the RO system. Maintenance must be applied regularly and before any fouling becomes a serious problem. When this happens depends on the quality of the feedwater, so constant supervision is required to maintain all parts in peak condition. Incorrect maintenance or late maintenance adversely affects the quality of water delivered.
The overall environmental cost of Reverse Osmosis technology, when taking into account replacement parts, waste water, chemical treatment and energy costs, is enormous. There are ways to reduce the environmental impacts of RO, but on the scale that will be required for water purification in the coming years, reduction is simply not good enough. RO technology requires a lot of maintenance and careful monitoring of parts and feedwater.
All of these variables mean that there is a constantly vacillating amount of care required to produce quality water and ensure the environment is protected. “...the quality of the raw seawater strongly influences the ultimate quality of the discharge water (concentrate) requiring disposal.” (Missimer) The effects on the environment are too high and the potential points of failure are too many for RO to be a significant player in the future of water purification.
Reverse Osmosis technology is the best of the current standards of water purification. But that does not mean it is the best choice overall. The pressing problems of consistent water purity and environmental change require a game changing solution. Reliance on reverse osmosis technology will be environmentally and fiscally costly in the long term, and it provides an inconsistent product. “Reduced productivity of RO plants exerts serious economic impact on the downstream production of steam, power, microelectronics, pharmaceuticals and beverages among other products.” (Ning)
Novus wants to break the water wheel by making the limitations of distance, maintenance and technical downtime irrelevant. Improvements are possible in RO technology but to address the potable water demands created by climate change and a swiftly increasing population, radical technological change is needed. Reverse Osmosis is not capable of handling these demands. Distillation can provide that change. It produces virtually no wastewater, requires limited maintenance and is energy efficient.