How Drinkable Is Your Water? The Reverse Osmosis Process Explained:
Currently, one in four people in the world drink water contaminated with faeces. What’s more, one in three lacks access to safe drinking water, resulting in 165 million cases of Dysentery each year – the equivalent of half the US population. If those of us in the UK find these figures shocking, it’s only because we have the good fortune of living in a land that provides water so clean that whenever we are thirsty, we just need to turn on the tap. Compare ourselves with the Ugandans and we can see how lucky we are: 51% of the country lacks access to safe water; the urban population spends as much as 22% of its income just to buy clean water from vendors.
Measuring Water Quality
Many different factors are considered when measuring water quality as it is essential we do not drink water that contains harmful chemicals or pathogens. Water can also be contaminated by human or chemical waste without proper disposal, especially in less developed countries, which leads to people falling ill. Lead poisoning affected the ancient Romans who failed to understand that their lead pipes caused health issues. In Victorian London, cholera ran rampant as there was no proper waste disposal at the time. Therefore, being the most convenient method, they would throw their waste out of the window and into the street; where the public water supplies got contaminated (and unfortunate members of the public received a nasty surprise). Although the worst may be in the past, the effects of poor water quality should not be understated as 15 million people are infected with typhoid every year, which is roughly equivalent to the size of Zimbabwe’s population. These figures show that we are still a long way from supplying safe drinking water worldwide, with less developed areas such as Africa and Southern Asia most affected.
So how could we measure the water to ensure that it is safe? Some common tests are to test the pH, conductivity, the dissolved oxygen and turbidity (suspended particles in water) which may reveal chemical contamination in the water. Biological tests are also carried out to identify harmful pathogens that may be present.
There are also sets of standards that are applied to regulate drinking water. When it comes to purifying water, more may not mean better. The WHO recommendation – that TDS (total dissolved solids) should be under 1000ppm (parts per million) for safe drinking – is a vague guideline at best. Government officials overseeing public health still have to decide what kind of balance to strike: anything above 600ppm should probably be filtered further, but sift below 250ppm and they risk depriving their citizens of some of the minerals needed to maintain optimal health. There are many different values that can be obtained as there are particles of many different sizes in the water.
To explain Reverse Osmosis, it is useful to first understand the process of Osmosis. Osmosis is the net movement of solvent molecules across a semipermeable membrane from areas of higher water potential to lower water potential. A common example of the process is the absorption of water by plant roots. If you haven’t already guessed that Reverse Osmosis is the exact reverse of osmosis, it’s exactly what is says on the tin.
Osmosis is the scientific principle underlying desalination and pathogen removal, and conversely Reverse Osmosis is induced by applying pressure on the side of a mass that has lower water potential, so that the solvents there will penetrate the semipermeable membrane and enter into the region with higher water potential. Or simply put, you can get cleaner water if you push dirty water through the membrane.
Effectiveness and Efficiency
However, how effective is this method? Around 90-99% of contaminants are removed, making this an extremely effective filtering method. The technology is well known for being able to remove a broad range of contaminants.
The systems nowadays are so compact that they can even be portable, so they can conveniently fit in your water jug. The regular systems are generally quite slow at filtering water at a painstakingly slow rate of only around 60-90 ml per minute. Though faster systems can filter 1 litre of water per minute, which can fill a regular glass of water in 12 seconds, still slow but not unreasonable. With multiple units, the process can actually be very quick though the cost would also be high, reaching £50,000 for one good industrial unit. The current systems are not very efficient either as the best systems waste around 4 litres of water to produce 1 litre of clean water, this means that it is only a 20% efficiency in volume of water filtered, which is quite a low figure compared to other filters. However, there is a new emerging technology that is capable of turning the volume of waste water to 0; coined “ZeroWaste”, it uses another system to circulate the water, removing all water waste. The future is still looking bright as we continue to innovate and optimise this technology as there is still much room to improve.
[Diagrams depicting the processes of Osmosis and Reverse Osmosis]
Applications of Reverse Osmosis
It seems that only a lack of imagination can put a limit on the ways Reverse Osmosis can be harnessed to improve our physical and mental wellbeing. Children who delight in the rich taste of maple syrup on their pancakes each morning should thank Reverse Osmosis (it thickens their syrup by removing its water content); pet owners can keep their fishes, toads and turtles happy with clean water in the aquarium; supermarket favourites like juice concentrate and low-alcohol beer wouldn’t have been existed without Reverse Osmosis. On a grander scale, with Reverse Osmosis gadgets in hand, the US military can convert any water source into drinking water; resource-starved Singapore relies on desalination to keep its population of 5.7 million quenched.
[US military setting up Reverse Osmosis system for desalination]
Although the technology sounds so powerful, there are still measures that need to be taken to ensure that the water is safe and the system stays well maintained. Reverse osmosis membranes that are used are now one-way membranes, which only allow one direction of flow. Some of the membranes today utilise filters with nanotechnology to accomplish this, with specialised Graphene or other materials. The one-way system also means that anything caught in the filter cannot be removed, which is why pre-treatment is needed to maintain the filter. Processes involved in pre-treatment include filtration of particles up to 1 μm in diameter, removal of biological material, bacterial dosing, and other filtration methods to prevent material from clogging the filter. As there are some pathogens that may have been able to bypass the filter, UV disinfection is also required after filtering and before distribution ensuring the cleanest water possible. As getting your sufficient intake of minerals can still be important, many of the filters come readily equipped with remineralisation systems solely for this purpose.
Reverse Osmosis systems are typically quite expensive which may not be suitable for some applications. Some of the most common filters are sediment filters which are the filters that are seen everywhere and are very common, they do not remove particles smaller than their filter size and can be maintained easily due to being packaged in cartridges usually. Activated Carbon filters are another good alternative, offering the benefits of retained minerals and removal of spores (beneficial to people with allergies), though they do not remove some viruses and a few other particles which may not produce the cleanest water. Distillation is another method that is used quite effectively, though it is more inconvenient as it requires high maintenance and is time consuming while heating and condensing the water. The more common filters rarely outperform Reverse Osmosis, showing why the latter should be used.
What the Future Holds
Reverse Osmosis is widely predicted to become the primary method of filtering water for drinking. This is especially the case with desalination, where it convincingly proves to be better than thermal desalination. The process of purifying groundwater will also be available widely. It will also probably be used to purify water for more effective and efficient Oil and Gas extraction to enhance recovery of the fuel. Only our imagination is holding us back from utilising this technology further, meaning that there are still new uses to be discovered. Overall, a very promising and exciting future awaits Reverse Osmosis.
Figures cited from who.int
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