Freshwater management is essential to ensure clean water is accessible to humans. They are also used for various daily tasks, including food production, sanitation, power generation, etc. This is specifically important in Australia as it has the second lowest water precipitation levels after Antarctica and because it has low rainfall that is unevenly distributed across the country. With the threat of climate change, the growing population of the country that is set to increase in the next thirty years, in addition to the creation of larger cities that will raise water consumption by 73 per cent, the need to protect freshwater bodies has been at an all-time high more than ever.
One aspect of maintaining freshwater bodies is to ensure a proper system is in place to clean them. Filtration is, in that respect, a process that removes particulate matter from water by forcing the water through a porous media. This porous media can be natural (ex: sand, gravel and clay) or a membrane wall made of various materials. Through this, contaminants can be separated from the water. Water treatment plants currently use a range of filtration processes, including ultrafiltration, nanofiltration and reverse osmosis. This article will explain the three, assessing their contribution to freshwater management.
Why Are There Three Types of Filtration?
There is no standard filtration method that can be deemed the best. This is because what may be suitable for one treatment plant may not be the most effective for another. The water source is the main factor in deciding which filtration process is better. Generally, water that enters treatment plants can be surface or groundwater. Surface water includes freshwater bodies like lakes, rivers and streams. However, they are filled with sediment, germs, chemicals and toxins much more than groundwater, requiring more filtration and treatment methods. The appropriate filtration method will further depend on the particles in the water. For instance, whether the water has radionuclides or a specific type of toxins may require more potent forms of filtration. The only actual difference between the three is the pore size. Hence, the ideal filtration method will depend on the level of water quality you want or need. Sometimes, however, water managers will use a mix of filtration methods to ensure water is thoroughly cleaned.
What is Ultrafiltration?
Ultrafiltration is a form of membrane filtration which uses a pore of 103-106 daltons (0.01 to 0.1 microns). It uses hydrostatic pressure to force water through a semi-permeable membrane. This results in suspending solids, endotoxins, bacteria, plastics, proteins, silica, silt, smog and others to produce water with slow silt density and high purity. Another way of understanding this is that ultrafiltration retains high molecular weight while water and low molecular weight solutes pass through the membrane. This aspect is known as ‘molecular weight cut off’. It is famous for producing stable water quality regardless of source water type. It moreover has a compact physical footprint which makes it environmentally sustainable. Ultrafiltration, moreover, wastes less water when draining. This form of filtration is regarded as best for water managers that want microscopic contaminants taken out but prefer minerals to be left in the water. Therefore, it is typically used to pretreat surface water, seawater and groundwater whilst also being used to prepare water for further treatment with filtration systems such as nanofiltration and reverse osmosis. It should be noted that they are most effective against dissolved substances when they are coagulated or absorbed. These systems usually need to be cleaned properly using standard hygienic procedures that integrate chemicals never otherwise used in this type of water treatment.
What is Nanofilteration?
Nanofiltration refers to the use of nanoporous membranes. It is used in waters with low total dissolved solids to take out polyvalent ions whilst retaining organic matter. Hence, it is also known as organic membranes or ceramic membranes. The membranes are pressure-controlled, dense and can repel ionic or molecular models. Hence, this is often seen as an intermediate filtration between ultrafiltration and reverse osmosis. It is worth noting that nanofiltration operates and is designed very similarly to reverse osmosis. However, it uses less pressure than reverse osmosis, a reason why monovalent ions can pass through. It is also not as ‘tight’ as the reverse osmosis membrane. It can, nevertheless, lower the concentration of dissolved solids, although not as much as reverse osmosis. Its pore size ranges between 1 to 10 nanometers, making it smaller than ultrafiltration but larger than reverse osmosis. One way it is regarded as better than reverse osmosis is that it offers high flux rates and uses less energy. This mode of filtration is used by those who want to remove heavy solids. Hence, those who produce dairy products or key players in drug manufacturing, textiles, drinking water, industrial kitchen and medical laboratory water would adopt nanofiltration mostly.
What is Reverse Osmosis?
Reverse osmosis uses a semi-permeable membrane to filter molecules, large particles, microorganisms and sediments from drinking water. To understand reverse osmosis, it is helpful to consider what osmosis amounts to. Osmosis refers to a process whereby water is passed via the membrane from a less concentrated solution to a more concentrated one. Hence, pure water is passed through contaminated water to equalise the concentrations. This does not make water suitable for drinking. Thus, in reverse osmosis, pressure is applied to overcome the osmotic pressure, pushing the water from a high concentration of contaminants to a low concentration of contaminants. This forces the contaminated water trying to move into the pure water to pass through a filter, thereby removing all contaminants as water passes through. Reverse osmosis is regarded as one of the most extensive filtration methods because it has the smallest pore size. As noted above, however, this does not make it the most preferred at all times. While some may consider ultrafiltration to be better because it retains beneficial minerals which reverse osmosis would typically remove, others would note that this is a drawback. In this respect, those that want particles, including dissolved substances, to be removed from water would prefer reverse osmosis as the best filtration process for them.
Does Filtration Guarantee Clean Water?
It is important to note that filtration is only one step out of other treatments used in the freshwater management process. Other treatments include coagulation, flocculation and sedimentation. Generally, after filtration occurs, the water treatment plants may add a few steps to disinfect the water using chemicals such as chlorine or chloramine. This is done to kill any remaining bacteria, parasites or viruses. The disinfectant process focuses on ensuring there is a low level of chemical disinfectant remaining even after the water is treated so that the amount of disinfectant existing in the water networks would help kill any germs present in the pipes as the water is transported across distributing systems to households. Sometimes, some water managers would disinfect water using ultraviolet (UV) light instead of adding a chemical. The only negative of using the latter would be that there would be no way to guarantee germs accumulated in the water as they travel through the pipes were free of bacteria/parasites or viruses when they reached your tap.
