Each of these approaches targets different water issues, and many filters actually combine these approaches to achieve multi-stage filtration
Mechanical
The basic idea of mechanical filtration is to physically remove sediment, dirt, or any particles from the water using a barrier. Mechanical filters can be any kind of filter, from a basic mesh that filters out large debris to ceramic filters with extremely complex pore structures for ultra-fine filtration of disease-causing organisms.
Absorption
Absorption in water filters is usually done by carbon, which is very effective at capturing contaminants in water. The reason carbon absorbs contaminants so quickly is because it has a huge internal surface that is packed with nooks and crannies that can capture chemical impurities such as chlorine.
The most common household filters contain granular activated carbon (GAC), which reduces unwanted tastes and odors through absorption. More expensive filters use carbon block elements, which are generally more effective and often have micron-level particle removal rates.
A variety of different substances can be used to make carbon for filters, including wood and coconut shells, with coconut shell filters being more effective but also more expensive.
Sequestration
Sequestration is the act of chemically isolating substances. Food-grade polyphosphates are often used in scale-inhibiting filters to sequester calcium and magnesium minerals that cause scale and corrosion. However, polyphosphates are usually added only in very small amounts, and they only inhibit scale rather than eliminate it. This means that polyphosphates do not soften the water, but rather hold the minerals in solution, preventing them from forming scale on any surfaces they come into contact with.
Scale inhibition is not suitable for all applications because hard minerals are still present in the water. Instead, water softening using a process such as ion exchange is often recommended in waters with alkalinity levels of 180ppm or more (very hard water) and in applications where the water temperature is maintained at 95°C or higher.
Ion Exchange
Ion exchange is a method of softening hard water by exchanging the magnesium and calcium ions in the hard water for other ions, such as sodium or hydrogen ions. Unlike scale inhibition, ion exchange physically removes hard minerals, reducing scale and making the water suitable for applications that maintain a constant high temperature, such as commercial coffee machines.
Ion exchange most often uses ion exchange resins, which are usually in the form of small beads. Some water softeners use a similar type of resin, which uses sodium ions and requires periodic recharging to prevent the resin from failing. Since water filters are usually sealed units, you can simply replace the filter with a new one, but it should be noted that calcium treatment units (CTUs) can be returned to the supplier and regenerated.
Resins that utilize sodium ions are not usually used in drinking water filters because the legal limit for salt (sodium) in drinking water is 200 mg/L. Since sodium ion exchange increases the salt content, hydrogen-based ion exchange resins are the preferred choice for filters.
Reverse Osmosis
Reverse Osmosis (RO) is the process of removing dissolved inorganic solids (such as magnesium and calcium ions) from water by forcing it through a semi-permeable membrane under pressure, so that the water can pass through but most contaminants are retained.
Reverse Osmosis is a highly effective method of water purification and is often combined with a number of other filters, such as mechanical (sediment) filters and absorption (activated carbon) filters, to return water that is virtually free of contaminants.
Reverse Osmosis systems use water pressure to force water through the membrane, so they do not consume electricity, but they do produce a certain amount of wastewater that must be discharged to the sewer. The additional filters involved in multi-stage water filtration can make reverse osmosis units more expensive than other filtration methods, but in applications where 99.9% pure water is required, RO offers the finest level of filtration, as it is increasingly used to treat water for coffee
