Filtering softened water doesn’t involve the same processes that are used to make hard or regular tap water taste better. Instead of removing chlorine and other contaminants, the filtration of softened water is concerned solely with removing excess ions from your drinking water.
When water is softened, the excess calcium and magnesium that make water hard are removed and replaced with sodium or potassium through a process called ion-exchange. To filter exchanged ions, reverse osmosis can be used to push the softened water through a membrane at high pressure. The ions cannot penetrate the membrane and are removed.
The aim of filtering, in this instance, is to provide safer water for those that need to remove the additional sodium or potassium from their diets. To understand the nuances of how to filter softened water, we also should consider what softening water entails and why we need to do it in the first place.
Do I Need a Special Filter for Softened Water?
Normal water filters will not remove the added sodium or potassium ions that were added during the softening process. To do this, you need a specialized system that can remove very small particulates.
Most common water filters are packed with micron-sized particles that are coated in activated carbon. They are designed to remove larger particulates that can’t pass through the small pore size. They also remove organic compounds like bleach that was used to purify the water by adhering them to the particle’s surface.
Neither of these mechanisms removes ions from water as they are too small and do not bind to carbon. The sodium chloride compounds that form (Na+ binds to Cl- to make an ionic compound) are between 15 and 60 nm in size.
How to Eradicate the Extra Sodium or Potassium in Softened Water?
The only feasible, large-scale way for you to remove the extra salt compounds from your softened water is through a process called reverse osmosis.
This method uses pressure (typically 40-100 psi) to push the water through a semi-permeable membrane that is nanometers in size. These elevated pressures are needed to force the water molecules through this minuscule pore size as such filters generate enormous backpressure.
To prevent the membrane from getting clogged, most reverse osmosis systems use 3-5 filtration stages, with decreasing pore size at each stage (5,000 to 0.1 nm). The result is 96-99% of all contaminants have been removed and the product is nearly pure drinking water.
It is not wise to directly filter hard water this way, as the hardness will dramatically decrease the lifespan of your reverse osmosis system. This is why water is often softened first.
Drawbacks to Using Reverse Osmosis
There are other things to consider when using reverse osmosis to prepare your drinking water.
It is roughly twice as expensive to produce reverse osmosis filtered water compared to regular groundwater. This upswing in price is primarily due to the wastewater associated with the process.
In a normal municipal water line, with a pressure of 40-45 psi, you can expect a maximum yield of 20%. That means 80% of all water used in the reverse osmosis filtration process is waste.
To still use this system effectively, this wastewater can be used for everything else in your home (dishwasher, shower, toilets, etc.) and the 20% that was filtered can be directed to a dedicated drinking faucet.
To increase the yield, expensive pumps can be installed to increase the driving pressure. Around 70 psi, under the right conditions, can produce a 1:1 ratio of filtered to wastewater. Personal reverse osmosis systems are available commercially.
Extra Maintenance Required
Another drawback to reverse osmosis is the maintenance of the filters/membranes. As they are so small, they can easily become clogged with particulate matter from the water. To continue having a functioning system, the filters need to be frequently backflushed and/or replaced.
The filters are so fine that they will trap bacteria that may grow on the filters’ surface. Therefore, the reverse osmosis system needs to be treated with chemicals and washed before use. Each of these concerns drives up the cost and the environmental impact of the process.
Are There Other Filtration Methods Other Than Reverse Osmosis?
You can distill water to remove salt (NaCl) from it. Distillation requires boiling your water, collecting the steam, and then condensing it.
Although it isn’t exactly filtration, it will remove the sodium or potassium you are trying to avoid in your softened water. Of course, this method is only suitable for very small batches, even if you have the specialized glassware to do this efficiently and continuously.
What Exactly Is Hard and Soft Water?
Water hardness is a term derived from the capacity of water to react with soap – the more soap you need to get a lather, the harder the water. The World Health Organization considers your drinking water “hard” if it contains >120 mg/L of calcium carbonate.
Although magnesium concentrations also affect the hardness, it is generally found in lower concentrations ( <50 mg/L) than calcium, therefore, it is considered to a lesser extent. The principal source of this hardness comes from limestone and chalk that groundwater interacts with. More than 85% of American homes have hard water, and the states with the hardest water are Texas, New Mexico, Kansas, Arizona, Utah, Colorado, Nevada, and California.
Soft water is water that contains <60 mg/L of calcium carbonate. This can be achieved naturally or by treating hard water to remove the excess calcium.
Softening water is normally done via ion-exchange, which swaps the calcium and magnesium ions for sodium or potassium. This is done by passing hard water resin beads which are coated with sodium (for example) ions. The calcium ions displace the sodium ions, causing the resin beads to now be coated in calcium ions and the sodium ions to be free in the water. Potassium is a common alternative to using sodium.
The resulting soft water doesn’t produce limescale or the “soapy scum” on your bathtub. Soft water is also better for elongating the lifespan of your pipes and appliances. Also, washing your clothes in softened water makes the detergent function more effectively and the result is cleaner clothes and brighter whites.
Why Does Softened Water Need to Be Filtered for Drinking?
Since processed foods infiltrated the diets of those living in the Western world, salt intake has been a health concern. Table salt (sodium chloride, NaCl) is made from sodium (Na) and chloride (Cl) ions, and it is the excess sodium that can lead to high blood pressure, heart conditions, poor kidney function, and diabetes. Often, doctors recommend low sodium (or salt) diets to their patients that are suffering from these conditions.
The average American now consumes ~3,400 mg of table salt each day, exceeding the government’s recommended daily allowance (RDA) of 2,300 mg. Therefore, to reduce your daily intake it may be necessary to remove some sodium from the softened water to make it safer to drink for those in the high-risk groups.
Often their doctors recommended only consuming 1,500 mg of table salt per day. It’s worth noting that, the harder your water, the more sodium will be present in your drinking water after it has been softened. On average, each 8 oz glass of sodium-softened water will increase your intake by ~20 mg of sodium compared to non-softened water.
Too Much Potassium Can Cause Problems
When the kidneys stop functioning normally, it is easier to have a build-up of potassium in the bloodstream (hyperkalemia). The excess potassium can cause arrhythmias and is particularly dangerous for someone with a history of heart issues.
Even in a healthy person, excessive consumption of potassium can induce lethal cardiac conditions. Therefore, potassium-based ion-exchange resins instead of sodium-based ones aren’t always a suitable alternative for softening water. Additionally, it’s worth considering that drinking water softened with these resins can account for 11% of your RDA of potassium (assuming eight glasses per day).
Soft Water Tastes Bad to Some People
For some, the taste of softened water can be the motivation for filtration. Despite the normal human palate being unable to taste sodium at concentrations below 300 mg/L, some people can detect the Na+Cl- in their drinking water.
Unless you have health issues requiring you to cut back on sodium or potassium intake, it is probably not worth the trouble to try to filter your softened water. The cost and hassle likely far outweigh the benefits.
Unfortunately, if you absolutely can’t stand the taste, the only viable solution for most is to either stick to bottled water or become really good at distillation.
Is drinking reverse osmosis water safe? The World Health Organization advises against drinking reverse osmosis water. While the process does remove impurities, it also removes many minerals that the body needs. Reverse osmosis removes up to 99% of the useful magnesium and calcium present in drinking water. If you drink only reverse osmosis water, you need to ensure you replace these minerals through your diet.
Aren’t there salt-free water softeners? Water softening is defined as removing Ca2+ and Mg2+ ions from water, however, salt-free softeners actually transform the ions so they will not adhere to surfaces. These systems do not remove the hardness, and therefore, these aren’t softeners, but descalers. The produced water shares some properties with soft water (soapier, less limescale), but isn’t as effective. Descaling systems are a good compromise if you want a maintenance-free system and soft-like water.