By Rehan Iqbal
Most companies that make reverse osmosis filter systems promote them as a way to get better tasting tap water. After all, if you check the labels of most bottled water, you will see reverse osmosis listed as the water treatment method.
However, these systems also have their place in well water treatment systems. Depending on the types of contaminates you need to remove from your water, an RO system may be more effective or affordable than a whole house solution. Here’s what you need to know about how these systems can be put to use, and their limitations for treating well water.
What is Reverse Osmosis?
Reverse Osmosis is a process in which chemicals pass through permeable barriers until saturation is equal on both sides. For example, if you have salty water on one side of this barrier and clean water on the other side, salt will pass from the salty side to the clean side until both sides have equal salinity. By applying pressure, this process can be reversed.
The result is clean water on one side of the membrane, which goes to the tap, and waste water on the other side, which goes down the drain. While an RO filter acts like a strainer, it’s also capable of removing dissolved chemicals in water. This lets it remove contaminates that normally require additional chemical processing.
Why are Reverse Osmosis Filters Sold as Part of a System?
RO systems are always sold as a kit with carbon filters, while some systems also include a storage tank, sediment filter and/or a KDF filter. Each part is there to improve the taste of water, protect the RO filter, or improve the RO filter’s performance.
Chlorine destroys RO membranes. By adding a KDF or activated carbon filter, this chemical is removed before it reaches the reverse osmosis filter.
Reverse osmosis filter media is expensive. Pre-filtering the water reduces the amount of contaminates the RO filter has to deal with. This lets you use a smaller, cheaper filter to get the results you want. Activated carbon is great at removing chemicals, including chlorine, phosphates, VOCs, THMs, pesticides and herbicides. Catalytic carbon removes hydrogen sulfide and chloramines, and it’s more effective than activated carbon at removing chlorine. KDF filters remove heavy metals, hydrogen sulfide, calcium and magnesium.
Reverse osmosis depends on water pressure to push water through the membrane. The membrane’s high resistance to flow greatly reduces the flow rate between the RO filter and the rest of the plumbing system. To compensate for this, many RO systems include a storage tank. This gives the system time to process water, so it’s ready to fill your glass.
Why are Most RO Systems Built for Single Tap Use Instead of Being Full House Treatment Solutions?
Reverse osmosis filters are expensive. Even if you have multiple problems with your well water, a regular multi-step filtration system is a fraction of the price of a full house RO system.
Using a whole house RO system can damage plumbing and lead to contaminated water coming out of your taps. Water is a powerful solvent, especially if it’s acidic. If it has low total dissolved solids (TDS,) water has no problem absorbing material from anything it comes in contact with. This includes lead solder and copper used in home plumbing systems. This eats away at pipes, causing leaks, while increasing the water’s lead and copper levels. To use a whole house system safely, you need to replace metal pipes with non-reactive polyethylene.
Water consumption is also an issue. The most efficient RO filters produce as much waste water as they do clean water. On less efficient systems, the ratio of waste water to clean water can be as high as 5 to 1. This dramatically increases the amount of water you need from your well. By using an under-sink system, you only generate waste water when you get tap water for drinking and cooking.
What is NSF/ANSI 58, and Why is Certification Important when Choosing an RO System?
There are no federal regulations covering home water filtration systems. State regulations exist, but they’re usually tied to waste water problems, like banning the use of salt-based water softeners to protect ground water. To help consumers find safe, reliable ways to treat water, ANSI and NSF have stepped in to develop voluntary standards.
NSF/ANSI 58 is the standard used to measure the performance of complete reverse osmosis systems. This standard requires filters to reduce total dissolved solids (TDS.) It also looks at the safety of the materials used in the system, their efficiency, and the information provided to the end user.
NSF/ANSI 58 also has testing standards for other water contaminates. These aren’t required, unless the manufacturer makes a claim that their filter removes a specific contaminate. Standard 58 includes testing requirements for these contaminates:
Standard 58 doesn’t cover everything a reverse osmosis system can treat. NSF/ANSI 42 covers systems that remove chlorine from water. Since chlorine must be removed before it reaches the RO membrane, most systems meet this standard.
Certification also doesn’t guarantee the filter will make your water safe. For example, an approved RO filter needs to remove at least 80% of arsenic from water. That’s fine for most homes, but it may not be enough to get contamination down to safe levels, if you have heavily contaminated well water.
Why Are RO Systems Recommended for Removing Nitrates?
Long term consumption of nitrates and nitrites can cause a long list of health problems, including cardiovascular issues. While our bodies break down most nitrates into nitrous oxide or nitrides, babies can’t break down this chemical.
Once it enters their blood stream, it bonds with red blood cells, keeping them from carrying oxygen. This results in a type of asphyxiation called methemoglobinemia, or “blue baby syndrome.” Nitrate levels are usually highest in agricultural areas, where ammonium nitrate and potassium nitrate are used as fertilizers.
There are two ways to remove nitrates from water: reverse osmosis and ion exchange. Ion exchange filters are usually used to soften water. They work by replacing unwanted ions with sodium ions. Standard ion exchange media favors multivalent ions, like sulfides. If your water has high sulfide levels, the media won’t do much to remove nitrates. Selective media ion exchange systems use media that prefers bonding with nitrates, but these tanks can be pricey.
Since nitrates are only an issue for water you consume, there isn’t much reason to treat all of the water used in your home. This makes an undersink reverse osmosis system a cheaper option for water treatment.
Are Reverse Osmosis Filters Really Effective at Removing Arsenic?
There are two types of arsenic: arsenic 3 (trivalent arsenic) and arsenic 5 (pentavalent arsenic.) There is no way to directly remove arsenic 3 from water. Instead, it must be oxidized, which turns it into arsenic 5.
Although there are separate NSF tests for arsenic and pentavalaent arsenic, reverse osmosis filters are only effective at removing arsenic 5. Pairing an RO system with an oxidation system is an effective way to remove all arsenic.
What is Perchlorate, and Why is it Seldom Mentioned in Water Treatment?
If you’re familiar with well water treatment systems, you probably recognize all the contaminates tested under NSF standards except this one. Perchlorate is a chemical naturally found in soil, mostly in arid regions like the American Southwest. It’s also used for chemical manufacturing, particularly explosives and rocket fuel. Perchlorate interrupts the absorption of iodine by the thyroid, and is used as a medicine to treat hyperthyroidism.
The EPA and related organizations have studied this water contaminate for over 20 years. They concluded that perchlorate is safe at levels up to 56 µg/L. In 2020, the EPA determined there was no need to make a national law limiting perchlorate levels, due to low levels of contamination.
In their studies, only two municipal water systems in the country were found to have more than 56 µg/L of perchlorate. Instead, their focus is on cleanup of perchlorate left over from manufacturing, stopping this pollutant at its source. Some states have their own rules for municipal water contamination, with limits as low as 1 µg/L.
If you’re on well water, you’re responsible for making your water safe. Perchlorate can be tested by a water lab like any other contaminate. If your water has high levels of this chemical, an NSF/ANSI certified reverse osmosis filter is an excellent way to treat it. Perchlorate has no effect on washing or showering, so there’s no need to filter it out of your entire home’s water system.
Can I Use an RO System to Remove Pathogens?
A reverse osmosis filter is fine enough to remove bacteria and cysts. However, the reliability of this filtration isn’t perfect, which means you’re always a risk of an infection. Instead, you should rely on purpose-built pathogen treatment solutions, like UV lights and oxidizers. While an NSF testing standard exists for cysts, manufacturers rarely claim their filters are effective on these pathogens.
Does Reverse Osmosis Remove Hydrogen Sulfide?
It depends on the system. While RO systems are promoted as a way to get the best tasting drinking water, reverse osmosis can’t remove this contaminate. However, catalytic carbon and KDF filters are effective at removing small amounts of this foul-smelling gas.
Since the rotten egg smell from hydrogen sulfide is a problem for all water used in your home, you’re usually better off getting a whole house treatment solution, like an aerator.
What Does a Remineralization Filter Do, and Do I Need One?
Reverse osmosis takes out all the dissolved solids out of water, including those that give the water its flavor. If you’re used to drinking regular tap water, water processed by an RO system will taste slightly acidic and flat. If your water is already acidic, this off taste will be more noticeable.
A remineralization filter contains manganese and calcium, which dissolve into the treated water. This restores the water’s flavor. Alternatively, you can add mineral drops or other flavorings to balance out the water’s taste. You usually won’t notice the difference between regular water and RO water in cooking, although it can make the flavors of juices made from concentrate more intense.
If the taste of your filtered water changes suddenly, it usually means there’s a problem with the filter system. A worn out reverse osmosis cartridge will let contaminates through, and it can become a breeding ground for bacteria. Saturated carbon filters stop filtering, and may release trapped contaminates into the water.
How Long Do Reverse Osmosis Filters Last?
Most systems require filter replacements every 6 to 12 months, while some systems have components that last up to two years. Longevity is mostly a matter of space: larger filters last longer, but they may not fit under your sink, especially if the system has a storage tank. Before you buy a system, price the replacement filters. You will probably spend more on these filters over the lifetime of the filtration system than on the system itself.
Can I Install a Reverse Osmosis System Myself?
Installing an RO system isn’t much harder than installing a faucet. You will need to drill a hole in your kitchen sink for the RO tap, and a hole in one of the sink drains to connect the waste water line. Other than that, it’s mostly a matter of fitting all the components in the space available under your sink. If you feel like this is too difficult, any plumber should be able install your RO system.