TDS tests are one of the most common water quality measurements used by professionals and homeowners alike. This is because TDS in water is naturally occurring, tough to remove, and easy to test. While TDS can illuminate elevated levels of dissolved salts or chemicals from industrial waste, it not a one-size-fits-all solution. Water possesses many contaminants that are not represented by a TDS test. Furthermore, many water filtration systems are addressing problems unrelated to TDS, so testing their water’s TDS is irrelevant. Read on to explore why people test for TDS, when TDS testing is useful, and how to gauge your water filter’s performance.
What is TDS?
TDS, an acronym for total dissolved solids, is an aggregate measurement of all the organic and inorganic material present in a sample of water. This includes dissolved organic compounds, inorganic compounds, salts, minerals, and ions. TDS meters measure the conductivity of the water in parts-per-million (ppm), which is equivalent to measuring the mass of contaminants present per liter of water (mg/L). TDS is a nonselective measurement. It does not differentiate between the salts dissolved in your well water and the naturally occurring mineral content, like magnesium and sodium. If your TDS levels are above 600, or even in the 1,000s, it is a huge indicator that there is a wildly problematic level of dissolved material present in your water. A reading between 300 and 500 is considered acceptable, but it is difficult to distinguish which dissolved materials are in your water from a TDS reading alone. To determine whether any dangerous TDS are in your water or not, you must rely on other tests.
The EPA considers TDS to be a secondary drinking water contaminant. Secondary drinking water contaminants pose no health risks and, though there are suggested optimal levels of these contaminants, the EPA does not enforce these standards. Instead, these are considered to be primarily cosmetic, aesthetic, and technical inconveniences. For example, acidic water is not dangerous to consume, but it is extremely corrosive and destroys household plumbing. Hard water similarly does not run the risk of making you and your family sick, but it will wreak havoc on your water heaters and household appliances. High levels of iron can turn your water a bright, unattractive orange color, but this does not mean that drinking this water will cause illness.
What does TDS not measure?
TDS does not measure the safety of your water. For example, you could receive a 0 TDS reading and have water with toxic levels of lead present. Other potentially dangerous drinking contaminants, like pharmaceuticals, pesticides, arsenic, hexavalent chromium, and VOCs will not appear. Potentially pathogenic viruses, cysts, and protozoa will similarly not appear in a TDS reading. TDS is strictly concerned with solid matter dissolved in the water, not the overall makeup of the water.
Why measure TDS in water?
TDS is a popular water quality measurement because it is simple, accessible, and provides the reader with a general understanding of their water’s makeup. If the TDS meter returns a 2,000ppm reading, your water is brimming with dissolved solids, many of which are likely to be undesirable contaminants. If the TDS meter shows a reading of 100, you know your tap water is generally low in organic and inorganic compounds and lacking high mineral content.
However, a TDS reading is also a common water salesman trick. If a homeowner is shown a 300ppm TDS reading and told their water is full of dangerous and unknown dissolved substances, this can be used to leverage the sale of an unnecessary and expensive water filtration system. The truth is a TDS meter alone cannot provide the salesman or the homeowner a complete picture of the water’s quality. A more in-depth water test kit can provide a much more complete picture of what is present in your water. Some specialized tests can be performed at home and give you estimated ranges of specific contaminants like copper or arsenic. A lab test kit will give you a complete assessment of the contaminant levels present in your water.
Learn more: Lab water tests vs at-home test kits
How to test TDS in water
You can easily test TDS in your water with a TDS meter. These are battery-powered devices that can test TDS, temperature, electrical conductivity, and other contaminants depending on the model of the meter. More advanced meters, such as the HM Digital COM-360, can measure pH, salinity, temperature, TDS, and electrical conductivity within the same device. If you only wish to know the TDS content in water, a simple, inexpensive TDS meter should do the trick.
Why does my filtered water contain TDS?
If you have an under-sink carbon filter installed to reduce the chlorine content of city water, your filtered water is likely going to have exactly the same TDS level as your unfiltered tap water. This is because carbon filtration in no way addresses dissolved solids. Through a process called adsorption, carbon filters reduce chlorine, chloramines, and VOCs from drinking water, restoring the overall taste and odor of your water.
Carbon filters also usually come with a micron rating. Micron ratings tell the size of the pores in the filter media. If the carbon filter has a 0.5 micron rating, it will prevent any particulate larger than half a micron from passing through the filter. Picture a screen door on a house. That screen door has a mesh to prevent insects and leaves from blowing inside of your home. But, if your next-door neighbor is grilling a barbeque, the smell of the grill is going to waft into your house. The screen door is like a micron filter. The particulate matter (like chlorine and cysts) will be blocked from entering. But the TDS is like the smell of your neighbor’s barbeque, it flows easily through the filter without any kind of reduction. This is because TDS is dissolved within the water, it cannot be filtered out physically by the minute pores on the carbon filter’s surface.
Reverse osmosis systems, also known as RO systems, remove TDS by forcing them through a semipermeable membrane that separates contaminants entirely from the water. This is because the reverse osmosis membrane’s pores are small enough to block even dissolved particles, allowing only clean water to flow through the membrane to the less concentrated side. If the TDS of your tap water is the same as the TDS of your RO system, there is definitely a performance issue with the filter, and you need to replace the membrane or have the unit serviced. For those with unsafe TDS levels or people desiring water of exceptional purity, a reverse osmosis system is an ideal solution. Water distillers also remove TDS, along with all other contaminants present in the water, by converting the source water into steam before cooling it back to a liquid state. This water treatment method also produces very high purity water.
Learn more: How reverse osmosis works | How activated carbon filters work
How do I accurately judge my water filter’s performance?
Understanding what contaminant or water quality issue your water filtration system is targetting is key to evaluating the performance of that filter. Challenging a carbon filter with a TDS meter isn’t going to provide you with any tangible information, because that filter is not addressing TDS.
Water softeners
A water softener is eliminating water hardness, through a process called ion exchange. Water hardness creates scale build-up in pipes and dramatically reduces the efficacy and lifespan of household appliances like dishwashers, laundry machines, and water heaters. To know if your water softener is working, measure the water’s hardness. If your water test reveals that you have more than 60 ppm of hardness in your water still, your softener probably is not performing as optimally as it should be.
Learn more: How water softeners work | How to prevent limescale buildup in your home
Acid neutralizers
Acid neutralizers boost the pH of acidic water by exposing them to mineral-rich, highly alkaline media. Acidic water is extremely corrosive and can strip the metal out of your pipes and deposit it into your sinks, leaving blue-green stains on your faucets and drains. To monitor your acid neutralizer, check your water’s pH balance. If the neutralizer has failed to raise your pH closer to the ideal neutral 7, you may need to use a more alkaline filter media to achieve your desired results.
Learn more: What is acidic water and how do you treat it?
Carbon filters
Carbon filters use adsorption to improve water’s taste and odor by reducing chlorine, a sterilization agent used by municipal water treatment plants to neutralize bacteria in water. The best way to tell if your carbon filter is working is by the taste and smell of the filtered water. Chlorine gives water a chemical taste and harsh smell. If you notice an unpleasant aftertaste creeping back into your drinking water, it is probably time to replace your carbon filter. You can also purchase test strips that will measure your water’s chlorine content. Many carbon filters have lead-reducing capabilities as well. If you use a carbon filter to reduce lead content, it is extremely important to test for lead periodically. A TDS meter will not convey the amount of lead in your water supply, and lead has a host of dangerous side effects.
Reverse osmosis systems
A reverse osmosis unit is an example of a unit that should be challenged with a TDS test. If your TDS levels are higher than 8, the membrane is allowing too many dissolved particles through, and you will need to troubleshoot what problems you may be facing. Evaluating your RO’s reduction capabilities also depends on what specific contaminants you are using it to eliminate. If your water is high in arsenic, use arsenic test strips to ensure that the toxin isn’t making its way into your water. You can purchase test strips for many contaminants that reverse osmosis reduces, like pesticides and nitrates. Using these will give you a more specific understanding of what is making its way past the membrane, a nuance that the TDS meter alone will not provide you.
Learn more: How to troubleshoot your reverse osmosis system