As an affiliate, we may earn a commission from qualifying purchases. We get commissions for purchases made through links on this website from Amazon and other third parties.
A reading of 20 ppm TDS is very low, but it does not mean the water is unsafe to drink. It also does not prove that the water is safe.
TDS measures the amount of dissolved material in water. It does not identify that material or check for many important drinking-water hazards. Water with 20 ppm TDS can be suitable for drinking if the water source, treatment, plumbing, storage, and laboratory testing show that it meets drinking-water requirements.
What Does 20 PPM TDS Mean?
TDS stands for total dissolved solids. These are dissolved substances such as calcium, magnesium, sodium, chloride, bicarbonate, and sulfate.
TDS is usually reported in parts per million (ppm). In ordinary drinking water, 20 ppm is roughly equal to 20 milligrams per liter (mg/L).
A reading of 20 ppm means the water contains only a small amount of dissolved material. This can happen with:
- Reverse osmosis water
- Distilled or highly purified water
- Some low-mineral source water
- Rainwater
- Water that has gone through strong demineralization treatment
The number alone does not tell you whether those 20 ppm of dissolved material are harmless.
Is 20 PPM TDS Too Low for Drinking Water?
There is no health-based minimum TDS level that makes drinking water safe.
The World Health Organization does not set a health-based guideline value for TDS. Its current drinking-water guidance says TDS at levels normally found in drinking water is not considered a health concern by itself. TDS mainly affects qualities such as taste and mineral content.
In the United States, the EPA lists 500 mg/L of TDS as a secondary drinking-water standard. This is not a health limit. Secondary standards mainly deal with issues such as taste, staining, deposits, and appearance.
So a 20 ppm reading is well below that EPA secondary level. That does not, however, mean the water has passed a safety test.
Why a Low TDS Reading Does Not Prove Water Is Safe
A TDS meter usually estimates dissolved solids by measuring how well the water conducts electricity.
It is useful for checking changes in mineral content. For example, it can help you see whether an RO membrane is still reducing dissolved ions.
It cannot provide a complete drinking-water test.
A low TDS reading does not rule out:
- Bacteria
- Viruses
- Parasites
- Some pesticides
- Fuel or solvent contamination
- Other organic chemicals
- Harmful contaminants present at very small concentrations
TDS also tells you the total amount, not the identity of each dissolved substance.
For example, two water samples could both measure 20 ppm while containing very different dissolved materials.
What a TDS meter tells you
| A TDS meter can help show | A TDS meter cannot establish |
|---|---|
| Overall dissolved mineral level | Whether water is safe to drink |
| Changes in dissolved-ion content | Whether bacteria are present |
| Whether an RO system is reducing many dissolved ions | Which specific chemicals are present |
| Changes in source or treated water over time | Whether treatment removed every health hazard |
Use TDS as one water-quality indicator, not as a pass-or-fail drinking-water test.
Does 20 PPM TDS Mean the Water Lacks Minerals?
It means the water has relatively little dissolved material.
That may include low amounts of calcium, magnesium, and other minerals. Water with very little mineral content may taste different from harder or more mineral-rich water.
Low TDS by itself is not a reason to assume the water is harmful.
You also should not assume that adding minerals makes questionable water safe. Remineralization may change taste, hardness, or water chemistry, but it does not replace testing and proper treatment.
If very low-TDS water is being used throughout a plumbing system, it can also be useful to check pH, alkalinity, and corrosivity. TDS alone cannot tell you whether water may contribute to pipe corrosion.
What If the 20 PPM Reading Comes From an RO System?
A low reading can be expected after reverse osmosis because an RO membrane removes many dissolved ions.
The more useful check is usually to compare the treated water with the untreated feed water.
For example, if the source water has a much higher TDS and the RO water measures 20 ppm, the reading can show that the membrane is reducing dissolved solids.
It still does not prove microbiological safety or confirm removal of every possible contaminant.
An RO system also needs proper:
- Filter changes
- Membrane maintenance
- Storage-tank sanitation
- Faucet and tubing cleanliness
- Treatment appropriate for contaminants actually present in the source water
If the original water supply is questionable, test for the contaminants of concern instead of relying only on the TDS number.
What If the 20 PPM Water Is Collected Rainwater?
Low TDS is common in rainwater because rain starts with relatively little dissolved mineral content.
But roof-collected rainwater can pick up contamination before reaching the tank.
Possible sources include:
- Bird and animal droppings
- Dust and debris
- Roofing materials
- Gutters and flashing
- Metals
- Smoke and airborne particles
- Dirty storage tanks
- Insects and other animals
The CDC warns that collected rainwater is not necessarily safe to drink even when it looks clean. For rainwater used for drinking, cooking, or bathing, the CDC recommends regular testing for germs and chemicals and maintaining the collection system.
A first-flush diverter can also improve incoming water quality. A first flush sends the first portion of roof runoff away from the storage tank because that water can carry more dirt and contamination from the roof. It is a useful prefiltration step, but it does not make rainwater potable by itself.
Evaluating EPA guidance for TDS in drinking water helps assess the appropriate follow-up check for likely contaminants.
For drinking-water use, consider the whole system:
roof and gutters → debris control → first flush → storage → filtration → treatment → protected plumbing → laboratory testing
A 20 ppm TDS reading at the end of that system is only one small piece of the water-quality picture.
What Should You Test Before Drinking Low-TDS Water?
The right tests depend on where the water came from.
Public tap water
Public water systems are routinely regulated and monitored. If you run that water through an RO system and get a reading of 20 ppm, review your water supplier's quality report and maintain the treatment system correctly.
A low post-RO TDS reading alone is not usually a reason to stop drinking the water.
Private well water
Private wells require more direct testing by the owner.
The CDC recommends testing private well water at least once each year for:
- Total coliform bacteria
- Nitrates
- TDS
- pH
Other tests depend on local geology, nearby land use, and possible contamination sources. The CDC recommends using a state-certified laboratory and asking the local health department which additional contaminants matter in your area.
Rainwater and cistern water
If collected rainwater will be swallowed, use laboratory testing appropriate for both germs and chemicals.
The testing plan may need to account for the roof, gutters, tank materials, local air quality, animals, treatment equipment, and other possible contamination sources. The CDC specifically recommends regular testing when rainwater is used for drinking, cooking, or bathing.
Do not use a TDS meter, test strip, single filter, UV unit, or purifier as proof that roof runoff is potable.
Should You Remineralize 20 PPM Water?
Not necessarily.
Some people prefer more mineral content because of taste. Some RO systems also add minerals after treatment to adjust water chemistry.
But there is no universal TDS number you must reach before water becomes drinkable.
If your water is already from a safe source and has been properly treated, remineralization is mainly a system-design and water-chemistry decision. If the water has not been shown to be safe, adding minerals does not solve the underlying problem.
The Bottom Line
20 ppm TDS is not automatically too low or unsafe for drinking water.
It simply means the water contains a small amount of dissolved material. Both WHO and EPA guidance treat TDS mainly as a water-quality and acceptability measure rather than a stand-alone health test.
What matters most is where the water came from and what else is in it.
If the 20 ppm reading comes from properly maintained treatment of an already safe water supply, the low number alone is usually not a concern. If it comes from a private well, rainwater tank, cistern, or another unregulated source, use appropriate laboratory testing before treating the TDS reading as evidence of drinking-water safety.
Frequently Asked Questions
Is 20 ppm TDS good for drinking water?
It can be. A TDS level of 20 ppm is very low, but TDS alone cannot determine whether water is safe. The source, treatment, plumbing, storage, and actual contaminants matter more.
The number is easier to interpret alongside a suitable TDS level for drinking water and the limits of TDS testing.
Is 20 ppm TDS too low?
There is no health-based minimum TDS level that water must meet to be drinkable. Very low-TDS water may taste flat or different because it contains fewer dissolved minerals.
Can I drink water if my TDS meter reads 20 ppm?
A reading of 20 ppm is not a reason by itself to avoid the water, but it is also not proof of safety. If the source is a private well, rainwater system, or other unregulated supply, use proper laboratory testing.
Does low TDS mean the water is pure?
No. Low TDS means there are few substances that the meter can detect through electrical conductivity. Germs and some chemicals may still be present.
Is 20 ppm normal after reverse osmosis?
A low TDS reading can occur after reverse osmosis because RO removes many dissolved ions. Use the reading to track treatment performance, but continue following the system's filter, membrane, and sanitation requirements.
Is rainwater with 20 ppm TDS safe to drink?
Not based on the TDS reading alone. Roof runoff can contain germs and chemicals even when its TDS is low. Drinking-water use requires suitable collection, treatment, regular maintenance, and appropriate laboratory testing.
Do I need to add minerals to 20 ppm water?
Not automatically. Adding minerals may change taste or water chemistry, but it is not required simply because the TDS is 20 ppm. It also cannot make contaminated water safe.
