You just installed a brand-new carbon filter.
The water tastes amazing. No more chlorine smell. Crystal clear.
But then you test it with your TDS meter. And your heart sinks.
The number hasn’t budged. Still 250 ppm. Still the same as before you spent $400 on the filter.
You think: Did I get scammed? Is my filter broken?
No. Your filter is working perfectly. The problem is you’re measuring the wrong thing.
What TDS Actually Measures (And What It Doesn’t)
TDS stands for Total Dissolved Solids.
Your TDS meter sends an electrical current through water. Minerals conduct electricity. More minerals mean higher numbers.
But here’s the critical detail most people miss: TDS only measures dissolved minerals.
It measures calcium. Magnesium. Sodium. Potassium.
It does NOT measure chlorine. It does NOT measure pesticides. It does NOT measure volatile organic compounds (VOCs). It does NOT measure pharmaceuticals.
These contaminants don’t conduct electricity the way minerals do. So your meter ignores them.
This is why bottled water with a TDS of 5 ppm can still be contaminated with chemicals.
Why Carbon Filters Don’t Lower TDS
Carbon filters work through a process called adsorption.
Think of activated carbon like a sponge with microscopic pores. Chemical molecules stick to the surface of these pores as water flows through.
But minerals like calcium and magnesium are too small and too charged to stick.
They pass right through.
What carbon DOES remove:
| Contaminant | Removal Rate |
| Chlorine | 95-99% |
| Chloroform (THM) | 90-98% |
| Volatile Organic Compounds | 85-95% |
| Pesticides | 70-90% |
| Calcium & Magnesium | 0% |
Notice the pattern?
Carbon removes chemicals. It ignores minerals. So your TDS stays the same.
The Only Filter That Lowers TDS: Reverse Osmosis
If you want your TDS to drop, you need a different technology.
Reverse osmosis (RO) uses a semipermeable membrane. Think of it like a microscopic strainer with holes so small that only water molecules can pass through.
Minerals get trapped. Chemicals get trapped. Everything gets trapped except H2O.
An RO system will drop your TDS from 250 ppm down to 10-20 ppm.
But RO comes with trade-offs:
Water waste. RO systems flush 3-4 gallons of water down the drain for every gallon they produce.
Slow flow rate. RO fills a storage tank because the membrane filters water drop by drop.
Higher cost. A quality RO system runs $300-$600, plus $80-$120 per year in filter replacements.
Pro Tip: Most whole-house RO systems are overkill. Install an under-sink RO unit for drinking water and use carbon filters for the rest of your home.
Why TDS Is Not a Measure of Water Purity
This is where most people get confused.
High TDS does not mean dirty water. Low TDS does not mean clean water.
Example 1: Mountain spring water.
It flows over rocks and picks up calcium and magnesium. TDS might be 300 ppm.
But it’s pristine. No chlorine. No pesticides. No lead.
Example 2: Distilled water spiked with chloroform.
TDS reads 0 ppm because chloroform doesn’t conduct electricity.
But it’s toxic.
According to the World Health Organization, the ideal TDS for drinking water is 150-500 ppm. Not because higher is dangerous, but because minerals improve taste and provide electrolytes.
The EPA does not regulate TDS as a health hazard. It’s listed as a secondary standard for taste, not safety.
What You Should Actually Test For
Forget the TDS meter.
If you want to know if your filter is working, test for the contaminants that matter:
Lead: Use a $15 lead test kit from Tap Score or 120Water. Carbon filters certified for NSF 53 should remove 99% of lead.
Chlorine: Drop a pool test strip in your water. Free chlorine should read 0 ppm after a carbon filter.
Chloramines: Chloramine test strips cost $10. Standard carbon removes 50%. Catalytic carbon removes 95%.
VOCs: Send a sample to a lab. Your local water utility may offer free testing.
Warning: If your filter claims to remove lead but is not NSF 53 certified, it’s lying. Check the NSF database at nsf.org before you buy.
When High TDS Actually Matters
There are two scenarios where you should care about TDS:
Scenario 1: Coffee and espresso machines.
High mineral content causes scale buildup. Manufacturers recommend TDS below 150 ppm.
Solution: Use an RO system or buy a water softener.
Scenario 2: Aquariums and hydroponics.
Fish and plants need precise mineral levels. TDS above 500 ppm can stress aquatic life.
Solution: Start with RO water (TDS near 0) and add minerals back manually.
For drinking water? Stop obsessing over TDS.
The Real Question: Is Your Carbon Filter Working?
Here’s how to verify your filter is doing its job:
Step 1: Taste test.
Fill two glasses. One from the tap before the filter. One after. If the chlorine taste is gone, your filter is working.
Step 2: Smell test.
Run hot water. If you smell rotten eggs or a chemical odor, your filter might be saturated.
Step 3: Check the install date.
Most carbon filters last 6-12 months. If yours is older, replace it.
Expired carbon doesn’t just stop working. It releases trapped contaminants back into your water.
Pro Tip: Mark your calendar. Set a reminder for filter replacement. Most filter failures happen because people forget.
The Bottom Line
Your TDS meter is not broken.
Your carbon filter is not defective.
Carbon filters remove chemicals. They don’t remove minerals. So TDS stays high.
If you want lower TDS, install an RO system.
But if you just want clean, safe water without chlorine taste? Your carbon filter is doing exactly what it should.
Stop chasing a number. Start testing what actually matters.
Your health will thank you.