Deconstructing Purity: Why Low TDS Isn't the Whole Story

In the quest for clean water, we’re taught to fear contaminants like PFAS, lead, and arsenic. This has led many to the world of water filtration, armed with a new tool: the TDS meter. We’ve been conditioned to believe that the lower the TDS (Total Dissolved Solids) number, the purer the water.

So, you buy a Reverse Osmosis (RO) system, and you’re thrilled to see an impressive “2 ppm” on the display. But then, you’re also told that “alkaline” or “mineral” water is healthier, and you find that this water has a TDS of “50 ppm” or higher.

This is the central paradox of modern water filtration. How can a low number be the goal, and a high number also be the goal? The answer is that a TDS meter is a “dumb” tool—it can’t tell the difference between “good” and “bad” solids.

Decoding “TDS”: What Are You Really Measuring?

TDS, or Total Dissolved Solids, is a measure of everything dissolved in your water. It’s a simple count of all minerals, salts, and metals, measured in parts per million (ppm).

• “Bad” TDS: These are the contaminants we want to remove. They include industrial chemicals (like PFAS), heavy metals (lead, arsenic, mercury), and excess salts (fluorides, chlorides).
• “Good” TDS: These are the healthy minerals we naturally find in spring water. They include calcium, magnesium, and potassium, which are essential electrolytes and contribute to water’s taste.

A TDS meter cannot distinguish between lead and calcium. It just counts the total. Your tap water might read 250 ppm, which is a mix of both good minerals and potential bad contaminants.

Stage 1: The Sieve (Reverse Osmosis)

This is where a high-tech filter comes in. The heart of a system like the Stokk T1 is its Reverse Osmosis (RO) membrane. With a pore size of 0.0001 micrometers, this membrane is a hyper-efficient physical sieve.

It is so fine that it blocks almost everything that isn’t a pure water molecule. This includes the bad (PFAS, lead) and the good (calcium, magnesium).

This process creates water of exceptional purity. It’s why a user might see a TDS reading drop from 250 ppm (tap) to an incredibly low 2 ppm (post-RO). This is the “blank canvas.”

However, this process also leads to the one common complaint of RO water: the taste. When you remove all the minerals, you get water that tastes “empty” or “flat,” as one user described it. It’s pure, but it’s not what we’re used to.

The Countertop Trade-Off: How “Plug & Use” Works

Before we get to the solution, it’s important to understand how “plug & use” countertop RO systems work without any plumbing.

These units use a two-tank system, often housed within one removable reservoir.
1. Raw Water Tank: You fill this (e.g., 6 liters) with tap water.
2. Filtration: The machine draws from this tank, pushes the water through its RO membrane, and stores the pure water in a separate, internal reservoir (e.g., 1.5 liters).
3. Concentrate Tank: The “bad stuff” (PFAS, lead, salts) that was rejected by the membrane is flushed back into a separate partition of the raw water tank.

This is why, as one user noted, you fill a 6L tank but must dump the “remaining 2L.” That remaining water is no longer tap water; it’s a highly concentrated waste solution. As the system’s manual clarifies, re-filtering this “risks damaging the RO membrane.”

This 3:1 (pure:drain) ratio is highly efficient, and the trade-off for not needing an installer to drill a hole in your drainpipe is the simple daily task of dumping this concentrate and refilling with fresh tap water.

Stage 2: The Enhancement (Alkaline Remineralization)

Now that you have your “blank canvas” of pure, 2 ppm water, you can solve the “empty” taste problem. This is where remineralization (or “alkaline boost”) comes in.

Systems like the Stokk T1 solve this with a separate alkaline mineral boost carafe.
1. You dispense the pure RO water from the machine into this special carafe.
2. The carafe contains a filter (Model RT1-MF) designed to add back beneficial minerals like calcium, magnesium, and potassium.

This does two things:
1. Improves Taste: It adds back the “good” TDS, changing the flavor from “flat” to “crisp,” “clean,” and “smooth,” as users describe.
2. Raises pH: By adding these alkaline minerals, the water’s pH balance is raised.

Now, if you were to use your TDS meter on this final glass of water, what would it read? It would no longer be “2 ppm.” It might be “30 ppm” or “50 ppm” or even “80 ppm.”

This is the solution to the paradox.

A low TDS reading after the RO membrane proves its purity. A higher TDS reading after the alkaline filter proves the remineralization is working. You have successfully used technology to first remove everything, and then selectively add back only what you want.

Conclusion: The Goal Isn’t Zero TDS

The quest for clean water isn’t about achieving a TDS reading of “0.” It’s about selective filtration.

The ideal process, as exemplified by a multi-stage countertop system, is to first use RO to create a “blank slate” by aggressively removing harmful contaminants like PFAS and lead. Then, it’s to use remineralization to turn that pure-but-flat water into healthy, great-tasting mineral water.

A TDS meter is a fantastic tool, but only if you know what it’s telling you. Use it to check the performance of your RO membrane (where low is good), not the final quality of your enhanced, mineralized water (where higher is the goal).