Why are TDS controllers harmful?

By Urban Company

5 min read

May 24, 2026

A marketing gimmick

Scroll through most water purifier listings online, and you'll see the MTDS controller. It's become one of those features that sounds advanced, and is often pitched as ‘the thing that makes your water mineral rich'. But once you understand what MTDS actually does, you might think twice before letting it influence your purchase.

So what is MTDS, really and why is it harmful?

MTDS stands for Manual TDS Controller. The mechanism behind it is quite simple and that's where the problem really lies. A typical RO purifier has a Reverse Osmosis (RO) membrane that does the heavy lifting. It pushes water through a membrane so fine that it removes most dissolved impurities: including heavy metals like lead, arsenic, mercury, along with pesticides, salts, and harmful chemicals. This is the part of your purifier that's actually keeping you safe. Here's where MTDS comes in. It takes a portion of your untreated source water, the same water that came out of your tap and

bypasses the RO membrane entirely

. That bypassed water is sent through only a UV (ultraviolet) lamp or a UF (ultrafiltration) filter, and then mixed back into the purified water before it reaches your glass. While UV and UF kill bacteria and viruses but they don't remove heavy metals. They don't remove dissolved chemicals and they don't remove pesticide residues. None of it. So if your tap water has lead or arsenic in it, MTDS lets a portion of those metals flow straight into your drinking glass. This matters because in many Indian cities, source water quality is genuinely concerning

[Read this report]

. The Bureau of Indian Standards (BIS) under IS 10500 sets a maximum permissible limit of 0.01 mg/L for lead and 0.01 mg/L for arsenic in drinking water, and exceeding these limits, even slightly, is linked to long-term health risks including kidney damage, neurological issues, and cancer. If your municipal water is anywhere near those thresholds, an MTDS controller could be the difference between water that's safe and water that isn't.

Not all TDS is the same - This is the part most people miss.

Imagine two glasses of water, both reading 100 TDS on a meter.

Glass A comes from a purifier with a proper mineraliser cartridge: where calcium, magnesium, and other beneficial minerals are added back to RO-purified water in controlled, measured amounts.

Glass B comes from a purifier with an MTDS controller: where the 100 TDS includes whatever was in your tap water to begin with: minerals, yes, but also potentially heavy metals, chlorine, and dissolved chemicals.

Same number on TDS meter, but completely different water.

Why do brands push TDS controllers so hard?

Firstly, it's a great way to inflate the TDS number on the final water. TDS (Total Dissolved Solids) is a measure of everything dissolved in your water, good and bad. Minerals contribute to TDS. So do heavy metals, salts, and chemicals. The TDS meter doesn't distinguish between them. For years, there's been a marketing-driven narrative that "low TDS water lacks minerals" and is somehow unhealthy. This isn't quite accurate; the minerals your body needs largely come from food, not water. But the narrative stuck. So brands needed a way to show higher TDS numbers on RO water without admitting it. MTDS solves that problem. By blending in unfiltered tap water, the final TDS reading goes up and iIt looks ‘mineral-rich’. Secondly, a bypass valve costs almost nothing to manufacture, but it lets brands list a feature that sounds premium.

What is the solution?

100% RO purified water. Choose a purifier that sends every drop through the full RO membrane: no bypass, no shortcuts. If taste or minerals matter to you, look for a built-in mineraliser cartridge that adds clean, food-grade calcium and magnesium back to the purified water in controlled amounts. The key difference: minerals are added to clean water, not retained from contaminated source water. Yes, 100% RO-purified water may taste different to you if you've been drinking tap or borewell water for years. Your palate has adjusted to its mineral profile and the change can feel noticeable at first. But taste is not a health metric. What tastes right to you today is simply what you're used to. People who switch to fully purified water typically adjust within a couple of weeks and stop noticing any difference at all. More importantly, the version of ‘tastier’ water that MTDS gives you is borrowed directly from your source: meaning the better it tastes, the more of your tap water (and whatever's in it) is reaching your glass. The bottom line: MTDS isn't a health feature. It's a feature designed to make the TDS number look better, not to make your water safer. The point of buying a water purifier is to remove what shouldn't be in your water. A feature that puts some of those impurities back in, just to chase a meter reading, is working against the very reason you bought the purifier in the first place. When in doubt, ask one simple question: "Does this water purifier give 100% RO purified water or does it bypass the RO in any way?" If the answer is bypass - walk away.

Video:

What is MTDS

FAQs

1. What is an MTDS controller in a water purifier?

MTDS stands for Manual TDS Controller. It is a mechanism that mixes a portion of non RO water back into RO-purified water to increase the final TDS level.

2. Does MTDS make water mineral-rich?

Not necessarily. MTDS increases the TDS number, but a TDS meter cannot tell the difference between good minerals and harmful contaminants like heavy metals, salts, pesticides, or chemicals.

3. Why can MTDS be harmful?

Water flowing through MTDS bypasses the RO membrane. This means dissolved impurities like lead, arsenic, mercury, pesticides, and chemicals may remain in the final drinking water.

4. Do UV and UF filters remove heavy metals?

No. UV can deactivate bacteria and viruses, while UF can filter some microorganisms and particles. But neither UV nor UF removes dissolved heavy metals, salts, or chemical contaminants the way RO does.

5. What should I look for instead of MTDS?

Look for a purifier that offers 100% RO purification, meaning every drop passes through the RO membrane. If you want better taste, choose one with a proper mineraliser rather than a bypass-based TDS controller like Native by Urban Company.