EXPLAINER
RO Terms: TDS, GPD & Recovery Ratio - What Actually Matters
Decode essential RO water purifier terminology: TDS levels, GPD ratings, and recovery ratios. Understand what these specs mean for real-world performance and.
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01
The three numbers on every RO listing
RO purifier listings lead with three specs and explain none of them. Here is what each one actually measures, before the detail below.
TDS
Total Dissolved Solids
Indian municipal supply runs 100–400 ppm. Borewells run much higher.
GPD
Gallons Per Day
A lab flow rating. Real output falls with cold water and low pressure.
RECOVERY
The clean-water share
Typical RO recovers 25–50%. The rest leaves as wastewater.
02
TDS, the water quality indicator
TDS is the total concentration of dissolved substances in water, measured in parts per million (ppm) or milligrams per litre. It is the number a ₹300 pen meter reads, and the one every salesperson quotes.
What the ranges mean
| TDS range | Verdict | What it means |
|---|---|---|
| 0–50 ppm | Excellent | Ultra-pure, though very low on minerals |
| 50–150 ppm | Good | The WHO-recommended band for drinking water |
| 150–300 ppm | Fair | Acceptable, but taste may suffer |
| 300+ ppm | Poor | Needs treatment; unpleasant taste or odour |
Scroll the table sideways to see all columns.
What Indian sources actually measure
| Source | Typical TDS |
|---|---|
| Municipal supply | 100–400 ppm |
| River water | 200–500 ppm |
| Borewell water | 300–800+ ppm |
| Deep groundwater | 500–1,500+ ppm |
Scroll the table sideways to see all columns.
What is actually dissolved in it
BENEFICIAL MINERALS
- Calcium. Bone health, and it carries taste.
- Magnesium. Essential mineral, muscle function.
- Potassium. Heart health, electrolyte balance.
- Sodium. An electrolyte, in moderate amounts.
HARMFUL CONTAMINANTS
- Lead. Neurological damage.
- Arsenic. Cancer risk and organ damage.
- Fluoride. Dental and skeletal fluorosis in excess.
- Heavy metals. A range of health effects.
THE POINT EVERYONE MISSES
TDS measures quantity, not quality. A high reading could be beneficial minerals or harmful contaminants. The meter cannot tell you which, and neither can the salesperson holding it.
03
GPD, and the flow-rate reality check
GPD is the theoretical maximum production under ideal laboratory conditions. It is not what your kitchen will deliver.
IDEAL LAB CONDITIONS
- Water temperature. 25°C
- Water pressure. 50–60 PSI
- TDS level. 500 ppm
- Membrane. Brand new
REAL HOME CONDITIONS
- Water temperature. 15–35°C, varying
- Water pressure. 20–40 PSI typically
- TDS level. 100–1,000+ ppm
- Membrane. Degrading from day one
What drags the number down
- Cold water. 20°C water yields roughly 15% less than 25°C.
- Low pressure. 30 PSI costs about 30 to 40% of the rated GPD.
- High TDS. Above 1,000 ppm can cut output 20 to 30%.
- Membrane age. A six-month-old membrane flows 10 to 15% less.
WHAT TO EXPECT IN PRACTICE
A 50 GPD system delivers about 30–40 GPD, or 1.2–1.7 litres an hour. A 75 GPD system delivers 45–60 GPD, or 2–2.5 litres an hour. A 100 GPD system delivers 65–80 GPD, or 2.7–3.3 litres an hour.
Sizing GPD for your household
| Family size | Daily need | Minimum GPD | Recommended GPD |
|---|---|---|---|
| 2–3 people | 10–15 L | 50 GPD | 75 GPD |
| 4–5 people | 20–25 L | 75 GPD | 100 GPD |
| 6 or more | 30+ L | 100 GPD | 150+ GPD |
Scroll the table sideways to see all columns.
04
Recovery ratio, and the water it wastes
Recovery is the percentage of input water that comes out as drinking water. Everything else goes down the drain, and in most Indian homes that is the majority of it.
| Recovery | Per litre of clean water | Ratio |
|---|---|---|
| 25% | 3 L wastewater | 1:3 |
| 33% | 2 L wastewater | 1:2 |
| 40% | 1.5 L wastewater | 2:3 |
| 50% | 1 L wastewater | 1:1 |
Scroll the table sideways to see all columns.
What moves the ratio
- Input TDS. Higher TDS means lower recovery.
- System design. Multi-stage systems often recover less.
- Membrane pressure. Higher pressure improves recovery.
- Flow rate. A slower flow gives a better ratio.
WHAT THE WASTE COSTS YOU
At a 20 L daily need, a 25% recovery system draws 80 L a day — 2,400 L a month, and roughly ₹180–300 extra on the water bill.
A 50% recovery system draws 40 L a day, or 1,200 L a month. That is ₹90–180 a month saved, which is how a higher-recovery system pays for itself.
Reusing the wastewater
WHERE IT WORKS
- Plant watering. Check salt tolerance first.
- Toilet flushing. The single biggest saving.
- Floor cleaning. Mopping and general cleaning.
- Car washing. Exterior only.
COLLECTING IT
- Tank. 50–100 L covers a day's wastewater.
- Diversion. Solenoid valve kits automate it.
- Freshness. Use within 24–48 hours.
- Quality. Expect 2–3x the input TDS.
05
What actually matters when buying
These three specs are not equally important, and they are not evaluated in the order the listing presents them.
-
1
Test your input TDS first
It decides whether you need RO at all, how much capacity to buy, what output to expect, and how often the system will need servicing. A digital TDS meter costs ₹200–500 and is about 95% accurate. A professional test runs ₹500–1,500 and identifies specific contaminants. Test every source you use — municipal, borewell and tank.
-
2
Then compare recovery ratios
Recovery drives the monthly water bill, the environmental cost, how fast the tank fills, and how much wastewater you have to manage. It is the spec with the largest running-cost impact and the one listings mention least.
-
3
Only then look at GPD, and derate it
Expected real GPD is roughly rated GPD × 0.6 to 0.8 in Indian conditions. Temperature costs 10–20%, pressure 15–30%, high TDS 10–25% and membrane age 10–15%. Oversize by 25 to 50%, plan for peak usage, and remember the storage tank affects perceived speed more than the rating does.
06
Two spec myths worth killing
MYTH — LOWER OUTPUT TDS IS ALWAYS HEALTHIER
50 to 150 ppm is the target. That band keeps beneficial minerals while removing harmful contaminants. Ultra-low TDS water strips the minerals along with everything else, and tastes flat.
MYTH — HIGHER GPD MEANS FASTER WATER
GPD is daily capacity, not flow rate. A 75 GPD system produces about 3 litres an hour in ideal conditions and less in real use. Storage tank size affects how fast water feels to arrive far more than the GPD rating does.
07
Frequently asked questions
What TDS level should I target for drinking water?
Aim for 50 to 150 ppm in the output. That range keeps beneficial minerals while staying safe. If your input water is below 300 ppm and tastes fine, you may not need RO at all — UV or UF purification may be enough, and for low-TDS municipal water a non-electric gravity water purifier can be enough too.
Why is my RO producing much less water than the GPD rating?
That is normal. GPD ratings assume 25°C water, 60 PSI and a new membrane. Real output is typically 60 to 80% of the rating once temperature, pressure, TDS and membrane age are accounted for. Size the system 25 to 50% above your calculated need.
Is RO wastewater safe to reuse?
For many uses, yes. It typically carries two to three times the input TDS but is free of bacteria and most contaminants. It suits toilet flushing, floor cleaning, gardening — check plant salt tolerance — and car washing. Use it within 24 to 48 hours.
How can I improve my system's recovery ratio?
Fit a booster pump if your pressure is below 40 PSI, and replace the membrane on schedule. Adjust the waste-flow valve if yours is adjustable, or look for a system with built-in recirculation. Do not trade water quality for recovery — the rejection ratio still has to hold.
Should I just buy the highest GPD system available?
No. Higher-GPD systems cost more and sometimes recover less. Work out your real daily need at three to four litres per person, add a 25 to 50% buffer, and buy to that. A 75 GPD system covers most four-person households. Recovery ratio and build quality matter more than the maximum rating.
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