Executive Summary
Buy a water filter with Ultrafiltration, Ultraviolet Light and Activated Carbon technologies.
If the TDS of water in your house is above 300 ppm, consider the Reverse Osmosis feature.
A water filter or purifier is a good investment to prevent the risk of waterborne diseases.
Water purifiers often come with technical specifications that can be difficult to understand. Relying on a salesman’s advice, or worse, that of a celebrity in an advertisement, can get you a lemon.
To complicate matters, the requirements vary depending on the town or area you live in. Here is a simple explanation to help you make the right choice.
Impurities in Water
There are three types of contaminants: Physical, biological and chemical.
Physical Impurities
Dirt particles need to be removed. Waterborne pathogens are also physical entities, but we will treat them as biological impurities.
Physical contaminants are removed by simple filtration, passing the water through a fine sieve.
Ultrafiltration
A more advanced technology called Ultrafiltration (UF) can clean deeper. Water is forced through hollow fibre membranes with small pores (0.1 to 0.01 microns, depending on the quality) to remove pollutants.
Get a water filter with ultrafiltration capabilities.
Pre-Filter
A global manufacturer of high-end water purifiers encountered a unique challenge when launching its filters in developing countries. Their UF filter works for a year in the USA. When tested in Thailand, it choked within one month. In New Delhi, India, the same lasted just one and a half days.
A choked UF filter cannot be cleaned and needs replacement. A simple solution is to install a Pre-filter, which does coarse filtration by removing large dirt particles. That cleaner water is then fed to a UF filter.
If you are based in a developing country, get a pre-filter.
Biological Impurities
Municipalities treat water with chlorine and ammonia, which destroy bacteria, viruses and other waterborne organisms. Yet, closer to homes, pathogens may enter the water supply through nearby sewer drains, leaky ducts and mucky overhead tanks.
UF membranes can physically remove bacteria and other large microbes. However, viruses are extremely tiny, some as small as 0.005 microns — far below the physical filtration limit.
Shining Ultraviolet (UV) light on these organisms for a few seconds can damage their DNA and destroy them.
Many countries guarantee potable tap water. In such places, you may not need the UV feature in your water filter. However, if you are worried about waterborne pathogens, UV filtration is desirable.
In India, you must have UV capability in your water filter.
Chemical Impurities
The UF and UV technologies can eliminate physical and biological impurities from water. Things get complicated with chemical pollutants because some contaminants are rather desirable, while others are detrimental or even toxic.
Unfortunately, there is no easy way of measuring which chemical compound — good or bad — is in your water and how much. So you will need some judgment. Let me help you with that.
There are two types of dissolved matter, each needing a different filtration method.
- Chlorine and ammonia are added to water to kill microbes. Additionally, certain undesirable odours, tastes and organic compounds — called Total Organic Compounds (TOCs) — can be dissolved in water. These are removed using Activated Carbon technology.
- In nature, various salts, minerals, metals and other substances may dissolve in water. They are removed using a technology called Reverse Osmosis (RO). However, it should be used only if the contaminants are above a certain limit.
Roughly speaking, reverse osmosis removes inorganic compounds while activated carbon removes organic ones. However, they have overlapping capabilities, needing more understanding.
Total Dissolved Solids (TDS)
The total amount of dissolved organic and inorganic molecules, ions or particulates smaller than 2 microns is called Total Dissolved Solids (TDS). Filtration aims to keep the TDS in a certain range — neither too high nor too low.
TDS concentration is measured in parts per million (ppm), which is milligrams of dissolved solids per 1,000 grams of water. That is almost equivalent to milligrams per litre (mg/L).
Water TDS originate from natural and human factors. Natural origins include soil minerals and land runoffs. Pesticides from agriculture, industrial production and waste disposal are the common human sources.
While some TDS are desirable, many others are not. Since it is not possible to selectively remove them, TDS filtration is a tricky concept. Generally, if we keep the water TDS level below a certain number, even the undesirable types of TDS do not cause harm.
Table 1 displays the guidelines for acceptable TDS levels in drinking water as per the World Health Organisation (WHO) and the Bureau of Indian Standards (BIS).
Table 1. Guidelines for acceptable TDS levels in drinking water.| TDS Level (ppm) | Rating | Comments |
|---|---|---|
| 0 – 50 | Unacceptable | Lacks minerals; Flat taste |
| 51 – 150 | Excellent | Ideal for drinking; Balanced Mineral levels |
| 151 – 300 | Good | Good taste; Good for health |
| 301 – 500 | Fair | Acceptable taste; Better if lesser mineral amount |
| 501 – 1000 | Poor | Poor taste; Avoid for long term consumption |
| Above 1000 | Very Poor | Unsafe for consumption |
The TDS of 80 to 150 ppm is ideal for taste and health.
Different cities around the world have varying levels of TDS. If the water source is a borewell or groundwater, the TDS is usually high. Cities that receive water supply from lakes generally have lower TDS. The average TDS (ppm) in some cities of India and the world is in Table 2.
Table 2. Average TDS (ppm) in Many Cities of India and the World.| City | Average TDS (ppm) |
|---|---|
| Ahmedabad, India | 400 to 1000 |
| Bengaluru, India | 250 to 600 |
| Chennai, India | 300 to 1200 |
| Delhi NCR, India | 200 to 800 |
| Hyderabad, India | 400 to 800 |
| Kolkata, India | 60 to 200 |
| Mumbai, India | 50 to 150 |
| Pune, India | 150 to 400 |
| Melbourne, Australia | 10 to 26 |
| Sydney, Australia | 39 to 60 |
| Toronto, Canada | 121 |
| Montreal, Canada | 116 |
| London, England | 275 |
| Manchester, England | 25 |
| Cape Town, South Africa | 44 to 217 |
This list is compiled from various sources.
Within a city, the number may vary widely depending on the location as well as the water source. So it is always good to check the TDS in your house water before deciding on a water purifier.
You can purchase a simple handheld TDS meter online (costing about Rs 250 or USD 3) to test your home supply.
As a thumb rule, unless the TDS levels in your home water are above 300, you do not need a RO filtration system.
Incidentally, high TDS water is not the same as hard water.
Hard Water
TDS includes any dissolved substance. Hard water refers to high concentrations of calcium and magnesium salts, a subset of TDS.
Water with a TDS of more than 120 ppm is considered hard, and above 180 ppm is very hard. The assumption is that the TDS number comes from dissolved calcium and magnesium compounds. That is often, but not always, the case.
Softening the water does not reduce its TDS. The process replaces hardness-causing calcium and magnesium minerals with those of sodium or potassium — the water hardness reduces, but its impurities do not.
Things to Keep in Mind
- Higher TDS does not necessarily lead to health issues, though the guidelines prohibit drinking such water. Water purifier companies claim that high TDS water can cause kidney stones, diabetes, heart disease or digestive disorders. However, studies have shown a weak correlation with such diseases. As a perspective, the entire animal kingdom drinks untreated water in nature. Poultry has a safe upper limit of 2,900 ppm TDS, whereas cattle have a safe upper limit of 7,100 ppm. Why would humans fall sick from 501 ppm water?
- The drinking water should have a minimum of 40 ppm calcium and 20 ppm magnesium, respectively. Thus, a minimum TDS of 60 ppm is recommended.
- Reverse Osmosis technology ends up wasting a lot of water. For every litre of water purified, 3 to 4 litres are discarded, a point to note if your area has a perennial water shortage.
- Some lower-end reverse osmosis systems remove almost the entire TDS of water, which is unhealthy. Modern RO systems ensure adequate remineralisation of filtered water. If your purifier does not, you may have to remineralise the filtered water separately.
To Read More
- Wikipedia: Total Dissolved Solids
- Ion Exchange: TDS in Water: Ideal Levels and Treatment Options
- RO Megamart: Safe TDS for Drinking Water in India
- Intec America: TDS Measurement: All Important Questions Answered
- Calsoft Water: What Is the Difference Between Hardness and TDS in Water?
- Drink Prime: Why RO Purifiers Waste Water: Understanding the Filtration
- Safe Drinking Water Foundation, Canada: TDS and pH
- Eureka Forbes: Effects of High TDS on Health and Home Appliances
- Waterdrop: How to Remineralize Reverse Osmosis Water?
First Published On: 4th August 2025
Image Credit: svetlanasokolova on Freepik
Last Updated on: 14th August 2025