Before buying a water purifier, first figure out what your water lacks

Double 11 has arrived again, and water purifiers are a "major annual purchase" for many families. The specs on product pages are prettier one after another: how many filtration stages, how large the flow rate, how many years of long-lasting performance. But many people only discover after bringing one home that the unit they installed doesn't solve their own household's problems.

There is no "best" water purifier, only the one that "matches." First figure out what your water lacks, then choose the equipment — this matters far more than comparing specs.

1. First, look at what condition your water is in

Step one: check the reports. Most cities' water supply companies regularly publish test results for treated water and distribution-network water, which can be found on the official websites or WeChat accounts of local water authorities or supply companies. In China, drinking water is governed by GB 5749-2022, which sets limits for total dissolved solids (TDS) at 1000 mg/L, total hardness at 450 mg/L, and oxygen consumption (an indicator of organic matter) at 3 mg/L.

Step two: look at the water from your own tap. Water meeting standards at the treatment plant doesn't mean your tap is necessarily the same. Old residential communities, secondary-supply water tanks, and household pipes can all make the water at the endpoint different from the treated water. You can start from a few everyday observations:

• Fill a glass with water and let it sit — is it cloudy or does it have sediment?
• Do your kettle, showerhead, and water heater scale up particularly quickly?
• Does the water have an obvious chlorine smell or earthy odor?
• Do you just find the taste poor and want "purer" water?

These observations correspond to different problems, and also to different purification technologies.

Two, four types of problems, corresponding to four types of technology

• Your household water: turbid, with sediment and rust; main problem: particulate matter; corresponding technology: pre-filter, PP cotton; which indicator to check for effectiveness: turbidity.
• Your household water: chlorine smell, earthy/musty odor, elevated organics; main problem: residual chlorine, organics; corresponding technology: activated carbon (or composite filter cartridge); which indicators to check for effectiveness: residual chlorine, TOC, UV254.
• Your household water: heavy scale, heavy soap scum; main problem: calcium and magnesium hardness; corresponding technology: water softener, or nanofiltration; which indicator to check for effectiveness: total hardness.
• Your household water: you want to drink low-mineral water; main problem: dissolved ions; corresponding technology: reverse osmosis (RO); which indicators to check for effectiveness: TDS, salt rejection rate.

A few notes:

• The pre-filter is the cheapest stage and the one you should install first. It blocks sediment and rust and protects all the filter cartridges downstream.
• Activated carbon is effective against residual chlorine, odors and some organics, and is the most direct stage for improving taste.
• Water softeners and nanofiltration deal with hardness. Nanofiltration removes most calcium and magnesium while retaining some minerals; water softeners replace calcium and magnesium with sodium.
• RO can remove the vast majority of dissolved ions, producing water close to pure water, and is suitable for households whose source water has high TDS or who care a lot about taste.

If your household water only has high TDS and everything else is normal, RO is not necessarily required: TDS is mainly minerals, and elevated levels affect taste and scaling more than safety.

Three: the three most common misunderstandings

Misunderstanding 1: If the TDS of the ultrafiltration purifier's output water doesn't drop, it's broken.

No. The pore size of an ultrafiltration membrane is about 0.01 microns; it can retain bacteria, colloids and large-molecule organic matter, but it cannot hold back ions dissolved in the water, so the output TDS is basically the same as the input. This is a technical characteristic of ultrafiltration. To judge whether ultrafiltration is working, look at turbidity, not TDS.

Misunderstanding 2: The TDS of a water softener's output water should decrease.

Also no. A water softener replaces calcium and magnesium with sodium; hardness drops substantially, but TDS does not drop and may even rise slightly. To judge a water softener, hardness alone is enough. Also remember one point: softened water is not suitable for direct drinking; the drinking line should be tapped before the softener, or an RO unit should be connected after softening.

Misunderstanding 3: The lower the TDS, the better the water.

TDS only reflects how much is dissolved in the water, mainly minerals; it cannot indicate whether the water contains bacteria, organic matter or heavy metals. Water with low TDS is not necessarily "safer"; water with high TDS is not necessarily "unclean" either.

Four: how to accept the installation once it is done

After the water purifier is installed, don't rush to drink; do an acceptance check first. The method is simple: draw input and output water at the same time and compare them.

• RO units: look at the salt rejection rate. Salt rejection rate = (1 − output TDS ÷ input TDS) × 100%. GB/T 19249-2017 "Reverse Osmosis Water Treatment Equipment" specifies a salt rejection rate of not less than 95% for spiral-wound membrane module equipment (except where users have special requirements); this value is not a universal acceptance threshold for all household RO units. Household models should be checked against the applicable standard, rated performance and specified test conditions. Looking only at the absolute output value can easily lead to misjudgment: in areas with high raw-water TDS, a slightly higher output value may be completely normal.
• Ultrafiltration: check whether turbidity drops noticeably.
• Activated carbon: check whether residual chlorine and organic-matter indicators drop.
• Water softener: check whether hardness drops substantially.

Write down the acceptance-test numbers, then retest every month or two and compare with this reading. To judge whether a filter cartridge needs replacing, look at the trend of change; it is more accurate and more economical than replacing by the calendar.

V. What to use for testing

If there is a public water-quality report for your area, read that first; if you are worried about bacteria or heavy metals, commission testing by a third-party laboratory with CMA accreditation. For routine inlet-versus-outlet comparison and trend observation, a portable testing tool is enough.

The Water Detective by AtomBit that we make is designed for exactly this kind of comparison: 9 the standard-parameter version includes an "RO water purifier" scenario, and the water-treatment-professional version can score pre-filter, activated carbon, ultrafiltration, nanofiltration, RO and water softener separately, and can also measure residual chlorine. We should also be clear about what it cannot do: it cannot measure bacteria, heavy metals or pH, and no matter how good the readings are, they cannot prove the water is safe to drink.

We have always hoped to make water quality understandable to everyone. If you understand your own water before buying a water purifier, you are less likely to buy the wrong one.

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About Us

Shenzhen AtomBit Technology Co., Ltd. focuses on micro-spectral sensing and intelligent water quality testing, with products covering water quality sensor chips, online monitoring sensors and portable testing terminals. AtomBit is a drafting unit of two association standards: the Technical Guidelines for Online Water Quality Monitoring Systems Using Spectroscopy (T/CWEC 13-2019) and the Technical Specification for Intelligent Modules for Online Water Quality Monitoring at the End of Urban Water Supply Networks (T/CAQI 93-2019).

References: GB 5749-2022 "Standards for Drinking Water Quality"; GB/T 19249 "Reverse Osmosis Water Treatment Equipment".