Thailand's drinkable tap-water criteria, 2026 edition: what changed and what you can test on site

Thailand's Department of Health has issued new drinkable tap-water criteria, formally the Department of Health Notification on Drinkable Tap Water Quality Criteria B.E. 2569 (2026). The Director-General signed it on 14 September 2026 and the Bureau of Food and Water Sanitation published it on 25 September 2026. It repeals the 13 July 2020 criteria in full. Most limits are unchanged, but two are tighter, total dissolved solids now has a single accepted method, and the notification says for the first time who may produce the results.

These criteria are what the Department uses to certify a water supply as "drinkable tap water", which the notification defines as tap water whose quality is controlled from the production system all the way to the user's home, not only at the treatment plant. They are issued under Section 32 of the State Administration Act, not under food law. Bottled drinking water falls under a different instrument, Ministry of Public Health Notification No. 462, which we cover in our article on bottled drinking-water testing. The two sets of numbers are close but not identical, so do not borrow limits across them.

The two tighter limits are not a paper change. In fiscal year 2025 the Department tested 4,873 village water supplies across 76 provinces. Only 9.5% met all 21 limits of the 2020 criteria; 779 exceeded 5 NTU for turbidity and 317 exceeded 0.3 mg/L for manganese. Those are exactly the two limits that have just come down. As for certification, the Department said on 20 November 2024 that of more than 69,028 village supplies run by local governments, 21 had been certified as drinkable tap water.

The 21 limits in the annex

The annex groups the limits into five sets. Every figure is a maximum, except pH, which is a range, and bacteria, which must not be detected. The figures below are read directly from the signed annex.

Physical

• Turbidity: not more than 4 NTU, nephelometric method
• Apparent colour: not more than 15 platinum-cobalt units
• pH: 6.5–8.5, electrometric method, which means a pH meter

General chemistry

• Total dissolved solids: not more than 500 mg/L; the only listed method is drying at 180 °C and weighing
• Hardness as CaCO₃: not more than 300 mg/L, by EDTA titration
• Sulfate: not more than 250 mg/L; chloride: not more than 250 mg/L
• Nitrate as NO₃: not more than 50 mg/L; nitrite as NO₂: not more than 3 mg/L
• Fluoride: not more than 0.7 mg/L

Metals

• Iron: not more than 0.3 mg/L; manganese: not more than 0.08 mg/L
• Copper: not more than 1 mg/L; zinc: not more than 3 mg/L

Toxic metals

• Lead 0.01 mg/L, total chromium 0.05 mg/L and cadmium 0.003 mg/L, all maxima
• Arsenic 0.01 mg/L and mercury 0.001 mg/L, both maxima

Microbiological

• Total coliforms and E. coli: not detected in 100 mL by a presence-absence test, or below 1.1 MPN per 100 mL by the MPN method

A note under the table sets free residual chlorine at the end of the distribution line at 0.2–0.5 mg/L. It is used for tap-water quality surveillance and is not one of the 21 certification limits, but it is the one number that tells an operator every day whether disinfection is still reaching the consumer's tap.

What changed from the 2020 edition

We compared the two tables line by line. Two limits changed and the other 19 did not, but the changes to methods and laboratories will affect day-to-day work more than the numbers.

• Turbidity: from 5 NTU to 4 NTU
• Manganese: from 0.3 mg/L to 0.08 mg/L, almost four times stricter
• Total dissolved solids: the 2020 edition accepted gravimetric and electrometric (conductivity-based) methods; the 2026 edition lists only drying at 180 °C and weighing
• Nitrate and nitrite: cadmium reduction is dropped; flow injection analysis is added for both, and enzymatic reduction for nitrate
• Clause 5: the old text cited the 23rd edition (2017) of Standard Methods for the Examination of Water and Wastewater; the new text requires the latest edition or an equivalent international method, performed by a government laboratory or one accredited to ISO/IEC 17025

The new turbidity and manganese limits now match what Notification No. 462 already applies to bottled water: 4 NTU and 0.08 mg/L. Hardness is still far apart, with 300 mg/L allowed for tap water against 100 mg/L for bottled water.

The two big utilities already work to these figures. The Provincial Waterworks Authority's water-quality standard of October 2024 sets turbidity at 4 NTU and manganese at 0.08 mg/L. The Metropolitan Waterworks Authority's 2022 criteria set manganese at 0.08 mg/L and turbidity at 1.0 NTU, stricter than the Department's. The new criteria change little in MWA or PWA service areas; they change a great deal for village and local-government supplies.

If we had to pick one thing to act on first, it would be manganese. A supply that passed with manganese between 0.1 and 0.3 mg/L passed last round and will fail the next without its water changing at all. In the 2025 report, supplies over the old manganese limit were spread across 68 provinces, with the same mix of groundwater and surface water as the supplies tested overall, so no group can skip this one. Turbidity is different: 60.7% of the supplies over 5 NTU drew on surface water, against 38.9% of all supplies tested. Surface-water systems should look at turbidity first.

One more source of confusion. The Department also has recommended drinking-water criteria for surveillance, issued in 2020, which cover every drinking-water source: tap water, shallow wells, groundwater and rainwater. The new notification does not repeal them, and on the day we checked no replacement had been published, so they still use 5 NTU and 0.3 mg/L. If a document quotes the old figures, check which of the two it is citing.

How certification works

The village-supply certification requirements published by the Bureau of Food and Water Sanitation call for samples at two points: the tap from the elevated tank in the production system, and the far end of the main distribution line, with one end-of-line sample per roughly 5,000 people. Samples are taken in two rounds, and both must pass. A certificate lasts two years. After certification, the full panel is tested once a year, and the end of the line is checked in the field at least weekly, either by measuring free residual chlorine against 0.2–0.5 mg/L or with the Department's อ.11 coliform kit, with immediate correction if the result is positive.

That requirements document was posted in 2021 and still refers to the 2010 criteria. The limits that decide certification now are the 2026 ones, but the sampling rounds may be revised too, so ask your regional health centre before planning. Separate requirements for municipal, hospital, school and hotel supplies sit on the same page; the municipal version adds that a bacteriological sample must be taken whenever chlorine at the end of the line falls below 0.2 mg/L.

What you can measure on site, and what goes to a lab

Clause 5 closes the question of certification results: they must come from a government laboratory or an ISO/IEC 17025-accredited one. But a water supply is not run on an annual certificate. It is run on numbers its operators measure themselves every week or every day, and only a few of the 21 can honestly be measured that way.

Free residual chlorine: measure it at the end of the line

If you buy one instrument, make it a free chlorine meter. The 2025 surveillance report cites a Department of Water Resources assessment that 83.1% of village supplies had never measured free residual chlorine at the end of the line, and 75.4% had no chlorine tester at all. In the Department's own testing in the same report, coliforms exceeded the limit in 76.6% of the supplies tested. Those two numbers describe the same problem.

Sample from the tap furthest out in the network, let the water run first, and read immediately, because chlorine starts to decay in the sample bottle. The Department of Health's own อ.31 kit compares the sample against sealed colour standards at three levels, 0.2, 0.5 and 1.0 mg/L. The Department's laboratory centre reports at least 97% agreement with its standard laboratory method, and one bottle of reagent covers 60 samples.

อ.31 tells you whether the water is in range, but not by how much. To see chlorine at the end of the line sliding from 0.4 towards 0.25 before it drops out, you need a meter. The Hanna HI701 free chlorine checker reads 0.00–2.50 mg/L with 0.01 mg/L resolution, using an adaptation of the DPD method in USEPA 330.5, which covers 0.2–0.5 comfortably.

Turbidity: a portable meter uses the same principle as the criteria

The nephelometric method measures light scattered sideways out of the sample cell, which is how portable turbidity meters already work. The Hanna HI93703 follows ISO 7027, reads 0.00–50.00 FTU at 0.01 FTU resolution and calibrates at three points: 0, 10 and 500 FTU. Its stated accuracy is ±0.5 FTU or ±5% of reading, whichever is greater.

That accuracy matters more now that the limit is 4 NTU: a reading of 3.6 could be 4.1. Send results close to the limit to a lab. Most field errors come from fingerprints or air bubbles on the sample cell, not the meter. ISO 7027 meters report FTU while the criteria are written in NTU; both are defined against formazin standards, but the instruments use different wavelengths of light.

pH: the electrometric method is a pH meter

Calibrate the pH meter against at least two buffers on the day of measurement and measure at the sample's own temperature. The criterion is a range, 6.5–8.5, so both ends matter. In the 2025 report, 565 supplies, 11.6% of those tested, fell outside it.

Total dissolved solids: measure it yourself, but you cannot submit it

A TDS pen converts conductivity using a factor. It is a good trend tool: if a reading that sat around 200 mg/L jumps to 450, the source or the system has changed. But since the 2026 edition, the only method accepted for certification is drying at 180 °C and weighing. That needs a drying oven that holds 180 °C steadily and an analytical balance, which makes it laboratory work rather than field work.

Coliforms: อ.11 screens, it does not certify

The อ.11 medium is a field kit the Department of Health developed for communities to use themselves. Wipe the tap mouth with 70% alcohol, run the water for about a minute, fill the bottle to its fourth mark, and leave it at room temperature, 25–40 °C, for 24–48 hours. If the medium turns from red to brown, orange or yellow, becomes turbid and releases gas when gently shaken, coliforms are present; the kit does not tell you whether they are E. coli. The Department's laboratory centre reports at least 84.5% agreement with the multiple-tube fermentation method.

That 84.5% is why อ.11 is a screening tool. A positive is the signal to fix the system and sample for a laboratory; a negative is not a certificate. A lab running the presence-absence or MPN tests in the criteria incubates samples in a temperature-controlled incubator, not at room temperature.

Everything else: send it to a lab

The five toxic metals have limits between 0.001 and 0.05 mg/L, and the annex lists graphite-furnace AAS, ICP or equivalent techniques. No field kit substitutes for them. Iron, manganese, nitrate, nitrite and fluoride can be screened with portable photometers and test kits that use the same colour-forming chemistry as the spectrophotometric methods in the annex, but check the instrument's resolution first. Manganese at 0.08 mg/L needs a low-range method; a meter that only resolves 0.1 mg/L cannot tell you whether the water passes the new limit.

This is how the Department itself works. Its 2023 report on verifying field water-testing techniques used a portable photometer for free chlorine, colour, iron, manganese, hardness, nitrate, nitrite, sulfate, fluoride, copper and zinc, alongside a portable turbidity meter, and required field readings to be compared with laboratory results. The Department's own field kits state that they were not developed to replace laboratory analysis.

The Department publishes a list of ISO/IEC 17025-accredited water testing laboratories with their addresses (opens in a new tab), as an Excel file last dated 25 April 2025. Use it to find a laboratory in your region before you sample.

Before you sample for a laboratory

Clause 5 applies Standard Methods to sample collection and preservation as well as analysis. Bacteriological bottles must be sterile and contain a chlorine neutraliser, and metals go in a separate bottle from general chemistry. Ask the laboratory before sampling which bottles to use, what volume, and how many hours you have to deliver them. A sample in the wrong bottle is a testing fee spent for nothing.

What we checked

We read the annex of the 2026 notification from the signed file and compared it line by line with the 2020 edition. Every Department of Health document cited here was downloaded from the Bureau of Food and Water Sanitation and Department of Health laboratory centre websites on 28 September 2026. The village-supply figures come from the fiscal 2025 surveillance report, which assessed supplies against the 2020 criteria. The MWA and PWA figures were read from both utilities' own criteria documents. Instrument specifications come from our product pages, which cite the manufacturers' documents.

What we have not verified is when each regional health centre will start applying the new criteria to certification rounds, or when certificates issued under the 2020 criteria will be reviewed. If your system is about to apply for certification, ask your regional health centre before you sample.