Drinking water quality testing (IS 10500 and WHO)

Drinking water can carry three kinds of hazard. Microbial hazards — bacteria, viruses and parasites from sewage or animal waste — cause the most illness and act within hours or days. Chemical hazards such as fluoride, arsenic, nitrate and lead usually harm health only after months or years of exposure. Acceptability problems — taste, colour, cloudiness, hardness, salinity — are not directly harmful but make people turn to other, possibly unsafe, sources.

In India the specification is IS 10500:2012 (Drinking water — Specification), published by the Bureau of Indian Standards. Internationally, the WHO Guidelines for Drinking-water Quality give health-based guideline values that countries use to set their own standards. Test methods come mainly from APHA Standard Methods for the Examination of Water and Wastewater and the IS 3025 (chemical) and IS 1622 (microbiological) series.

Microbiological safety comes first. A sample that fails for E. coli is unsafe to drink no matter how good its chemistry looks.

IS 10500 gives two numbers for many parameters: an acceptable limit, which water should meet, and a permissible limit, which may be tolerated only in the absence of an alternative source. Some parameters, mostly health-related ones, have no relaxation. The values below are those commonly quoted from IS 10500:2012 — always check the current edition and its amendments before using them.

• Health-related parameters (microbiology, nitrate, fluoride, toxic metals) decide whether water is safe.
• Aesthetic parameters (TDS, hardness, chloride, turbidity) decide whether people will accept it.
• Operational parameters (pH, residual chlorine, turbidity) tell the operator whether treatment is working.

A perfect analysis cannot rescue a bad sample. The container, how it is filled, what is added and how long it waits all change the result. Plan sampling with the lab beforehand so the right bottles and preservatives go to the field. ISO 5667-3 and the 'sample collection and preservation' section of Standard Methods give detailed tables; the table below summarises common practice — your SOP and method take priority.

1. 1Label every bottle with a unique sample ID, location, date and time before filling.
2. 2At a tap, remove any aerator or filter, run the water until the temperature is steady, then reduce the flow and fill. For microbiology, disinfect or flame the spout first.
3. 3Do not rinse bottles that contain a preservative (thiosulphate or acid). Leave an air space in micro bottles so the sample can be shaken; fill chemistry bottles to the brim for pH, alkalinity and dissolved gases.
4. 4Keep samples cool and dark in an insulated box, and complete a chain-of-custody form recording who collected, transported and received them.
5. 5On receipt, check temperature, bottle type, preservative and elapsed time. Reject or flag samples that are outside the requirements — never test them silently.

Chlorine neutralisation: sodium thiosulphate in micro bottles stops residual chlorine from killing bacteria during transport, which would give a falsely clean result. The same thiosulphate makes the sample useless for chlorine testing — so residual chlorine is always measured on site, from a separate portion.

Some parameters change so quickly that only a field result is meaningful. Free chlorine decays within minutes to hours, pH drifts as CO₂ escapes or enters, and temperature changes as soon as the bottle leaves the tap. These are measured at the sampling point with a portable photometer, pH meter or multiparameter probe.

Field test kits (strip or tablet kits for fluoride, nitrate, iron, hardness and H₂S-strip bacteriological screening) are valuable for village surveys and rapid screening, but they are semi-quantitative. A positive or borderline field result should be confirmed by a laboratory method before any official decision.

Compare each result with the limit for the same form and unit. Nitrate as NO₃⁻ and nitrate as N differ by a factor of 4.43; hardness and alkalinity are expressed as CaCO₃; metals are often measured in µg/L but limits are written in mg/L.

Think about measurement uncertainty when a result is close to a limit. A fluoride result of 1.48 mg/L with an expanded uncertainty of ±0.08 mg/L could truly be above 1.5 mg/L. Your lab's decision rule — agreed with the customer and, for accredited labs, required by ISO/IEC 17025 — says how such borderline results are stated.

• Below the acceptable limit: complies.
• Between acceptable and permissible: complies only where no alternative source exists; report both limits so the user can judge.
• Above the permissible limit (or above a no-relaxation limit): does not comply.
• Any E. coli or total coliforms detected in 100 mL: does not comply — notify the customer promptly and recommend resampling and investigation.
• Check internal consistency: TDS should be roughly 0.55–0.75 × conductivity; total hardness cannot exceed the sum of calcium and magnesium as CaCO₃; cation and anion totals (in meq/L) should balance within a few percent.

A result that breaks the ion balance or the TDS/conductivity ratio is a reason to recheck the data before reporting, not after the customer asks.

• Calibrate instruments daily or before use (pH buffers, turbidity standards, conductivity standard, ISE standards, ICP-MS calibration).
• Run a method blank, a check standard from an independent source and one duplicate in about every ten samples.
• Spike one sample per batch for chemistry methods and record recovery.
• For microbiology, run positive and negative culture controls, media sterility checks and incubator / water-bath temperature records.
• Take part in proficiency-testing schemes and trend the results.

A good report lets a non-specialist see at once whether the water is fit to drink, and lets a specialist trace exactly how each number was produced.

• Sample identity: source, location, sampling date and time, who sampled, and the condition on receipt.
• For each parameter: result, unit, form (e.g. 'as CaCO₃', 'as NO₃⁻'), test method reference, and the IS 10500 acceptable and permissible limits.
• Results below the reporting limit written as '< LOQ value' (e.g. '< 0.001 mg/L'), and microbiology as 'not detected in 100 mL' or '< 1.1 MPN/100 mL' as appropriate.
• A clear compliance statement per parameter and overall, with the decision rule used.
• Remarks on any deviation: late receipt, wrong preservative, field results reported from the sampler's sheet, or tests subcontracted.
• Authorised signature and date; for accredited labs, the accreditation mark only on accredited parameters.

Never round a failing result into compliance. Report to the precision of the method and compare the reported value with the limit.