Chemical disinfection of water with chlorine and iodine

Summary

Adding a measured dose of chlorine (bleach or calcium hypochlorite) or iodine to clear water and waiting a contact time inactivates bacteria, viruses and most Giardia cysts, without fuel and for any volume: from a 20 L jerrycan to a tank of several cubic meters. Expected, measurable result: with clear water at 15-30 °C, 20 L are disinfected with about 1.6 mL of 5 % bleach (4 mg/L) waiting 60 min at 15 °C or 45 min at 30 °C (30 min is enough for bacteria and viruses), and the water afterward keeps a free chlorine residual of 0.2-0.5 mg/L that protects it from recontamination during storage, something neither boiling nor solar disinfection achieves. Honest limits: chlorine and iodine at field doses do not inactivate Cryptosporidium, work poorly in turbid water and do not remove chemical contaminants. Iodine is only suitable for emergency use lasting a few weeks.

Prerequisites

• Knowledge: none essential. It helps to know how to clarify the water beforehand (settling and filtering through cloth, see “Procedure”, step 1) and to know the alternative methods for when the reagent runs out or Cryptosporidium is suspected: Boiling water, solar disinfection (SODIS) and ceramic or sand filters, in this same folder.
• Dependence on technological level (read before planning): the procedure is N1 — measuring volumes with ceramic containers, counting drops, waiting — which is why the article carries nivel: 1. But the reagent is not: bleach and calcium hypochlorite are products of industrial chemistry (N3-N4), and iodine tincture or iodine crystals are too. In a reconstruction scenario the reagent will be recovered from existing stock (supermarkets, swimming pools, pharmacies, agricultural stores) and runs out; its local manufacture (e.g. hypochlorite by electrolysis of brine) is a separate chemistry topic. While stock lasts, this is the cheapest method per liter; once it runs out, one must go back to boiling, sun or filtration. That is why it is worth storing mainly granular calcium hypochlorite, which lasts years, rather than liquid bleach, which degrades within months (see “How it works”).
• Materials (depending on the option chosen):
  • Bleach (sodium hypochlorite) with no additives: no fragrance, no detergent, no thickeners (“gel”). In Spain, look on the label for “suitable for disinfecting drinking water” and the concentration in grams of active chlorine per liter. Not suitable: “bleach-free bleaches”, “active oxygen” products or “color-safe” bleaches (these are peroxides, not chlorine).
  • Granular calcium hypochlorite at 65-70 % (HTH, pool “granular chlorine”) with no stabilizer (cyanuric acid) or added algaecides. 100 g treat about 17 m³ at 4 mg/L.
  • Iodine (alternative): 2 % iodine tincture, 10 % povidone-iodine (Betadine and similar), 5 % Lugol's solution, or resublimed iodine crystals (4-8 g) in a 30-60 mL glass bottle with a lid.
  • Enameled ceramic, glass or food-grade plastic containers for the water; never metal for concentrated solutions (chlorine and iodine corrode them).
  • Tightly woven cotton cloth for pre-filtering.
  • Optional: vitamin C (ascorbic acid) powder or charcoal to remove the taste.
• Tools:
  • A calibrated dropper (N3-N4 if plastic; in N1 a thin reed or a straw cut at an angle): count how many drops fill a known measure. Reference: 20 drops ≈ 1 mL, but it varies between 16 and 24 depending on the dropper (WMS 2024, table 7).
  • A set of ceramic or gourd volume measures marked out (a “cup” and a pot or pitcher whose capacity has been counted in cups). With these, 1:100 dilutions can be made without instruments (N1).
  • Optional (recovered N4): free chlorine DPD kit with color comparator or test strips; 1 and 10 mL syringes; kitchen scale for the HTH.
• Time and people: one person. Preparing the stock solution: 15-30 min every 2-7 days (see B.2 and C.5). Dosing a jerrycan: 2 min. Contact time: from 30 min (warm, clear water) to 4 h (water at 5 °C with a low dose).

How it works

How chlorine kills, and which variables matter

Sodium hypochlorite (bleach) and calcium hypochlorite (HTH) dissolve in water and form hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). Both oxidize the enzymes, membranes and nucleic acids of microorganisms. Together they are measured as free chlorine. Disinfection depends on the C·t product (free chlorine concentration in mg/L times contact time in minutes): less chlorine can be used by waiting longer, or more chlorine by waiting less. Four factors modify it:

1. Type of microorganism. For a 99.9 % reduction (WMS 2024, table 4): E. coli, C·t ≈ 0.016 mg·min/L (seconds); enteric viruses, C·t ≈ 0.5-30; Giardia cysts, C·t ≈ 77-153 depending on temperature and pH (ameba cysts, ≈ 35); Cryptosporidium oocysts, C·t ≈ 10,400-15,300 (20 mg/L for 8-13 h: impractical in drinking water).
2. Temperature. Cold slows the reaction down. Oxfam's field rule: 30 min at 25 °C and double the time for every 10 °C less. This rule gives shorter times than table 2 (at 5 °C, 120 min versus 180 min with 4 mg/L): if they do not match, table 2 governs, being the more conservative one.
3. pH. HOCl is much more effective than OCl⁻ (tens of times), and the proportion of HOCl falls as pH rises: 92 % at pH 6.5, 52 % at pH 7.5 and 26 % at pH 8 (at 25 °C, with pKa 7.54); at pH 9 barely 3 % is left (same calculation). That is why WHO requires pH < 8 to accept the standard residuals. Practical rule in alkaline waters: if it gets close to pH 8, double the contact time; between 8 and 9, also raise the residual to ≥ 0.4-0.6 mg/L and extend the contact; above 9 chlorination is not reliable (Oxfam calls for ≥ 0.4 mg/L with pH > 8; the pH 9 limit is derived from the 3 % HOCl calculated above). In N1 no one measures pH: in waters known to be very hard (they leave a lot of white scale in pots), apply directly double dose and double time.
4. Chlorine demand and turbidity. Organic matter, ammonium, iron, manganese and particles consume chlorine before it acts, and particles shield microbes inside them. The part left over after covering that demand is the free chlorine residual: this is what disinfects and what is measured. Above 10 NTU chlorination loses effectiveness and requires double dose; above 100 NTU the water must be clarified first (Oxfam). Test without a turbidimeter: single dose only if in a transparent glass, held against the light, no turbidity at all is visible (a person starts to see turbidity around 5 NTU, WMS 2024; that is, under roughly 5-10 NTU). If it looks somewhat turbid but printed text can be read through 10 cm of water, double dose (under roughly 30 NTU, the criterion used for solar disinfection). If the text cannot be read, clarify first.

Target dose

• Clear water of doubtful origin (surface water, open well): 3-4 mg/L of chlorine (WMS 2024, table 5). CDC/EPA instructions (2 drops of 5-9 % bleach per liter) give in practice about 5-9 mg/L.
• Good-quality water (protected well, unreliable piped supply): 2 mg/L. The CDC/WHO Safe Water System uses 1.875 mg/L for water under 10 NTU and 3.75 mg/L for 10-100 NTU (Lantagne et al. 2018).
• Turbid, colored or very cold water: double dose, 4-8 mg/L (CDC: water “cloudy, murky, colored, or very cold“; WMS 2024, table 5). The EPA no longer calls for a double dose for turbid water: only settling and filtering first.
• Ceiling: WHO sets 5 mg/L as the guideline value for chlorine in lifetime consumption; most people notice the taste well before that (from about 3 mg/L, WMS 2024). Exceeding it briefly in an emergency is not dangerous (“chlorine has no known toxicity at the concentrations used for disinfection”, WMS 2024), but it makes the water unpleasant and people reject it, which is a real health risk.

Target free chlorine residual

Why bleach loses strength and HTH not so much

Sodium hypochlorite in solution decomposes continuously into chloride and chlorate. The rate increases with temperature, with concentration, with light and with metal ions: a 15 % solution decomposes about 10 times faster than a 5 % one, and at 45 °C about 5 times faster than at 25 °C (Solenis). With heat, household bleach loses strength within months: if it is old or has been in the sun, check the residual or, without a kit, double the dose when the bleach's chlorine smell is weak. The EPA recommends using bleach that has been stored at room temperature for less than a year. Solid, dry calcium hypochlorite is much more stable: 3-5 years closed, cool and dry (Oxfam). Practical consequences: check the residual (or at least the smell) when the bleach is old, and prepare diluted stock solutions frequently.

How iodine works and why it is only for emergencies

Elemental iodine (I₂) and hypoiodous acid (HOI) oxidize microorganisms like chlorine does. Advantages: it is less affected by pH, reacts less with nitrogenous organic matter and works somewhat better in poorer-quality water (WHO 2018). Disadvantages: it needs higher doses (4-8 mg/L), it is ineffective against Cryptosporidium and, above all, it is physiologically active: the thyroid gland concentrates it. Drinking 3 L a day with a residual of 2-4 mg/L means 6-12 mg of iodine per day; with the 4-8 mg/L doses from table 4, 12-24 mg/day (WMS 2024 calculates 16-32 mg/day with tablets). That is many times more than the recommended intake of 0.15 mg/day for an adult and the tolerable upper limit of 0.6-1.1 mg/day (WHO 2018; WMS 2024). That is why WHO recommends it only for short-term emergency use; WMS 2024 limits it to “short periods (months)” and calls for a thyroid check for anyone who will use it a long time, and the CDC advises against drinking iodinated water for more than a few weeks. This article adopts the prudent limit of 3 weeks to 1 month, knowing that no time limit has a firm experimental basis (Backer and Hollowell, 2000). Tincture and Lugol's solution also carry iodide, which does not disinfect but does add iodine to the body. Colorless iodine solutions (iodide only) do not work.

Procedure

A. Chlorinating a jerrycan with bleach (20-25 L, household use)

1. Clarify. If the water is turbid, let it settle for 30-60 min and pour off the clear part without disturbing the sediment, or filter it through a cotton cloth folded 4-8 times. Success criterion: in a transparent glass, held against the light, no turbidity at all is visible (under roughly 5-10 NTU, WMS 2024): single dose in step 4. If it looks somewhat turbid but printed text can be read through 10 cm of water (under roughly 30 NTU), double dose. If the text cannot be read, do not chlorinate yet: clarify more or boil.
2. Identify the bleach's concentration. Read the label. If it comes in g/L of active chlorine, divide by 10 to get it in %: 40 g/L = 4 %. If it comes in % sodium hypochlorite, use it as is (the error of this approximation is under 15 %, well below the dose margin). Success criterion: you have a figure C in % and have confirmed it carries no fragrance, detergent or “active oxygen”. If the label of a bleach suitable for drinking water gives its own dose (drops per liter), follow it and check it against table 1: if they differ a lot, re-read the concentration. If there is no label, assume 3 % and check the residual (step 7).
3. Measure the container's volume. Count how many known measures (a 1.5 L bottle, a marked saucepan) fit up to the fill mark. Success criterion: volume V known within ±10 % and marked with a line on the container so the measurement need not be repeated.
4. Calculate and add the dose. Use table 1 or the formula mL of bleach = V (L) × dose (mg/L) ÷ (C (%) × 10). Dose 4 mg/L for clear water at 10 °C or above; 8 mg/L for turbid, colored water or water below 10 °C. Pour the bleach onto the water (not into the empty container), with a calibrated dropper or syringe. Success criterion: the dose added is noted or remembered; the freshly dosed water clearly smells of chlorine when the nose is brought close to the container's mouth.
5. Mix. Cover and shake, or stir with a clean rod for 30 s; if the container has a tap or screw cap, let some water run out through it so the thread is also disinfected. Success criterion: 30 s of mixing and the tap or thread wetted with chlorinated water.
6. Wait the contact time per table 2, covered and in the shade. If the water's temperature falls between two columns, use the lower temperature column closest to it (water at 12 °C with 4 mg/L: the 5 °C column, 180 min). Absolute minimum: 30 min. Success criterion: the timed interval (clock, or a calibrated candle or shadow mark) has been met.
7. Check the residual.
   • With a DPD kit: see “Verification”. Target 0.5-2 mg/L free chlorine.
   • Without a kit (minimum criterion, EPA): the water should have a slight chlorine smell. If there is no smell at all, repeat the same dose, wait 15 more minutes and smell again. If it still does not smell after the second dose, the water has high demand (organic matter, iron): clarify it better or boil it. Success criterion: a slight chlorine smell is perceptible (or a DPD reading in range).
8. Serve and store without putting hands or ladles in, in the same covered container. Success criterion: the water still smells slightly of chlorine 24 h later; if it no longer does, drink it that same day or re-chlorinate at half dose.

Table 1. Bleach dose by concentration (1 drop = 0.05 mL; chlorine in g/L ≈ C % × 10)

Comparison with the CDC's simplified rule: 2 drops per liter of 5-9 % bleach in clear water and 4 drops in turbid, colored or very cold water; with 1 % bleach, 10 drops per liter. The EPA also gives 2 drops per liter of 6 % or 8.25 % bleach, but for turbid water it only asks to settle and filter it first. That rule is valid and easy to remember, but with 8.25 % bleach in turbid water it gives about 16 mg/L: safe short-term, but very strong tasting. For volumes of 1-2 L drops are imprecise: if possible, treat at least 5-10 L at a time.

Worked example. Bleach labeled “40 g/L active chlorine” (C = 4 %). 25 L jerrycan with clarified river water, at 8 °C. Being cold: dose 8 mg/L. mL = 25 × 8 ÷ (4 × 10) = 200 ÷ 40 = 5 mL → a level teaspoon of 5 mL, or 100 drops. Contact time (table 2, 5 °C column, 8 mg/L): 60 min. After 60 min it should smell slightly of chlorine.

Table 2. Minimum recommended field contact time, chlorine or iodine (WMS 2024, table 6; calculated for Giardia cysts, well above what bacteria and viruses need)

Practical rule: warm, clear water with 4-5 mg/L, 30 min is the CDC/EPA minimum and is enough for bacteria and viruses; if Giardia is a concern (mountain water, livestock upstream) or the water is cold, use the table. In alkaline waters apply the pH rule from “How it works” (close to pH 8, double the time; between 8 and 9, more residual and more time; above 9, do not rely on chlorine); without measuring pH, in known very hard waters, double dose and double time.

B. Stock solution without instruments (1:100 dilution, N1 level)

Useful for dosing many containers with ceramic measures instead of drops.

1. Prepare the stock solution (~500 mg/L chlorine). In a clean, enameled ceramic or glass pitcher, pour in 1 measure of bleach and the measures of clear water indicated in table 3. Stir with a wooden or glass rod. Success criterion: a strong chlorine smell on uncovering it; the liquid is transparent.
2. Label and store covered, in the shade, in a cool place, out of children's reach, with a visible mark (e.g. an engraved cross) that distinguishes it from the drinking-water pitchers. Prepare only enough solution for a few days: 2-4 in a hot climate, 7 at most in a cool, dark place. Dose with it only if it smells clearly of chlorine on uncovering; with a DPD kit, check the residual of the treated water. Success criterion: an unmistakable container, a known preparation date and a clear chlorine smell before use.
3. Dose: 1 measure of stock solution per 100 measures of clear water (≈5 mg/L), or 2 per 100 if the water is turbid or cold (≈10 mg/L). Example: 1 cup of 200 mL per 20 L. Mix, wait and check as in A.5-A.8. Success criterion: a slight chlorine smell after the contact time.

Table 3. Measures of water per measure of bleach to obtain ~500 mg/L

C. Granular calcium hypochlorite (HTH 65-70 %)

1. Prepare the 1 % stock solution (10 g/L chlorine). Wearing gloves and goggles, outdoors, pour 14 g of 70 % HTH (≈2 heaping teaspoons: the EPA gives ≈7 g, ¼ ounce, per heaping teaspoon; if you have a scale, weigh it the first time, since the granular product's density varies) into 1 L of water in a plastic or glass container. Always the product onto the water, never the water onto the product. Stir 1 min with a non-metallic rod. Success criterion: the granules have dissolved except for an insoluble white residue (normal: calcium carbonate and hydroxide).
2. Decant. Let it settle for 30-60 min and pour the clear liquid into an opaque bottle with a non-metallic cap. Success criterion: transparent liquid with no particles; the residue is disposed of diluted in plenty of water, away from water sources and crops.
3. Dose as with the 1 % bleach in table 1: 8 drops/L (4 mg/L), 8 mL per 20 L, 400 mL per 1000 L. Success criterion: the same as A.7.
4. Alternative without a scale (EPA): 1 heaping teaspoon (≈7 g, ¼ ounce) of HTH in 2 gallons (≈7.6 L) of water gives a stock solution that the EPA rates at ≈500 mg/L (with real 65-70 % HTH it comes out around 600-650 mg/L); it is dosed at 1 part per 100 parts of water (≈5-6.5 mg/L). Example: 1 cup of 200 mL of solution per 20 L. Success criterion: a slight chlorine smell after 30 min.
5. Storage: the 1 % stock solution degrades faster than the solid. Prepare solution for 2-4 days (Oxfam); in an opaque, cool bottle it can last longer, but then check the residual of the treated water or the chlorine smell of the solution with each batch. Solid HTH, well closed, lasts 3-5 years.

D. Tanks, cisterns and reservoirs

D.1. Calculate the volume (WHO/WEDC, technical note 3):

• Rectangular: V (L) = length (m) × width (m) × water height (m) × 1000.
• Vertical cylinder: V (L) = 3.14 × diameter² (m²) ÷ 4 × water height (m) × 1000.

D.2. Chlorinate the stored water (routine treatment).

1. Calculate the volume of water (not that of the empty tank). Success criterion: a figure in liters.
2. Dose 4 mg/L (8 mg/L if turbid or cold): g of chlorine = V (L) × dose (mg/L) ÷ 1000; with bleach, mL = g of chlorine × 100 ÷ C (%); with 70 % HTH, g of HTH = g of chlorine ÷ 0.7. Success criterion: the amount calculated and checked by a second person.
3. Dissolve the product first in a 10 L bucket of water taken from the tank itself (decant the HTH, in addition) and pour it spread over the surface or through the inlet opening while the tank is being filled. Success criterion: no lumps left at the bottom of the bucket.
4. Mix (stirring with a long pole, or recirculating with buckets for 5-10 min) and wait per table 2. Success criterion: free chlorine 0.5-2 mg/L in a sample taken from the outlet tap.

Worked example. Rectangular reservoir of 2.0 × 1.5 m with 1.2 m of clear water at 18 °C. V = 2.0 × 1.5 × 1.2 × 1000 = 3600 L. Chlorine at 4 mg/L: 3600 × 4 ÷ 1000 = 14.4 g. With 6 % bleach: 14.4 × 100 ÷ 6 = 240 mL. With 70 % HTH: 14.4 ÷ 0.7 = 20.6 g. Contact: 60 min (table 2, 15 °C, 4 mg/L).

D.3. Shock disinfection of an empty tank (before first use, after contamination or after cleaning; WHO/WEDC technical note 3): 0. Safety inside the tank (WHO/WEDC technical note 3, box 3.2): open lids and manholes and ventilate before entering; gloves, boots and goggles; never enter alone: always another person outside, at the opening, who can see or hear you. Do not enter cisterns or underground tanks if there is a smell of gas or rot, or if a lit candle lowered on a rope goes out: there may be gases that suffocate without warning. Success criterion: tank ventilated, candle lit at the bottom, gear on and a helper at the opening.

1. Empty, scrape and scrub the walls with a brush, water and soap; rinse until no foam remains. Use only tanks that have held water or food-grade liquids, never fuel, pesticides or wastewater. Success criterion: no slippery biofilm to the touch and no residue in the corners.
2. Fill 1/4 of the capacity with clean water and add 80 g of HTH per 1000 L of total capacity (≈55 mg/L chlorine). Do not sprinkle it in: dissolve the granules first in a bucket with 10 L of water (product onto the water), let it settle 5-10 min and pour in the liquid. Equivalent with 5 % bleach: 1.1 L per 1000 L (80 g × 0.7 = 56 g chlorine; 56 g ÷ 50 g/L = 1.12 L); with 35-40 g/L bleach, 1.4-1.6 L. Success criterion: product dissolved, no lumps at the bottom of the bucket, and spread evenly.
3. Finish filling, close and leave for 24 h. If urgent, double dose (160 g/1000 L) and 8 h. If it has a pump and hoses, recirculate the chlorinated water through them for 1 h. Success criterion: time met and a strong chlorine smell on opening.
4. Empty the shock water away from homes, rivers and ponds (it kills fish and plants), fill with potable water, leave for 30 min and empty again. Success criterion: the rinse water smells at most slightly of chlorine; the tank is ready.

E. Iodine (alternative for short emergencies)

1. Clarify as in A.1. Success criterion: the same.
2. Dose according to the product (table 4). Success criterion: the water takes on a very faint straw-yellow tint (with 4-8 mg/L).
3. Wait per table 2 (the same one used for chlorine): 30 min minimum in warm, clear water (EPA); at 5 °C, 60 min with 8 mg/L or 180 min with 4 mg/L. Success criterion: time met.
4. Remove the taste (optional), only after the contact time: a small pinch of vitamin C powder per liter (WMS 2024). Stoichiometrically very little is needed: about 20 mg for 8 mg/L of chlorine and about 6 mg for 8 mg/L of iodine; a little extra does no harm. It converts iodine into iodide and chlorine into chloride, with no color or taste. It removes taste and color, not the iodine: the body still absorbs the iodide, so the contraindications and the week-based limit still apply. Drink the water that same day, since no residual is left. Success criterion: colorless water with no medicinal taste.

Table 4. Iodine dose per liter (WMS 2024, table 7; WHO 2018, table 6)

The WMS counts 20 drops per mL (it varies from 16 to 24). The EPA gives the simplified rule: 5 drops of 2 % tincture per liter, 10 if the water is turbid or colored, and wait at least 30 min. The Lugol's figure is a calculation made for this article (no published recommendation) from its free-iodine content (50 mg/mL at 5 %): use it only if there is nothing else. Watch the strength: pharmacies often sell 1-2 % Lugol's, and with those drops the dose would fall 2.5-5 times short. Check the label to confirm it is 5 % iodine; if it is 2 %, use the 2 % tincture doses; if the strength is not stated, do not use it. 5 % Lugol's also carries 10 % potassium iodide, which adds about 76 mg/mL of iodine as iodide, 1.5 times the free iodine: it does not disinfect, but it does add iodine to the body.

Crystal method (Polar Pure type).

1. Put 4-8 g of resublimed iodine crystals in a 30-60 mL glass bottle with an airtight lid; fill it with water, cap it and shake for 30-60 s. Wait 1 h. Success criterion: a brownish-orange liquid over the crystals, which do not fully dissolve.
2. Pour only the liquid into the water to be treated (13 mL per liter of clear water, 26 mL if it is cold or turbid), without letting any crystal fall in. Iodine's solubility drops with cold: refill and shake the bottle at pocket temperature (about 20-25 °C); if the bottle is cold (< 10 °C), use double the volume. Refill the bottle with water for next time. Success criterion: no crystal in the drinking water; bottle refilled. The crystals last hundreds of refills before dissolving completely (WMS 2024, table 7).
3. Wait per table 2. Success criterion: time met.

Verification

• Smell (minimum criterion, no equipment): after the contact time, chlorinated water should have a slight chlorine smell (EPA). No smell = insufficient or no residual: re-dose (step A.7). A strong smell that stings the nose = overdose: let it stand for a few hours in the shade, covered only with a clean cloth, aerate it by pouring it between containers, or mix it with already treated water that has no smell. The sense of smell is imprecise and quickly becomes accustomed: have someone who has not handled the bleach do the smelling.
• Free chlorine DPD kit (if available): fill the comparator cell with 10 mL of treated water (or the volume the kit specifies), add a DPD No. 1 tablet, crush it and mix. Read the pink color right away (in under 1 min) against the scale. Good range: 0.5-2 mg/L after 30 min; ≥ 0.2 mg/L at the time of drinking. Water with a lot of chlorine (> 10 mg/L) can bleach out the reagent and give a falsely low reading: if the color appears and then disappears, dilute the sample by half with chlorine-free water (e.g. boiled and cooled water) and multiply the reading by 2, per the kit's instructions.
• 4-bucket demand test (village level, with DPD kit; Oxfam): 4 buckets of 10 L with water from the source; doses of 1, 2, 3 and 4 mg/L (1, 2, 3 and 4 mL of 1 % solution, or 0.2, 0.4, 0.6 and 0.8 mL of 5 % bleach); after 30 min measure the free chlorine in each. The correct dose is the lowest one that leaves ≥ 0.5 mg/L at 30 min. Repeat if the source changes (rains, dry season / low flow).
• Iodine with starch (N1, indicative): boil a chopped potato or a handful of rice in a little water and keep the cooled broth. A few drops of that broth in some iodinated water turn blue-violet if free iodine is present. This is an indicative test: a negative result (no color) does not prove that no iodine remains.
• What none of these tests verify: the absence of Cryptosporidium, chemical contaminants or metals. The residual proves that there was enough disinfectant, not that the water is entirely harmless.

Common mistakes

Safety

• Never mix bleach or HTH with acids or with ammonia.
  • With acids (vinegar, muriatic acid or hydrochloric acid, descalers, bathroom and limescale cleaners, concentrated lemon juice) chlorine gas is released: a yellowish-green gas with a suffocating smell.
  • With ammonia or ammonia-based products (glass cleaner, some cleaners, accumulated urine) chloramines are released.
  • Both gases burn eyes, throat and lungs; at high concentration, in an enclosed space, they can kill or cause pulmonary edema that sometimes appears hours after an exposure that seemed mild.
  • What to do: leave immediately for the open air and ventilate from outside; remove soaked clothing; wash eyes and skin with plenty of water for 15 min; rest in a half-upright position; seek medical attention if there is persistent cough, difficulty breathing, wheezing or chest pain, and keep under medical observation for 24 h even if symptoms improve.
• Calcium hypochlorite (HTH): oxidizer and reactive (ICSC 0638).
  • It does not burn, but it feeds fire in anything nearby. In contact with oil, grease, diesel, brake fluid, glycerin, sugar, sawdust, paper or rags it can start a fire or an explosion. It reacts violently with acids, ammonia and amines, and with many metals. It decomposes from about 100 °C onward and rapidly above 175 °C, releasing chlorine and oxygen.
  • Storage: in its original container, well closed and dry, in a cool, ventilated place, in the shade, kept away from fuels, acids, fertilizers, paint and food, out of children's reach and never inside the home. Do not return leftover product to the container, and do not use wet or dirty scoops (a few drops of water or organic matter inside the container can cause heating and decomposition).
  • Never mix HTH with stabilized chlorines: trichloroisocyanuric acid (pool “tablets” and “multi-action” products) or sodium dichloroisocyanurate (NaDCC, “fast” granular chlorine and water disinfection tablets). Together they react violently: heat, fire or explosion and toxic gases. Not in the same bucket, nor the same dosing device, nor with the same scoop, nor on the same shelf. When recovering pool stock, read every label and keep the containers separate; an unlabeled container is not used or mixed with anything.
  • Handling: gloves, goggles, outdoors, product onto the water, never water onto the product.
  • Spill: collect it dry with a clean shovel into a separate, clean, dry container, and dilute the remains with plenty of water; do not sweep with sawdust or rags.
  • Fire involving HTH (ICSC 0638): water in large amounts, from a distance; do not use dry chemical powder extinguishers; cool exposed containers with water. Do not try to smother it with blankets or dirt: HTH supplies its own oxygen and keeps burning.
• Accidental ingestion of bleach or of concentrated solution. Diluted household bleach (≤ 5-6 %) usually causes irritation of the mouth, throat and stomach, vomiting and pain; concentrated bleach, the HTH stock solution or the granules can cause corrosive burns of the esophagus and stomach, shock and death.
  • What to do: rinse the mouth; do not induce vomiting; do not give vinegar, baking soda or “neutralizers”. Only if the person is conscious, swallows without difficulty and is not vomiting or drooling: a glass of water in sips (≈200 mL in an adult; in children, a few sips, ≈50-100 mL) (ICSC 0638: “rinse the mouth and give one or two glasses of water to drink”; Spain's Servicio de Información Toxicológica (Toxicological Information Service) advises against giving liquids without medical assessment, hence the limited amount). If there is drooling, difficulty swallowing or vomiting, nothing by mouth. Seek urgent medical attention, more so if there is drooling, difficulty swallowing, chest pain, vomiting blood or difficulty breathing.
• Eyes and skin: bleach and the HTH solution cause chemical burns, and in the eyes they can leave permanent damage. Wash with plenty of water for 15 min, removing contact lenses; seek medical attention if pain, redness or blurred vision persist.
• Mixed-up containers: the most common cause of serious poisoning is drinking from a water or soda bottle refilled with bleach or stock solution. Never store chlorine solutions in beverage containers; mark stock-solution pitchers unmistakably and keep them out of children's reach.
• Iodine: contraindications. Do not use for drinking water in: pregnancy (risk of goiter and hypothyroidism in the newborn); current or past thyroid disease, even if controlled; strong family history of thyroid disease; allergy or hypersensitivity to iodine; people from areas with chronic iodine deficiency (WMS 2024; CDC). As an additional precaution from this article (not found in those guidelines), also not during breastfeeding nor for infants: iodine passes into breast milk. No one should drink iodinated water for more than a few weeks in a row (CDC); this article's prudent limit, 3 weeks to 1 month; WMS 2024 speaks of “months” and calls for a thyroid check if use will be prolonged. Signs of excess to watch for: lumps in the neck, abnormal tiredness and feeling cold, palpitations or nervousness: stop the iodine and switch to chlorine, boiling or filtration.
• Iodine crystals: toxic and corrosive if swallowed; a single ingested crystal is a poisoning, not a dose. Well-closed glass bottle, out of children's reach, and care when decanting.
• Disinfection by-products (risk context). Chlorine reacts with the water's natural organic matter and forms trihalomethanes (such as chloroform) and other compounds; old or heated hypochlorite solutions build up chlorate. WHO sets guideline values (chloroform 0.3 mg/L; chlorate 0.7 mg/L, provisional) calculated for lifetime consumption, and their risk is small compared with drinking undisinfected water, which can kill within days from cholera or dysentery. WHO insists that disinfection must not be compromised for the sake of controlling by-products. To reduce them without losing effectiveness: clarify before chlorinating (removes organic matter), do not overdose, and use fresh bleach and stock solutions kept cool.
• What this method does not solve: Cryptosporidium (outbreaks of persistent watery diarrhea despite chlorinating: boil or filter), water suspected of fuel, pesticides, metals or industrial residue contamination (do not drink: find another source).
• Out of scope: the production of chlorine gas and its use in water treatment plants is not documented here due to its hazard. The manufacture of hypochlorite by electrolysis of brine belongs to a chemistry article.

Variants

• With fewer resources:
  • No dropper or syringes: method B (1:100 dilutions with ceramic measures). Precision is lost (±30 %), which is enough given the margin between 2 and 8 mg/L.
  • No reagent: there is no chemical substitute equivalent that is simple to make (WMS 2024). 3 % hydrogen peroxide, citrus juice or silver do not work as a primary disinfectant. One must switch to boiling, solar disinfection or filtration.
  • Chlorinated lime or old-style “chloride of lime” (bleaching powder with ≈25-35 % available chlorine when fresh): used like HTH, recalculating with its strength and decanting. It degrades quickly: check it by the residual or smell and adjust the dose.
• With more resources:
  • Sodium dichloroisocyanurate tablets (NaDCC, e.g. Aquatabs): exact dose, stable for years, lightweight; follow the dose on the package. Store them away from HTH and never mix them with it (see “Safety”).
  • Chlorine dioxide in tablets or two-component form: it does act on Cryptosporidium, but needs 2-4 h and leaves no lasting residual (WMS 2024).
  • DPD kit and pH meter: allows lowering the dose to what is strictly needed (better taste, fewer by-products) and checking each batch.
  • Prior filtration (ceramic, slow sand) followed by chlorination: a reference combination, because the filter removes turbidity and resistant parasites and the chlorine provides a residual.
  • Local manufacture of hypochlorite by electrolysis of brine (salt, water and direct current): gives independence from recovered stock; to be covered in a chemistry article.
• Scaling up:
  • Family: 20-25 L jerrycans with a tap, bleach or stock solution, smell as the check.
  • Village: batch chlorination in a communal tank (1-10 m³) or chlorine dispensers next to the source (a tank of 1 % solution with a valve that releases a fixed dose per bucket). This requires a responsible person, a 4-bucket demand test whenever the season changes, a DPD kit, a log of doses and residuals, and a reserve of HTH. Clarify first if the sources are surface water.
  • City: continuous dosing with pumps or hypochlorinators proportional to flow, a contact tank sized by C·t, turbidity control (ideally < 1 NTU) and pH, a residual of 0.2-0.5 mg/L at the taps, periodic microbiological testing and trained operators. Chlorine gas is used by large industrial plants under specific regulation, outside the scope of this article.

Sources

• CDC. «How to Make Water Safe in an Emergency». Water, Sanitation, and Hygiene (WASH)-related Emergencies and Outbreaks. https://www.cdc.gov/water-emergency/about/index.html
• US EPA. «Emergency Disinfection of Drinking Water». https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water
• Backer, H.D., Derlet, R.W. y Hill, V.R. (2024). «Wilderness Medical Society Clinical Practice Guidelines for Water Disinfection for Wilderness, International Travel, and Austere Situations: 2024 Update». Wilderness & Environmental Medicine, 35(1 Suppl), 45S-66S. doi:10.1177/10806032231218722. Manuscrito en https://stacks.cdc.gov/view/cdc/153527 (tabla 4, C·t; tabla 5, factores; tabla 6, tiempos de contacto; tabla 7, dosis de yodo y cloro; apartados sobre clarificación, toxicidad del yodo y mejora del sabor). Sustituye a la edición de 2019 (30(4S), S100-S120).
• Backer, H. y Hollowell, J. (2000). «Use of iodine for water disinfection: iodine toxicity and maximum recommended dose». Environmental Health Perspectives, 108(8), 679-684.
• OMS (2018). Alternative drinking-water disinfectants: bromine, iodine and silver (sección «Iodine as a drinking-water disinfectant»), apartados 2.2 y 3.2 y tabla 6.
• OMS. Guidelines for Drinking-water Quality, 4.ª ed. con addenda: cloro (valor guía 5 mg/L; residual ≥ 0,5 mg/L tras 30 min a pH < 8), trihalometanos y clorato.
• OMS/WEDC (2013). Technical Notes on Drinking-Water, Sanitation and Hygiene in Emergencies, n.º 3: «Cleaning and disinfecting water storage tanks and tankers».
• Lantagne, D. et al. (2018). «Sodium hypochlorite dosage for household and emergency water treatment: updated recommendations». Journal of Water and Health, 16(1), 112-125.
• Oxfam WASH. «Chlorination in Emergencies». https://www.oxfamwash.org/chlorination-in-emergencies/
• OIT/OMS. Ficha Internacional de Seguridad Química ICSC 0638: Hipoclorito de calcio (incendio, derrames, primeros auxilios).
• Instituto Nacional de Toxicología y Ciencias Forenses (España). Servicio de Información Toxicológica: recomendaciones ante la ingestión de productos cáusticos.
• Solenis. «The Stability of Chlorine Bleach (Sodium Hypochlorite) Solutions» (descomposición según temperatura y concentración).