Coagulation, flocculation and sedimentation with alum

Summary

Adding a small dose of alum to turbid water, stirring it fast and then slowly, and letting it settle makes the mud and clay that would not settle for days form clumps ("flocs") that fall to the bottom in 30 min to 2 h. The water on top is decanted clear. Reference measurable result: in a field test, water at 109 NTU ended up at ≈ 3 NTU with 30 mg/L of aluminum sulfate (Dorea 2009, fig. 3), which is equivalent to ≈ 1 g of potassium alum per 20 L bucket (see “Calculations”). The dose for your water is given by the jar test. Clarification does not make water potable: it removes part of the microbes (in the field, on the order of 90-95 % of bacteria; CAWST 2009), but disinfection is always needed afterward (chlorine, boiling, SODIS or a filter). Its great usefulness is that chlorine, sun and filters work poorly or clog with turbid water.

Prerequisites

• Knowledge: none essential. For the final step you need to know how to disinfect: Chemical disinfection of water with chlorine and iodine, Boiling water or Solar disinfection (SODIS).
• Dependence on technological level (read before planning): the procedure is N1 (containers, a stick for stirring, ceramic volume measures, a pan balance), which is why it carries nivel: 1. The reagent is not always so: commercial aluminum sulfate is industrial chemistry (N3-N4) and in a reconstruction scenario it will mostly be recovered from existing stock. Potassium alum has indeed been made artisanally since Antiquity (N2; see “Variants”). Moringa seeds are N0-N1 where the tree grows.
• Materials (for a 20 L bucket):
  • Coagulant, in order of preference:
    • Potassium alum (aluminum potassium double sulfate, KAl(SO4)2·12H2O): colorless crystals or white powder. Sold as “alum stone” (deodorant, aftershave), in pharmacies, natural dye shops and pickling supply stores (additive E522). Needs 0.2-2.5 g per bucket depending on turbidity.
    • Aluminum sulfate (Al2(SO4)3·14-18H2O, what water treatment calls plain “alum”): granules, blocks or powder from chemical suppliers, treatment plants and pool stores. Yields 60 % more per gram than potassium alum: the dose in grams is about 0.6 times that of the potassium one.
    • Ammonium alum (NH4Al(SO4)2·12H2O): many cheap “alum stones” are of this type. Only if there is no potassium alum or aluminum sulfate. It coagulates the same (same dose in grams), but it adds ammonium: ≈ 0.04 mg of NH4⁺ per mg of alum, that is, at 48 mg/L it adds ≈ 1.9 mg/L of ammonium (the EU allows 0.50 mg/L). Chlorine reacts with the ammonium and forms chloramines, much worse disinfectants; to get free chlorine you would have to pass the “breakpoint,” which at that dose requires ≈ 11 mg/L of chlorine (see “Calculations”). With ammonium alum, disinfect by boiling or SODIS; if you can only chlorinate, check with a free chlorine DPD kit (not total) that there is ≥ 0.5 mg/L after 30 min. How to tell them apart: a pinch of alum heated with a little slaked lime or soda and a few drops of water smells of ammonia if it is ammonium alum; potassium alum does not smell.
    • Dried Moringa oleifera seeds (natural alternative): 5-40 kernels per bucket depending on turbidity.
  • Clean water to prepare the stock solution: 1 L.
  • Optional, for soft water (rain, granite streams): slaked lime (calcium hydroxide), sodium carbonate (Solvay soda) or sodium bicarbonate; amounts in “Calculations.”
• Tools:
  • 2 buckets or pitchers of 20-25 L, ideally one with a tap 4-5 cm from the bottom (N1 with a pitcher and a wooden stopper; N3-N4 food-grade plastic bucket with a tap). In a 20 L bucket (≈ 29 cm in diameter) each centimeter of height is ≈ 0.66 L: a tap at 8-10 cm would leave 5-7 L unused.
  • A clean stick or paddle 50-60 cm long for stirring.
  • For the jar test: 5 equal transparent jars or bottles of 1 L (glass N2; recovered, cut PET bottles).
  • Volume measures: a 5 mL teaspoon and a 250 mL cup (calibrated with water; see step 1).
  • A homemade pan balance (N1) or a kitchen scale (N4 recovered) to weigh 10 g.
  • Optional: a rubber tube or a 1-1.5 m reed for siphoning; a tightly woven cotton cloth.
• Time and people: one person. Jar test: 45-60 min, once per water source and every time its appearance changes (floods, low water). Treating a bucket: 15 min of work plus 1-2 h of settling.

How it works

Why turbid water does not clear on its own. Turbidity is caused by colloidal particles of clay, fine silt, organic matter and microbes, from 1 nm to 10 µm. Almost all carry a negative electrical charge on their surface, repel each other and do not clump together (CAWST 2009). They are also so small that they fall extremely slowly: by Stokes' law, a clay particle of 0.002 mm and density 2650 kg/m³ in water at 20 °C falls about 0.3 m per day (calculation in “Calculations”). A 40 cm bucket would take days to clear, and thermal agitation and any current prevent it entirely with the finest particles.

The three stages.

1. Coagulation (seconds, rapid mixing). As it dissolves, alum releases aluminum ions (Al³⁺) that react with the water and form positively charged species and, afterward, aluminum hydroxide (Al(OH)3), a whitish gel. The positive charges neutralize the negative charges of the particles, which stop repelling each other (CAWST 2009). This happens in seconds, which is why the coagulant must be spread immediately and vigorously through the whole volume: if it stays concentrated in one spot, it reacts there and the rest of the water is not treated. At low doses charge neutralization dominates; at high doses and pH 6-8, "sweep flocculation" takes over: the particles get trapped in the precipitating hydroxide gel.
2. Flocculation (minutes, slow mixing). The already destabilized particles collide and stick together, forming flocs of 0.5-2 mm, visible as flakes or "snow" in the water. It needs gentle stirring: it favors collisions; vigorous stirring breaks up flocs already formed.
3. Sedimentation (30 min to several hours, absolute rest). The flocs, much larger than the original particles, fall within minutes or hours. They carry with them part of the bacteria, viruses and cysts attached to the particles, which end up in the sludge at the bottom.

The variables that matter.

• Dose. There is an optimal range that depends on each water. Too little dose: no flocs form. Too much: the particles become positively charged, repel each other again and the water ends up worse (Dorea 2009, fig. 3: with 109 NTU to start, 30 mg/L of aluminum sulfate gave about 3 NTU and 90 mg/L gave about 14 NTU). That is why the dose is determined with a jar test, not guessed. More turbid waters usually need more dose, but not in proportion. With less turbid water there are fewer particles to collide and flocs form more slowly: slow mixing may need to be extended; the jar test shows it.
• pH and alkalinity. The reaction of aluminum with water releases acid (3 H⁺ per Al). The water's alkalinity (mainly dissolved bicarbonates, in mg/L of CaCO3) neutralizes that acid. Every mg/L of aluminum sulfate consumes 0.5 mg/L of alkalinity (TDEC 2022) and every mg/L of potassium alum about 0.32 mg/L (stoichiometric calculation, see “Calculations”). If the water is very soft (rain, meltwater, streams from granite terrain), the pH drops, the aluminum stays dissolved instead of precipitating, no flocs form and the treated water carries more aluminum. The range for good coagulation with aluminum is pH 6.0-7.5, and the minimum solubility of aluminum (least residual) is between pH 6.0 and 7.0 (Dorea 2009). The fix is to add alkalinity (lime, soda, bicarbonate) before the alum.
• Temperature. Below 5 °C coagulation with aluminum is slower, clarifies worse and leaves more residual aluminum (Dorea 2009). In winter: more flocculation and settling time, and do the jar test with the water at its real temperature.
• Undisturbed settling time. Direct sun on one side of the container, wind or knocks create currents that keep the flocs suspended.

How much aluminum each coagulant carries. What coagulates is the aluminum, not the product's name:

(Masses calculated with IUPAC atomic weights. Commercial aluminum sulfate varies in its water content; the jar test corrects for any difference.)

What it does and does not do. It effectively removes turbidity, reduces color, part of the organic matter and part of the microbes: in the laboratory, more than 90 % to more than 99 % of bacteria, viruses, protozoa and helminth eggs (1-2 log units), but in the field, less than 90 % of bacteria in one study and 95 % in another (less than 1 to 1.3 log units; CAWST 2009, Key Data). Water with 10,000 E. coli per 100 mL can end up with more than 500-1000. It does not reliably disinfect, does not remove dissolved salts (brackish water), nitrates or most dissolved chemicals.

Procedure

A. Preparing the 1 % stock solution

Working with a solution of known concentration is much more precise than throwing in "a pinch" of powder, and dissolved alum spreads within seconds.

1. Calibrate the measures. What matters is consistency, not absolute precision: if the stock, the jars and the bucket are all measured with the same cup and the same teaspoon, an error in the cup cancels out. Fill the teaspoon with water and pour it into the cup: count how many teaspoons fill the cup (a 5 mL teaspoon gives 50 in 250 mL). If you have a real reference (a commercial 1 L bottle, a 250 mL container), use it to find the cup's volume. Mark the 1 L level (4 cups) on a pitcher and also measure the treatment bucket with the cup, marking the 20 L level (80 cups) on the inside. Always calculate with the marked volume, not the bucket's nominal one. Success criterion: teaspoons per cup recorded, 1 L mark on the pitcher and 20 L mark on the bucket.
2. Weigh 10 g of alum. With a scale, directly. Without a scale, with a pan balance (N1): on one pan, a small cup with 10 mL of water (2 teaspoons of 5 mL, which weigh 10 g) and on the other an identical empty cup; balance it by adding ground alum to the empty cup. Alternative with recovered euro coins: two 5-cent coins and one 1-cent coin weigh 10.14 g. If later on you will measure alum with a spoon, weigh once a level teaspoon of the same ground alum and note its mass. Success criterion: pans balanced with 10 g (± 1 g) of alum.
3. Dissolve. Crush the crystals to a fine powder in a stone or ceramic mortar. Pour the 10 g into the pitcher, add clean water up to the 1 L mark and stir for 2-3 min. 1 % dissolves easily: potassium alum can hold ≈ 11 g of hydrated crystal (≈ 6 g expressed as KAl(SO4)2) per 100 g of water at 20 °C, and 5-6 g of crystal at 0 °C (CRC Handbook; “Potassium alum”); in cold water it takes longer to dissolve. Result: 10 g/L = 10 mg of alum per mL. Success criterion: clear liquid with no crystals at the bottom after 5 min of settling.
4. Store. In a glass, glazed ceramic or plastic container with a lid, labeled (“ALUM 1 % — DO NOT DRINK”), in the shade and out of children's reach. The solution is acidic (pH ≈ 3.5). CAWST (2009, Key Data) gives a shelf life of up to 6 months for liquid coagulants and 1 year for solids; renew the solution sooner if it becomes cloudy, mold appears or crystals form. Do not use metal containers: the solution attacks them. Success criterion: closed, labeled bottle with the preparation date.

B. Homemade jar test (determines the dose)

Do it with each new source and each time the water's appearance changes (turbidity spikes after heavy rain). For the 1 L jars it helps to dilute the stock to 0.2 % so it can be measured with a teaspoon.

5. Prepare the 0.2 % dilution. 1 cup (250 mL) of 1 % stock solution + 4 cups (1 L) of clean water = 1.25 L at 0.2 % (2 mg per mL, 10 mg per 5 mL teaspoon). Success criterion: bottle labeled “0.2 %.”
6. Fill the jars. Take 5-6 L of the water to be treated, well stirred (the turbidity of the bucket should be the same as that of the jar). Fill 5 identical jars with 1 L each (4 cups) and number them. If the water is very soft (rain, meltwater, granite streams) prepare a second batch with added alkalinity (see “Calculations,” alkalinity section). Success criterion: 5 jars with the same volume and the same appearance.
7. Dose. Add 0, 1, 2, 4 and 6 teaspoons of the 0.2 % dilution to the jars = 0, 10, 20, 40 and 60 mg/L of alum. If the 60 mg/L jar turns out best, repeat with the high series: 0, 4, 8, 12 and 16 teaspoons (0, 40, 80, 120 and 160 mg/L). With aluminum sulfate the same series works, knowing that each teaspoon delivers 60 % more aluminum. Success criterion: each jar with its dose noted.
8. Rapid mixing: 1 min. Stir each jar vigorously for 1 min, either at the same time with several hands or one after another at the same pace, with a fork or a small stick, making a swirl. All jars must receive the same treatment (Dorea 2009). Success criterion: a vigorous swirl for 60 s in each jar.
9. Slow mixing: 3-5 min. Stir gently, one turn every 3-4 s (15-20 turns per minute), without splashing. In the field test described by Dorea (2009) 2-3 min is enough; extend to 5 min if it is cold. Success criterion: visible flocs of 0.5-2 mm appear in the jars with the right dose.
10. Settle: 30 min without touching the jars, in the shade (Dorea 2009 uses 20-30 min). Success criterion: a layer of sludge is distinguishable at the bottom and clearer water above in at least one jar.
11. Choose the dose. Look at each jar against the light and against a sheet of printed text held behind it. Choose the lowest dose that gives the clearest water and the largest, already settled flocs. If two jars look equally clear, choose the one with less alum. If the best is the highest dose, repeat with higher doses; if the best is the 10 mg/L one, repeat with 0, 5, 10 and 15 mg/L. If none forms flocs, suspect very soft water (low pH) and repeat with added alkalinity. Success criterion: a dose chosen and recorded ("River, July, turbid: 40 mg/L").

Worked numerical example (real data). Dorea (2009, fig. 3) describes a field test with water at 109 NTU, rapid mixing (1 min in the text, 30 s in the figure), 2 min of slow mixing and settling. Turbidity after settling, read approximately from the graph:

The chosen dose is 30 mg/L of aluminum sulfate: the clearest, and with more dose the water gets worse. If instead of aluminum sulfate you use potassium alum, multiply by 1.60 (table in “How it works,” assuming 14-hydrate sulfate; Dorea does not state the hydration, and with 18-hydrate it would be ≈ 43 mg/L): 30 × 1.60 = 48 mg/L, which you round to the nearest jar in your series (40 or 60 mg/L; repeat with 50 mg/L to refine). The jar test with your own product corrects this uncertainty. For a 20 L bucket: 48 mg/L × 20 L = 960 mg ≈ 1 g of alum = 96 mL of 1 % stock solution (about 19 teaspoons of 5 mL, or about 2/5 of a 250 mL cup). For a 1 m³ tank: 48 mg/L × 1000 L = 48 g.

C. Treating a 20 L bucket

12. Optional pretreatment. If the water carries leaves, sand or coarse mud, let it settle for 1-2 h and pour it into another bucket through a folded cloth. If the jar test called for alkalinity, add it now and stir for 1 min. Success criterion: no floating solids or sand at the bottom of the bucket.
13. Add the alum. Fill the bucket up to the 20 L mark (step 1). Measure the 1 % stock solution according to the chosen dose: 1 mL of stock per liter for every 10 mg/L (for 20 L: 20 mL = 4 teaspoons per 10 mg/L; at 40 mg/L, 80 mL = 16 teaspoons; at 48 mg/L, 96 mL ≈ 19 teaspoons). If your marked bucket has a different volume, use the formula in “Calculations” with that volume. Pour it in a thin stream while someone else stirs, or stir yourself immediately. Success criterion: volume measured and poured in under 15 s.
14. Rapid mixing: 1 min. Stir with the stick vigorously, making a swirl that reaches the bottom. Success criterion: a swirl that pulls water from the bottom for 60 s.
15. Slow mixing: 5-10 min. Stir gently, one full turn of the stick every 3-4 s. In a bucket the flocs take longer than in the jar to grow. Stop when you see large flocs; if you break them up, they do not re-form well. Success criterion: visible flocs of 0.5-2 mm among which the water starts to look clearer.
16. Settle: at least 1-2 h, covered (against dust and insects), in the shade and without moving the bucket. If it is cold or the water was very turbid, extend it to overnight; in tanks it reaches 12 h (Dorea 2009, sec. 3). Below 5 °C alum performs worse (Dorea 2009). Success criterion: a well-defined sludge layer at the bottom and the water above with no apparent turbidity against the light in a transparent glass. There is no reliable homemade test to confirm the water is below 5 NTU (see “Verification”): if in doubt, disinfect with double the chlorine dose.
17. Decant without disturbing the sludge. Three options, from best to worst:
    • Tap 4-5 cm from the bottom: open it slowly and close it when about 3 L remain or it comes out turbid.
    • Siphon: a tube filled with water, the intake end held to a stick so it stays 5-10 cm below the surface and lowering as the level drops, but never less than 5 cm from the sludge; the outlet end lower than the bottom of the bucket, into a clean container. Do not suck the tube with your mouth: fill it by submerging it fully and covering one end with your finger.
    • Pouring very slowly, tilting the bucket, stopping as soon as you see turbid water rising. Leave the last ≈ 3 L (≈ 15 %, about 4-5 cm of height) with the sludge in the bucket. Success criterion: the water collected is as clear as that at the top of the bucket, with no flocs.
18. Filter (recommended) and disinfect (mandatory). If you have a sand or ceramic filter, pass the clarified water through it: now it will not clog. Then disinfect: chlorine (at the clear-water dose if the turbidity is clearly low; double it if in doubt), boiling or SODIS (see linked articles). If you used ammonium alum, boil or use SODIS; chlorine only works if a DPD kit confirms ≥ 0.5 mg/L of free chlorine after 30 min (see “Materials”). Disinfect the same day: clarified water that has not been disinfected does not keep. Success criterion: water both clarified and disinfected by the chosen method.
19. Manage the sludge. The sludge concentrates mud, aluminum and the removed microbes. Pour it into a hole more than 30 m from wells and sources, with the bottom of the hole at least 1.5 m above the water table (Sphere Manual 2018 criteria for excreta), and downstream of any intake (Dorea 2009); cover it with soil. Wash the bucket with already treated water. Wash your hands. Success criterion: clean bucket and sludge buried away from the water.

D. Clarifying with moringa seeds (without alum)

20. Approximate dose by turbidity (ECHO 2005, p. 2; adjust with a jar test): < 50 NTU, 1 seed kernel per 4 L; 50-150 NTU, 1 per 2 L; 150-250 NTU, 1 per liter; > 250 NTU, 2 per liter. Expressed as mass of ground seed (Schwarz 2000, p. 3): < 50 NTU, 10-50 mg/L; 50-150 NTU, 30-100 mg/L; > 150 NTU, 50-200 mg/L. For a moderately turbid 20 L bucket: 10 kernels. For the jar test, prepare the solution in step 21 with a known amount of seed and dose it by teaspoons, as in section B. Success criterion: number of kernels (or grams) counted.
21. Prepare. Use mature, dry seeds; remove the husk and discard dark or moldy kernels. Grind to a fine powder (mortar) and sieve. Make a paste with a few drops of clean water, mix it with 250 mL of clean water in a bottle and shake for 1 min. Strain through fine cloth directly over the bucket. Prepare the solution right before use; use it within 8 h at most (Schwarz 2000; ECHO 2005). Success criterion: milky liquid with no lumps.
22. Mix and settle. Rapid mixing for at least 1 min; slow mixing (15-20 turns per minute) for 5-10 min; settle for at least 1-2 h; decant as in step 17 and disinfect as in step 18 (ECHO 2005). Moringa adds organic matter to the water and any remaining microbes can multiply again (ECHO 2005, p. 3; Schwarz 2000, p. 4): disinfect and consume the water the same day. Success criterion: clear water over sludge at the bottom.

Verification

• Floc formation (immediate). Flakes should be visible during slow mixing. Without flocs there is no treatment: check dose, pH and temperature.
• Turbidity without instruments.
  • Clarified water in a transparent glass against the light: no cloudiness or flocs floating.
  • Bottle test (as in SODIS, step 3): a transparent 1.5-2 L bottle, filled, standing on a newspaper headline and looking straight down from the mouth. If the letters can be read, the turbidity is under about 30 NTU (see Solar disinfection (SODIS)). It is a minimum, not the goal.
  • Coin test (Reed, Smith and Shaw 2017, p. 8): put a coin of ≈ 2.5 cm at the bottom of a tall, dark-walled container and add water while watching from above. If the coin disappears before 32 cm of water, more than 20 NTU; between 32 and 50 cm, 10-20 NTU; if it is still visible past 50 cm, less than 10 NTU.
  • Turbidity tube: a transparent tube 80 cm or more with a black mark on a white bottom; water is poured in until the mark disappears and the scale is read. Commercial tubes measure from 5 to 2000 NTU on a non-linear scale (MSF Unicat 11301; Reed, Smith and Shaw 2017) and do not measure below 5 NTU. An uncalibrated homemade tube is only good for comparing one water with another or one day with another.
  • Chlorination requires < 5 NTU (Sphere Manual). No homemade test guarantees it: if you have no turbidimeter and the water is not clearly crystal clear, use double the chlorine dose.
• Turbidimeter (N4 recovered): target < 5 NTU, better < 1 NTU before disinfecting.
• pH. Strips or a meter (N4): the treated water should end up between 6.0 and 7.5. The red cabbage indicator (N1: water from boiling red cabbage) is too coarse for this: it turns violet across a wide range, pH ≈ 5 to 7, and pink or red only at pH ≤ ≈ 4. A violet color does not guarantee pH ≥ 6; it only catches gross errors (pink = severe overdose or very acidic water). Strips or a meter are needed to monitor dissolved aluminum.
• Taste. Treated water with an acidic, astringent or metallic taste indicates overdose or low pH: repeat the jar test.
• Residual aluminum. It cannot be measured without a kit. It is controlled indirectly: minimum effective dose, pH 6.0-7.0, well-settled flocs and clean decanting (much of the residual aluminum goes into the flocs that escape; Dorea 2009). With means (N4): colorimetric aluminum kit; target ≤ 0.2 mg/L (WHO for small facilities) or 0.1 mg/L in large plants. Remember that a kit measures poorly with phosphates, fluoride, iron, manganese or high alkalinity (Dorea 2009, sec. 4).
• Microbiological. Clarification is not verified by appearance: the safety test is the subsequent disinfection (0.2-0.5 mg/L free chlorine residual or correct boiling) and, if there are means, a count of E. coli (0 per 100 mL).

Common mistakes

Safety

• Main risk: believing clear water is potable. It can kill. Clarification lets through bacteria, viruses and cysts (in the field, bacteria removal on the order of 90-95 %; CAWST 2009). Cholera, typhoid fever, hepatitis A or Cryptosporidium are still present. Always disinfect afterward and teach it this way: "alum to clarify, chlorine or fire to drink."
• Ammonium alum and chlorine. The ammonium it contributes consumes the chlorine and turns it into chloramines, which disinfect much worse: the water may seem chlorinated (smells of chlorine) and not be properly disinfected. With ammonium alum, boil or use SODIS; if you chlorinate, require ≥ 0.5 mg/L of free chlorine measured with DPD after 30 min. Always prefer potassium alum or aluminum sulfate.
• Contaminated sludge. It concentrates the pathogens from the water. Handle it like wastewater: without touching it with bare hands if possible, buried more than 30 m from wells and sources and downstream of the intakes (step 19); wash hands with soap. Do not pour it into a river from which others draw water: also, the aluminum concentrated in the sludge is toxic to the aquatic ecosystem (Dorea 2009).
• Aluminum in treated water. WHO has not set a health-based guideline value because there is no conclusive evidence of harm at the concentrations usual in drinking water (nor of a link to Alzheimer's disease), but it calls for keeping it at ≤ 0.1-0.2 mg/L as good practice (WHO 2010; WHO 2022); the EU sets 0.2 mg/L (Directive 2020/2184). This is achieved with a minimum effective dose, pH 6.0-7.0 and good decanting. Never use alum-treated water for dialysis or to prepare intravenous fluids: in patients on renal dialysis, aluminum in the water has caused severe encephalopathy and bone disease (WHO 2010). For formula-fed infants and people with kidney failure, as a precaution from this article (WHO gives no specific recommendation), prefer water from a clear source that does not need coagulant or be especially strict with the minimum dose: reduced kidney function favors aluminum accumulation (WHO 2010).
• The solid reagent and the stock solution. Potassium alum is mildly toxic (used in food as E522), but if ingested in quantity (grams) it irritates the stomach and intestine and causes vomiting; the stock solution is acidic and stings the eyes. Label the bottles, keep them away from children and food, and do not use the same containers for drinking. If it gets in the eyes: rinse with plenty of clean water for 15 min. If someone drinks the stock solution or eats alum: rinse the mouth, drink one or two glasses of water, do not induce vomiting and seek medical attention if there is persistent discomfort (the usual guidance in safety data sheets for aluminum potassium sulfate).
• Industrial aluminum sulfate and pool flocculants. They may contain impurities (iron, heavy metals) or polymers not suitable for drinking water. Use only product labeled for drinking-water or food treatment; if the label does not say so or lists other ingredients, do not use it for drinking. Aluminum sulfate powder irritates eyes and airways: handle it without raising dust and with a cloth over the nose and mouth.
• Homemade alum (made from minerals): may carry iron (tints the crystals) and other metals from the mineral. Only for drinking water after recrystallizing it at least once and using colorless, transparent crystals.
• Alkaline additives. Quicklime (calcium oxide) reacts with water, heating up and burning skin and eyes; slaked lime and sodium carbonate also irritate as powders. Add them dissolved or as a slurry, in the small amounts given in “Calculations,” with eye protection. Caustic soda (sodium hydroxide, an N3-N4 variant) is very corrosive: it destroys skin and eyes and heats up a lot when dissolving; always pour it slowly onto cold water (never water onto the soda), with goggles and gloves. If any of them splashes into the eyes: rinse for 15-20 min with water and seek medical attention.
• Moringa seeds. Only Moringa oleifera or M. stenopetala identified with certainty; do not use seeds from trees you cannot identify. Moringa does not fully disinfect either (ECHO 2005).
• Limits of the method. It does not remove salt, nitrates, fluoride, dissolved arsenic or most dissolved pesticides and metals. If the water comes from mines, industry or heavily treated fields, look for another source.

Variants

• With fewer resources (no coagulant, N0):
  • Simple settling or the three-vessel method: let the water rest 24-48 h in a covered container, transfer the water on top to another container and use that (check timing with Sobsey 2002, Managing Water in the Home, WHO). It removes sand, silt and part of the turbidity, but not fine clay, and requires three times as many containers.
  • Vegetable coagulants: besides moringa (step D), Strychnos potatorum seeds (nirmali, in India) and the mucilage of Opuntia cactus (nopal, in the Americas) have traditionally been used, rubbed or crushed into the water. No reliable doses are given here: always do a jar test. Moringa stenopetala is more effective than M. oleifera (ECHO 2005). Collect moringa seeds in the dry season: some studies found less active ingredient in seeds collected in the rainy season (ECHO 2005).
• Obtaining or making alum (N1-N3), mention only:
  • Recovery (the realistic option in the short term): pharmacies and drugstores (“alum stone,” powdered alum), natural dye and tanning shops (alum is a mordant), food stores (pickling supplies), treatment plants and water-treatment reagent suppliers (aluminum sulfate). CAWST (2009, Key Data) estimates a shelf life of 1 year for solid coagulants; store them dry and closed.
  • Historical manufacture (N2): since Antiquity it was obtained from alunite (calcined and leached) and, in England from the seventeenth to the nineteenth centuries, from aluminous shale with pyrite: it was roasted in heaps for months, leached with water, potassium was added (seaweed or wood ash) or ammonium (aged urine), and alum crystallized on cooling (Alum industry in North Yorkshire). Alum made with ammonium from urine has the chlorine problem described above: for drinking water the potassium route is preferable. This is extraction and crystallization chemistry; its detailed procedure belongs in a dedicated chemistry article.
  • Aluminum sulfate (N3): by attacking kaolinitic clay or bauxite with sulfuric acid; requires producing and handling sulfuric acid. Refer to basic industrial chemistry.
• With more resources (N3-N4):
  • Other coagulants: polyaluminum chloride (PAC), which consumes less alkalinity and tolerates cold water better (verify). Ferric chloride, with a useful pH range of 5.0-9.0 and easy to dissolve, used by Médecins Sans Frontières, though it is harder to obtain and can leave the water yellowish (Dorea 2009). Commercial "flocculant-disinfectant" sachets for household use combine coagulant and chlorine in a single dose; follow the manufacturer's instructions.
  • Filtration after sedimentation: rapid sand filter or slow filter; lowers turbidity to < 1 NTU and greatly improves disinfection.
  • Control: turbidimeter, pH meter, aluminum kit; adjust the coagulation pH to 6.0-7.0 with lime or caustic soda before the coagulant (Dorea 2009; precautions on caustic soda in “Safety”).
• Scaling up:
  • Family (20-100 L/day): buckets from section C; 1 jar test per source and season.
  • Village or camp (1-50 m³/day), batch treatment: the most common and simplest method in emergencies (Dorea 2009). Tanks of 1-10 m³ (or sheet-metal tanks with a liner of 11-90 m³). The stock solution is added to the water as it enters while the tank fills (the stream provides the rapid mixing), or an “alum cage” (a box of wire mesh and wooden slats) is hung in the inlet mouth with the weighed amount of stone alum, which dissolves as the water enters (Dorea 2009, fig. 5). Once the tank is full, it is left to settle up to 12 h, decanted through a floating outlet that draws water from the surface (Dorea 2007, Waterlines 26:17-19, cited in Dorea 2009) and disinfected with chlorine before distribution. The sludge is purged through a bottom valve when it approaches the outlet. Example: 10 m³ tank, dose 48 mg/L of potassium alum → 480 g per batch = 48 L of 1 % stock or 9.6 L at 5 %.
  • Village with continuous flow: constant-flow dosing of stock solution (drip from a constant-level container or at a pump's suction, which provides the rapid mixing; Dorea 2009, fig. 4), a channel or tank with baffles for slow flocculation, and a settling basin. Example: 1 m³/h at 48 mg/L = 48 g/h of alum = 0.96 L/h (16 mL/min) of 5 % solution. The 5 % solution is close to the solubility limit in cold water (5-6 g per 100 g at 0 °C): in winter use a 1-2 % solution (4.8-2.4 L/h) or prepare it with lukewarm water. Gravity drip changes with the container's level: check the flow rate with a cup and an hourglass or sundial every 2-4 h.
  • City: conventional plant (rapid mixer, baffle or paddle flocculators, settling tanks, rapid sand filters and chlorination), daily jar tests and laboratory monitoring of pH and residual aluminum. Sizing settling tanks and flocculators is the subject of a dedicated engineering article.

Calculations and reference tables

Stock solution and dose.

• 1 % solution = 10 g/L = 10 mg/mL. Volume of stock (mL) = dose (mg/L) × water volume (L) ÷ 10. Example: 40 mg/L in 20 L → 40 × 20 ÷ 10 = 80 mL. Quick rule: 1 mL of stock per liter for every 10 mg/L.
• 5 % solution = 50 g/L = 50 mg/mL. Volume (mL) = dose × volume ÷ 50. Useful for tanks (in lukewarm water; in cold weather, use 1-2 %).
• 0.2 % dilution for 1 L jars: 1 teaspoon (5 mL) = 10 mg → 10 mg/L per teaspoon.

Conversion between coagulants (by aluminum content). Potassium alum dose = aluminum sulfate ·14H2O dose × 1.60 (·18H2O × 1.42). Reverse: × 0.625. Example: 30 mg/L of 14-hydrate sulfate → 48 mg/L of potassium alum; from 18-hydrate → ≈ 43 mg/L.

Ammonium from ammonium alum. NH4⁺ = 18.04 ÷ 453.3 ≈ 0.040 mg per mg of alum. At 48 mg/L: 48 × 0.040 ≈ 1.9 mg/L of NH4⁺ (≈ 1.5 mg/L as nitrogen, 1.9 × 14.0 ÷ 18.04). The breakpoint of chlorination requires on the order of 7.6 mg of Cl2 per mg of ammoniacal nitrogen: 1.5 × 7.6 ≈ 11 mg/L of chlorine before free chlorine appears, several times the usual clear-water dose.

Alkalinity consumed (stoichiometry: each Al releases 3 H⁺, neutralized by 1.5 CaCO3).

• Aluminum sulfate ·14H2O: 3 × 100.1 ÷ 594.4 = 0.50 mg/L of alkalinity (as CaCO3) per mg/L of dose (matches TDEC 2022).
• Potassium alum: 1.5 × 100.1 ÷ 474.4 = 0.32 mg/L per mg/L of dose.
• Example: 48 mg/L of potassium alum consume 48 × 0.32 ≈ 15 mg/L of alkalinity. The water should keep at least about 10-20 mg/L afterward (verify), so the starting water should have about 25-35 mg/L or more. Rainwater has practically zero alkalinity (pH ≈ 5.6 in equilibrium with atmospheric CO2): with rainwater, alkalinity almost always needs to be added. Water from granite terrain is usually low; confirm it with a local analysis or with the jar test with and without added alkalinity.

Alkalinity to add in soft water (for each mg/L of alkalinity as CaCO3 that is missing; calculated by equivalent masses):

These amounts are so small that it helps to dissolve them first in 1 cup of water (a slurry in the case of lime) and add that cup to the bucket before the alum. Check with the jar test: with added alkalinity flocs should appear where there were none before. Do not add spoonfuls of lime "by eye": too high a pH also worsens coagulation with aluminum and increases dissolved aluminum, which is at a minimum around pH 6-7 (WHO 2010).

Settling velocity by Stokes' law (why coagulation is needed). v = g · (ρp − ρw) · d² ÷ (18 · μ). Clay with d = 2 µm = 2 × 10⁻⁶ m, ρp − ρw = 1650 kg/m³, μ = 1.0 × 10⁻³ Pa·s at 20 °C: v = 9.81 × 1650 × (2 × 10⁻⁶)² ÷ (18 × 10⁻³) ≈ 3.6 × 10⁻⁶ m/s ≈ 0.31 m/day. A 1 mm floc is porous and only slightly denser than water, so Stokes does not apply to it; its settling velocities are on the order of 0.5-3 m/h (verify), so that a 40 cm column clears in minutes to an hour.

Sources

• Dorea, C.C. (2009). «Coagulant-based emergency water treatment». Desalination, 248, 83-90, doi:10.1016/j.desal.2008.05.041. Apartados 2 (pH 6,0-7,5; mínima solubilidad del aluminio a pH 6,0-7,0; frío < 5 °C; cloruro férrico pH 5,0-9,0), 3 (tanques por lotes, reposo hasta 12 h; prueba de jarras de campo: 1 min rápido, 2-3 min lento, 20-30 min de reposo, fig. 3; dosificación y «jaula de alumbre», figs. 4 y 5), 4 (residuales e interferencias de medida) y 5 (lodos). Cita a Dorea, C.C. (2007), «Simple improvements for emergency batch water treatment», Waterlines, 26, 17-19 (salida flotante).
• CAWST (2009). Household Water Treatment and Safe Storage Fact Sheet: Chemical Coagulants y hoja Key Data (eficacia en laboratorio y en campo; vida útil de 6 meses en líquido y 1 año en sólido). Centre for Affordable Water and Sanitation Technology, Calgary.
• OMS (2022). Guidelines for Drinking-water Quality, 4.ª ed. con la primera y la segunda addenda, hoja química del aluminio.
• OMS (2010). Aluminium in drinking-water. Documento de base WHO/HSE/WSH/10.01/13.
• Directiva (UE) 2020/2184 del Parlamento Europeo y del Consejo, de 16 de diciembre de 2020, relativa a la calidad de las aguas destinadas al consumo humano, anexo I, parte C (aluminio y amonio).
• Tennessee Department of Environment and Conservation, Fleming Training Center (2022). Coagulation/Flocculation Workshop, Course #307.
• Doerr, B. y ECHO Staff (2005). Moringa Water Treatment. ECHO Technical Note, North Fort Myers (Florida), pp. 2-3.
• Schwarz, D. (2000). Water Clarification using Moringa oleifera. GATE Technical Information W1e, GTZ, Eschborn, pp. 3-4.
• Folkard, G.K., Sutherland, J.P. y Shaw, R. (1999). Water clarification using Moringa oleifera seed coagulant. WELL Technical Brief, WEDC, Loughborough University (citado en ECHO 2005; no consultado directamente).
• Reed, B., Smith, M. y Shaw, R. (2017). Measuring turbidity. WEDC Guide 31, Water, Engineering and Development Centre, Loughborough University, p. 8.
• Médicos Sin Fronteras. Catálogo Unicat, artículo 11301 (tubo de turbidez, 5-2000 NTU).
• The Sphere Project (2018). Manual Esfera, normas de abastecimiento de agua (turbidez < 5 NTU para la cloración) y de gestión de excretas (distancias a fuentes de agua y al nivel freático).
• IUPAC. Standard atomic weights (masas molares de la tabla de coagulantes y de los cálculos de alcalinidad y amonio).
• Banco Central Europeo. Características técnicas de las monedas en euros (masas de las monedas de 1 y 5 céntimos, usadas como pesas de referencia).
• Haynes, W.M. (ed.). CRC Handbook of Chemistry and Physics, tabla de solubilidad de compuestos inorgánicos (sulfato de aluminio y potasio); y «Potassium alum», Wikipedia en inglés.
• «Alum industry in North Yorkshire». Wikipedia en inglés, consultado en septiembre de 2026 (fabricación histórica del alumbre; fuente de orientación, a sustituir por una monografía de historia de la química).