Softening water with cation resin: how it works, how to build a softener and how to regenerate it with salt
How to soften water yourself with a cation exchange resin, using Purolex PRC107E as the example, the kind sold loose by the litre in bags. This article covers:
- what each line of the seller's specification means, in plain words;
- how softening works and how much water a litre of resin gives at your hardness;
- how to build a softener: a filter housing or a pressure vessel, how much resin, what flow rate and pressure;
- regeneration with salt step by step: how much salt and water, how long, at what flow;
- whether softened water is safe to drink unboiled, whether it protects against kidney and bladder stones, and whether it is better for the body;
- what other resins there are and where to buy them.
draft — under discussion and being expanded
What hard water is and what the resin does
Hardness is dissolved calcium and magnesium salts. They are what deposits as scale on heating elements, in the kettle, on taps and in the boiler. In Ukraine hardness is measured in meq/L (the same as the mmol/L in the standards): 1 meq/L = 2.8 German degrees (°dH) = 50 mg/L as CaCO₃. The limit for tap water is 7 meq/L; wells often have 8–15.
A cation resin in the sodium form is small polymer beads with sodium ions "sitting" on them. When hard water flows through, the beads hold on to calcium and magnesium more readily than to sodium: they take them out of the water and release sodium in exchange. The water coming out is soft, and the resin gradually fills up with calcium. Once it is full, a strong solution of table salt is passed through it: it has so much sodium that it pushes the calcium back off, and the resin is ready to work again. This is regeneration.
⚠ Softening does not remove salts from the water; it swaps calcium and magnesium for sodium. So a TDS meter shows almost the same reading after a softener as before it: you cannot check hardness with one. You need a hardness test (a drop kit or strips).
What the PRC107E specification means
Purolex PRC107E is a strong acid, gel-type cation resin: sulphonic groups on polystyrene cross-linked with divinylbenzene. Do not confuse it with Purolite C107E, which is a completely different, weak acid resin. Distributors list Purolite C100E and Dowex HCR-S/S as equivalents of PRC107E, and the maker states it is intended specifically for drinking water and food processing.
| Seller's line | Value | What it means |
|---|---|---|
| appearance | light yellow to brown | clear amber beads; the colour does not affect performance |
| ionic form as supplied | Na form | already charged with sodium: you can fill it and start softening without a first regeneration |
| total volume capacity | ≥ 1.9 meq/mL | the total capacity: in theory 1 L of resin holds 1900 meq; in practice less (section below) |
| moisture retention | 45–50 % | half the bead's weight is water; dried-out resin cracks, so keep it moist |
| bulk density | 0.77–0.87 g/mL | 1 L of resin weighs ≈ 0.8 kg, hence "sold from 1 L (0.8 kg)" |
| specific gravity | 1.25–1.29 g/mL | the bead is heavier than water, so it sinks and stays as a bed, and backwashing only lifts it |
| particle size | 0.4–1.25 mm, at least 95 % | a standard grade; the smallest beads would pass through a coarse distributor screen |
| uniformity coefficient | ≤ 1.7 | how alike the beads are; the lower, the more evenly water flows through the bed |
| whole beads after attrition | ≥ 90 % | mechanical strength: at least 90 % of beads stay whole after an attrition test |
| pH range | 0–14 | chemically stable in any water |
| maximum temperature | H form ≤ 100 °C, Na form ≤ 120 °C | the resin can take hot water, but housings and vessels are usually rated for cold water only |
| total reversible swelling (Na⁺ → H⁺) | 8–10 % | beads grow in the acid form; irrelevant for softening with salt |
| operating capacity | NaCl: 8–10; HCl: 4–5 | as the seller lists it; this looks like the regenerant strengths in per cent, not a capacity |
| regenerant concentration | NaCl 8–10 %, 1.5–2 volumes per resin volume | for 1 L of resin, 1.5–2 L of 10 % brine, i.e. ≈ 150–200 g of salt |
| regenerant flow | 4–6 m/h | the brine is passed through slowly (how to turn metres per hour into litres is below) |
| regeneration contact time | 30–60 min | how long the brine should be in the resin |
| rinse flow | 10–20 m/h | the rinse rate after the brine |
| soaking time | ≈ 30 min | soak new dry resin in water before first use |
| service flow | 10–45 m/h | the allowed water velocity during softening |
Metres per hour is a velocity, not a volume. To get litres per hour, multiply the velocity by the column's cross-section: Q (L/h) = v (m/h) × A (m²) × 1000. For example, a column 20 cm in diameter has a cross-section of 0.0314 m², and at 20 m/h ≈ 630 L/h flows through it, about 10 L/min. Purolite datasheets also give flow in bed volumes per hour (BV/h): 20 BV/h for 10 L of resin is 200 L/h. Converting is simple: BV/h = m/h ÷ bed depth in metres. With a 0.8 m bed, 10 m/h is ≈ 12 BV/h.
HCl (hydrochloric acid) in the table is for demineralisation in industrial plants. At home acid regeneration is unnecessary and dangerous: soften with salt only.
How much water the resin will give
The total capacity of 1.9 eq/L is a ceiling you never reach in service: each regeneration restores only part of it. How much depends on how generous you are with salt.
| Salt per 1 L of resin | Operating capacity, eq/L of resin (meq/mL) | Note |
|---|---|---|
| 80 g | ≈ 0.9–1.0 | economical but lower capacity; approximate, from a typical curve |
| 120 g | ≈ 1.1–1.2 | approximate |
| 160 g | ≈ 1.4 | the Purolite C100E datasheet example: 1.39 eq/L |
| 240 g | ≈ 1.6 | the salt is now used inefficiently; approximate |
How much water the resin softens before the next regeneration:
water (L) = operating capacity (eq/L) × resin (L) × 1000 ÷ hardness (meq/L)
Example: 10 L of resin, regenerated with 160 g of salt per litre (1.6 kg of salt each time), water at 7 meq/L: 1.4 × 10 × 1000 ÷ 7 = 2000 L. In practice you keep a 15–20 % margin and regenerate after about 1600–1700 L. The first signs that it is time: soap lathers less, scale reappears in the kettle, a test shows hardness at the outlet.
How to build a softener
| Option | How much resin | What for |
|---|---|---|
| Big Blue 10″ housing with a refillable cartridge | ≈ 2 L (depends on the cartridge) | a boiler, water heater, washing machine; regenerate by hand with the cartridge out |
| Big Blue 20″ housing | ≈ 4–5 L | the same, for more water |
| a fibreglass pressure vessel (e.g. 0844, 1054) with a control valve | 15–30 L | the whole home; the valve regenerates automatically on a timer or a meter |
| a home-made column (a pipe with distributors top and bottom) and three valves | as much as fits | an experiment or a boiler room; regenerate by hand as in the diagram above |
The basic rules for any option:
- Bed depth of at least 0.6–0.8 m in vessels and columns: in a shallow bed water slips through before it is softened. So a narrow, tall column is better than a wide, short one. In Big Blue housings the bed is shorter (≈ 0.25 m in a 10″, ≈ 0.5 m in a 20″), so water is run through them more slowly.
- Free space above the resin. During backwash the bed grows by 50–75 %; the Purolite datasheet allows headspace for backwash equal to the bed height itself. Domestic vessels are filled to 2/3–3/4 of their volume, so they are backwashed more gently.
- A distributor (screen or slotted nozzle) at the bottom and top, with 0.2–0.25 mm slots: up to 5 % of the beads are smaller than 0.4 mm and would pass through wider slots into the water.
- Pressure. Ordinary mains pressure is 2–6 bar. Fibreglass vessels and domestic control valves are rated with a margin for that, housings too, but check the limit in your housing's datasheet. Above 6 bar, fit a pressure reducer before the softener.
- Temperature: cold water only. The resin can take hot water, but housings and vessels usually cannot.
- A sediment filter before the softener (20–50 µm), so sand does not clog the resin.
- Iron above ~0.3 mg/L fouls the resin and does not always rinse out with salt. Remove the iron from such water first.
- Chlorine gradually breaks the resin down; if the water is chlorinated, fit a carbon filter before the softener.
First start. Pour new resin into water (into a housing or vessel half filled with water, so the beads do not knock together and shed dust), let it stand for ≈ 30 min as the seller advises, then backwash and rinse until the water has no smell or foam. Run the first few tens of litres to drain.
Softening
Water flows top to bottom through the resin bed. The velocity can be anywhere in the seller's 10–45 m/h; to keep hardness from slipping through, stay in the lower part of the range, about 10–20 m/h (with a 0.8 m bed, ≈ 12–25 bed volumes per hour). Run a softener faster than it is designed for and hardness slips through to the outlet.
Regeneration step by step
This is co-current regeneration (the brine flows in the same direction as the water), as in many domestic valves and in a home-made column. Some valves use counter-current (brine flowing upwards); for those, follow the valve's manual. The figures are for 10 L of resin and a dose of 160 g of salt per litre.
| Step | Direction | What and how much | Flow | Time |
|---|---|---|---|---|
| 1. Backwash | bottom to top, to drain | ordinary water; the bed should lift by 50–75 %, but no resin should be carried out | enough to lift the bed without carrying beads out | 10–15 min |
| 2. Brine | top to bottom, to drain | 8–10 % NaCl brine: 1.5–2 resin volumes, i.e. 15–20 L for 10 L of resin (≈ 1.5–2 kg of salt) | slowly: 4–6 m/h (seller); Purolite datasheet: 2–7 BV/h | 30–60 min |
| 3. Slow rinse | top to bottom, to drain | ordinary water that pushes the brine out of the bed; ≈ 2 resin volumes | the same slow rate as the brine | ≈ 15–20 min |
| 4. Fast rinse | top to bottom, to drain | ordinary water until the outlet no longer tastes salty and its TDS matches the inlet water | 10–20 m/h | 5–15 min |
Making 10 % brine: dissolve 100 g of salt and top up with water to 1 L. For 10 L of resin, 1.5–2 kg of salt topped up to 15–20 L. Or more simply: mix 1 part of saturated brine (≈ 26 %, the point where extra salt stays undissolved at the bottom) with 1.5–2 parts of water.
Which salt: tablet salt for water softeners is best. Table salt with no iodine or additives also works. Rock salt with insoluble residue clogs the distributor and the resin.
How much water a regeneration uses: for 10 L of resin, roughly 100–250 L including backwash and rinse (an estimate; we will measure it).
After regeneration, check the water with a hardness test and by taste. Salty water at the outlet means the resin has not been rinsed enough.
Can you drink softened water?
In short:
- Drink it unboiled only if the water before the softener is safe to drink unboiled. The resin does not disinfect water.
- No scale in the kettle does not mean the water is kind to your kidneys. Softened water does not protect against kidney or bladder stones, and the sodium it picks up may even raise the risk slightly.
- Softening does not make water healthier: calcium and magnesium go, and sodium comes in.
| What changes | By how much | What it means |
|---|---|---|
| more sodium | +23 mg/L for each 1 meq/L of hardness removed | after full softening, water at 7 meq/L gains ≈ 160 mg/L of sodium (7 × 23 = 161) on top of what it already had; the limit is 200 mg/L in total |
| almost no calcium or magnesium left | instead of the 1.5–7 meq/L that the standards consider physiologically adequate | not the best water to drink every day |
| if the resin is neglected | bacteria can grow in the bed where water stands for a long time | regular regeneration and a clean salt tank are a must |
| new resin | the first water may smell | run the first water to drain |
Unboiled softened water: what changes and what does not
The main thing the resin does is swap the calcium and magnesium in the water for sodium. Most other things stay as they were:
- microbes: the resin does not kill them, and the resin bed, where water stands, can even become a home for them;
- chlorine, nitrates, pesticides, odours: not removed (chlorine, if anything, slowly destroys the resin);
- iron and manganese: the resin holds back small amounts too, but it clogs faster as a result; it partly holds back lead and other heavy metals, but you cannot rely on it for that.
So if you drink your tap water unboiled, the softened water is fine in terms of microbes too, as long as the resin is regenerated regularly, the salt tank is clean and the first water is run off after the softener has stood unused for a few days. If the tap water is not safe to drink unboiled (a well without a test, old pipes), the softener will not fix that: you need a separate drinking-water filter and, ideally, a water test.
Scale in the kettle and kidney stones are different things
Where scale comes from. Calcium is dissolved in water as calcium bicarbonate, together with carbon dioxide. When the water boils, the gas escapes and the calcium falls out as white stone: scale. Nobody boils water inside the body: calcium from water is absorbed just like calcium from food, and the kidneys remove any excess. So what water does in a kettle tells you nothing about your kidneys.
Where kidney stones come from. Most often (about three cases in four) they are calcium oxalate. A stone grows when urine is supersaturated, and the causes mostly lie elsewhere than the tap water:
- drinking too little, so there is little urine: the main cause;
- a lot of salt in food: sodium makes the kidneys pass more calcium into the urine;
- a lot of animal protein, oxalates (spinach, rhubarb, beetroot, nuts) and sugary drinks;
- too little calcium in food: yes, too little; in the gut calcium binds oxalate, so less of it reaches the urine;
- heredity, excess weight, some illnesses and medicines.
Bladder stones more often have other causes, such as obstructed urine flow, infections or uric acid, and there is almost no data on how tap water affects them.
What research says about hard water. No convincing link has been found between the hardness of tap water and kidney stones; the research is inconclusive. Water contains far less calcium than food. Hard water raises urinary calcium a little, but it also brings magnesium, which probably slows stone formation.
And softened water? It does not protect against stones. The magnesium that may hinder stone growth is gone, and sodium is added: at a hardness of 7 meq/L, about 160 mg of sodium per litre, roughly 0.4 g of salt in every litre (0.6–0.8 g in 1.5–2 L you drink). That is usually little compared with the salt in food (the daily limit is about 5 g), but if anything it is a small step towards the risk, not away from it.
What really helps avoid stones: drink enough to pass 2–2.5 L of urine a day (pale, almost colourless); less salt; calcium from food, in normal amounts. If softened or filtered water tastes better and you drink more because of it, that is a plus, but thanks to the drinking, not the softening. Anyone who has already had stones should know their type: the doctor's advice depends on it.
Is softened water better for the body?
There is no reason to think so. Calcium and magnesium from water are a small but useful part of what we take in each day. The World Health Organization's review of calcium and magnesium in drinking water (2009) brings together data linking very soft water with little magnesium to a somewhat higher risk of heart disease; the authors word their conclusions cautiously, but on the side of "better to have magnesium in the water". Ukrainian standards consider water with a hardness of 1.5–7 meq/L physiologically adequate.
Who should not drink sodium-softened water at all:
- people on a low-salt diet, with high blood pressure, heart or kidney disease;
- babies, or anyone mixing infant formula with it: infants do not need the extra sodium.
If the resin is regenerated with potassium chloride instead of salt, potassium goes into the water instead of sodium. It costs more, and potassium is restricted for people with kidney disease too.
This is not medical advice: if you have stones or kidney disease, discuss your water and diet with a doctor.
Verdict. For a water heater, boiler, washing machine, dishwasher or shower, softened water is exactly right. For drinking and cooking it is better to:
- blend softened water with unsoftened (softener valves have a mixing screw for this) to keep 1.5–2.5 meq/L of hardness;
- or drink water from a separate filter (reverse osmosis, for example) and put the softener only on the hot water and appliances;
- people on a low-salt diet should not drink softened water.
The resin must be food grade and approved for drinking water: in Ukraine, ask the seller for the certificate of conformity for contact with drinking water.
Other resins
| Type | Examples | What for |
|---|---|---|
| strong acid gel cation, Na form (like PRC107E) | Purolite C100E, Dowex HCR-S/S, Amberlite IR120 Na, Lewatit S 1567 | ordinary softening with salt; the most common |
| the same in a fine or uniform grade | Purolite C100EFM; uniform-bead resins (Monosphere) | higher capacity for the same salt, less hardness slippage; more expensive |
| strong acid macroporous | Purolite C150 | usually more resistant to oxidation and mechanical stress; industrial use |
| weak acid cation | Purolite C104, C107E | removes only "temporary" (carbonate) hardness, regenerated with acid; not for home use |
| multi-component media | Ecomix and similar | remove hardness, iron, manganese and organics in one go; regenerated with the same salt |
| nitrate-selective anion | Purolite A520E | remove nitrates, not hardness; regenerated with salt |
| mixed bed (cation H⁺ + anion OH⁻) | various | demineralisation to "distilled" water; single use or regenerated with acid and caustic |
Where to buy resin
- From water treatment shops and online marketplaces, loose from 1 L or in 25 L sacks.
- Before buying, ask for the resin datasheet (the manufacturer's technical sheet) and the certificate of conformity for contact with drinking water. Unbranded resin with no paperwork is fit only for a boiler.
- The resin should be moist in the bag: dried-out beads crack.
- Purolex PRC107E, Purolite C100E and Dowex HCR-S/S are interchangeable for domestic softening; look at the type (strong acid gel, Na form), the bead size and the price per litre rather than per kilogram (1 L ≈ 0.8 kg).
- Store it in a closed bag at 0–40 °C and do not let it freeze: the water in the beads freezes and splits them.
[TO ADD: what we bought, how much, our water hardness before and after, salt use]
Article changelog
- 2026-11-10 — the drinking section: drinking it unboiled, kettle scale versus kidney stones, whether softened water is healthier, who should not drink it.
- 2026-11-10 — first draft: the PRC107E specification in plain words, operating capacity and a worked calculation, how to build a softener, regeneration step by step, whether it is safe to drink, other resins, where to buy.