---
title: "Effects of hardness; zeolite softener"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:hardness-zeolite-softener"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/hardness-zeolite-softener
site: BCT Engineering
---

# Effects of hardness; zeolite softener

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2072 Kartik, 8 marks
- Video: [What is Zeolite Process for Water Softening  | Permutit Process  | Water Softening Processes](https://www.youtube.com/watch?v=helEH29bwQ0) — Chemistry Learners
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/hardness-zeolite-softener
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Describe the effects of hardness. Explain the zeolite water softener with its advantages and disadvantages.

## Short answer

Hardness causes soap waste, scale, and industrial problems. Zeolite softeners exchange Na⁺ for Ca²⁺/Mg²⁺, producing zero-hardness water, regenerated with brine. Advantages: compact, no sludge, simple. Disadvantages: increases sodium, not suitable for turbid/iron water, produces brine waste.

## Answer

#### Effects of hardness
- **Wastes soap**: Ca and Mg form an insoluble scum with soap, which makes washing difficult and increases soap consumption.
- **Scale in boilers, heaters and pipes** (CaCO$_3$, CaSO$_4$): it reduces heat transfer, wastes fuel and can cause boiler explosions; it also reduces the carrying capacity of pipes.
- Bad for the **textile, paper, dyeing, laundry and food** industries; harms fabrics and skin.
- **Bad taste** and poor cooking (pulses and vegetables do not soften well), and a scum forms on tea.
- Very hard water may cause digestive upset and, in some people, kidney stones.

#### Zeolite (base-exchange) water softener
**Zeolite** is a hydrated sodium aluminium silicate (Na$_2$Z), either natural (green sand) or synthetic (permutit), which exchanges its sodium ions for Ca and Mg ions. Water passes down through the zeolite bed in a closed pressure vessel.

```text
   raw hard water / brine (regeneration)
             |
     +-------v-------+
     |:::::::::::::::|  zeolite bed
     |:::::::::::::::|  (on gravel support)
     +-------|-------+
             v
   soft water out / waste brine to drain
```
**Softening**:
$$
Ca(HCO_3)_2 + Na_2Z \rightarrow CaZ + 2NaHCO_3
$$
$$
MgSO_4 + Na_2Z \rightarrow MgZ + Na_2SO_4
$$
**Regeneration** (when the bed is exhausted): the bed is washed with 10 % common-salt brine:
$$
CaZ + 2NaCl \rightarrow Na_2Z + CaCl_2
$$
and then rinsed with water. Both temporary and permanent hardness are removed.

**Advantages**
- Gives water of nearly zero hardness; the final hardness is easy to control by bypassing part of the flow.
- No sludge is formed; the unit is compact, can be automatic and is easy to operate.
- Removes both carbonate and non-carbonate hardness; regeneration is cheap and quick.

**Disadvantages**
- The sodium content of the treated water increases (a problem for people with heart or kidney trouble), and the total dissolved solids are not reduced.
- Cannot treat turbid or coloured water, or water containing iron and manganese, because the bed gets clogged or fouled.
- Not economical for very hard water; regeneration needs salt and produces brine waste.
- The water may become corrosive and alkaline (sodium bicarbonate).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is an effect of hard water?
   A. Increases soap lather
   B. Prevents pipe scaling
   C. Improves the taste of tea
   D. Wastes soap by forming scum, and forms scale in boilers and pipes
   Answer: D. Ca and Mg form insoluble scum with soap and scale (CaCO3, CaSO4) on heating. (Key point: Effects of hardness)

2. What happens in a zeolite softener?
   A. Sodium of the zeolite is exchanged for Ca and Mg in the water
   B. Ca and Mg are precipitated as sludge with lime
   C. Hardness ions are boiled off
   D. Zeolite adsorbs sodium from the water
   Answer: A. Na2Z + Ca(HCO3)2 gives CaZ + 2NaHCO3, so hardness ions stay in the bed. (Key point: Zeolite action)

3. How is an exhausted zeolite bed regenerated?
   A. By backwashing with lime
   B. By washing with about 10 % common salt (NaCl) brine, then rinsing
   C. By adding alum
   D. By heating the bed
   Answer: B. CaZ + 2NaCl gives Na2Z + CaCl2, restoring the sodium form. (Key point: Regeneration)

4. Which is a disadvantage of the zeolite softener?
   A. It leaves sludge needing disposal
   B. It cannot remove permanent hardness
   C. Sodium in treated water increases and TDS is not reduced; turbid or iron-bearing water clogs the bed
   D. It removes all dissolved solids
   Answer: C. Base exchange swaps ions rather than removing them, and bed fouls with turbidity, Fe or Mn. (Key point: Disadvantage)

## Related in Water Supply Engineering

- [Design of an ion-exchange water softener](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awater-softener-design) — Water Supply Engineering, chapter 6
- [Causes and effects of hardness](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-causes-effects) — Water Supply Engineering, chapter 4
- [Hardness of well water with strontium 25.26 mg/l](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-bk81) — Water Supply Engineering, chapter 4
- [Living organisms in water; health effects](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aliving-organisms-in-water) — Water Supply Engineering, chapter 4
- [Ca and Mg concentration, alkalinity (300 mg/l hardness)](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-ca-mg-equal-300) — Water Supply Engineering, chapter 4, asked 2×
- [Non-carbonate hardness: TH 150, CH 60](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ahardness-c72) — Water Supply Engineering, chapter 4, asked 2×

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