Chemistry · Solutions and electrolytes · Grade 7

Water purification and softening

How does cloudy canal water come out clean from a tap, and why does scale form in a kettle?

The situation

Most of the water that comes out of the taps in Tashkent first flowed as muddy water in the Bozsu canal. At the Qodiriya waterworks in the Qibray district it is settled with alum, filtered and chlorinated. At home, white scale collects at the bottom of the kettle, and soap does not lather well in well water. How does muddy water become drinking water, and how can “hard” water be softened?

Topic

Water purification and softening. Tashkent water treatment plant and limescale in a kettle: settling, coagulation, filtration, chlorination, softening

Key ideas

Coagulation and filtration
Chlorination
Hardness of water

Formulas

Al^{3 +} + 3 HCO_{3}^{-} \overset{}{\to} Al (OH)_{3} ↓ + 3 CO_{2} ↑

Coagulation: alum ( $Al_{2} (SO_{4})_{3}$ ) forms loose flocs of Al(OH)₃ with the hydrogencarbonates in the water — they stick the clay particles together and settle.

Cl_{2} + H_{2} O \overset{}{⇌} HCl + HClO

Chlorination: hypochlorous acid HClO kills microbes; the “residual chlorine” left in the water also protects it in the pipes.

Ca (HCO_{3})_{2} \overset{\overset{}{t}}{\to} CaCO_{3} ↓ + CO_{2} ↑ + H_{2} O

Temporary (carbonate) hardness disappears on boiling — the scale in a kettle is CaCO₃.

CaSO_{4} + Na_{2} CO_{3} \overset{}{\to} CaCO_{3} ↓ + Na_{2} SO_{4}

Permanent hardness is removed with soda; an ion-exchange resin (a cation exchanger) gives $Na^{+}$ in place of $Ca^{2 +}$ : $2 R - Na + Ca^{2 +} \overset{}{\to} R_{2} Ca + 2 Na^{+}$ .

Hardness = \frac{c_{N} V_{titrant}}{V_{sample}} \cdot 1000 mmol-eq/L

The titration result: $c_{N}$ is the normal concentration of the titrant (Trilon B 0.025 M = 0.05 N; HCl 0.05 M = 0.05 N).

Theory

How water becomes polluted

Natural water always contains something: particles of clay and sand (turbidity is measured in NTU), plant debris, dissolved salts and microorganisms. Human activity adds more: domestic sewage — intestinal bacteria (E. coli is a sign of faecal pollution), industrial waste — heavy metals and oil products, and from the fields — fertilisers (nitrates) and pesticides. Drinking such water carries a risk of intestinal diseases and poisoning. Stopping the pollutant at its source is the cheapest and most reliable way: an ordinary treatment plant cannot hold back dissolved heavy metals.

How drinking water is treated

1) A screen holds back large debris. 2) Coagulation: alum ( $Al_{2} (SO_{4})_{3}$ ) is added to the water; it hydrolyses to form loose flocs of $Al (OH)_{3}$ , and the flocs trap fine clay particles and bacteria. 3) They settle in the settling tank. 4) A sand filter holds back the remaining fine particles. 5) Disinfection: chlorine ( $Cl_{2}$ or NaClO) or ozone ( $O_{3}$ ). Ozone is powerful but breaks down quickly in water — so a little chlorine is still added to protect the pipes. WHO recommendations: turbidity ideally below 1 NTU; E. coli must not be detectable in 100 mL of water; after 30 minutes of contact, free chlorine ≥ 0.5 mg/L, and at the consumer ≥ 0.2 mg/L. In the CIS standards the residual free chlorine is 0.3–0.5 mg/L. In Tashkent this work is done by the Qodiriya plant in Qibray (since 1969).

Hardness of water

Hardness is the amount of calcium and magnesium ions in water. In Uzbekistan and the CIS it is measured in mmol-eq/L: 1 mmol-eq/L = 20.04 mg/L Ca²⁺ = 12.15 mg/L Mg²⁺ ≈ 50 mg/L CaCO₃. Temporary (carbonate) hardness is caused by hydrogencarbonates — $Ca (HCO_{3})_{2}$ , $Mg (HCO_{3})_{2}$ , permanent hardness by sulfates and chlorides — $CaSO_{4}$ , $MgCl_{2}$ . The sum of the two is the total hardness. The scale of the US Geological Survey (USGS): up to 60 mg/L CaCO₃ — soft, 61–120 — moderately hard, 121–180 — hard, more — very hard. In the CIS standards, drinking water may have up to 7 mmol-eq/L. Hard water forms scale in kettles and boilers (heat passes through badly, the boiler overheats) and wastes soap: $2 C_{17} H_{35} COONa + Ca^{2 +} \overset{}{\to} (C_{17} H_{35} COO)_{2} Ca ↓ + 2 Na^{+}$ — the soap does not lather but settles as white flakes.

Ways of softening water

Boiling removes temporary hardness: $Ca (HCO_{3})_{2} \overset{}{\to} CaCO_{3} ↓ + CO_{2} ↑ + H_{2} O$ (CaCO₃ dissolves slightly — about 0.3 mmol-eq/L remains). Lime does the same: $Ca (HCO_{3})_{2} + Ca (OH)_{2} \overset{}{\to} 2 CaCO_{3} ↓ + 2 H_{2} O$ . Soda ( $Na_{2} CO_{3}$ ) removes both kinds: $CaSO_{4} + Na_{2} CO_{3} \overset{}{\to} CaCO_{3} ↓ + Na_{2} SO_{4}$ ; the mass needed is $m = \frac{1}{2} hardness \cdot V \cdot M (Na_{2} CO_{3})$ , that is, 53 mg of soda per 1 L of water for each mmol-eq. Ion exchange: a cation exchanger (sulfonated polystyrene resin) takes up $Ca^{2 +}$ and $Mg^{2 +}$ and gives $Na^{+}$ in exchange; when the resin is saturated, it is regenerated with table salt solution: $R_{2} Ca + 2 NaCl \overset{}{\to} 2 R - Na + CaCl_{2}$ . Filter jugs at home work on the same principle.

University: the water treatment plant

Total hardness is determined by complexometric titration: to a 100 mL sample are added an ammonia buffer (pH 10) and Eriochrome Black T — its complex with $Mg^{2 +}$ is wine-red. Trilon B ( $Na_{2} H_{2} Y$ ) binds metals into a stable complex: $Me^{2 +} + H_{2} Y^{2 -} \overset{}{\to} MeY^{2 -} + 2 H^{+}$ ; when the last metal ion is also bound, the indicator is freed and the colour changes to blue. Carbonate hardness is determined with HCl and methyl orange: $HCO_{3}^{-} + H^{+} \overset{}{\to} CO_{2} + H_{2} O$ , from yellow to orange. In the lime–soda method the doses (per 1 L, simplified) are: lime ≈ 37 mg × carbonate hardness, soda ≈ 53 mg × permanent hardness. The ion-exchange capacity is the mmol-eq absorbed by 1 mL of resin: $E = \sum (H_{in} - H_{out}) Δ V / V_{resin}$ ; for strongly acidic cation exchangers the full capacity is usually ≈ 1.8–2 mmol-eq/mL. If an H-cation exchanger and an anion exchanger work one after the other, the water is demineralised. Reverse osmosis: salt water is forced through a semipermeable membrane at a pressure higher than its osmotic pressure (≈ 2.7 MPa for seawater); in electrodialysis an electric current pulls the ions out through ion-selective membranes. Industrial wastewater is treated by mechanical, chemical (neutralisation, precipitating heavy metals as hydroxides) and biological (activated sludge in aeration tanks) methods.

Protecting the hydrosphere, safety and the model

The Aral Sea tragedy: in 1960 the Aral Sea was a lake of about 68 000 km² with a salinity of about 10 g/L. After a great deal of water from the Amu Darya and the Syr Darya was taken for irrigation, it dried up and split into several parts; the salinity of the southern (Large) Aral exceeded 100 g/L in the 2000s, and the wind blows salt and dust from the dried-up bed. That is why saving water, treating wastewater (in Tashkent wastewater is treated mechanically and biologically at aeration stations) and protecting water sources are essential. Safety: chlorine is a poisonous gas; work with alkalis and acids in goggles and gloves; open the ammonia buffer in a fume cupboard; do not taste the water. Model: the plant indicators are simplified typical curves (the quality of the sources is approximate); the composition of the samples is typical; solutions are ideal, 25 °C; in the Trilon B titration a single conventional stability constant ( $10^{8.7}$ ) is used; the flow in the column is sped up.

Did you know?

The water that most of Tashkent’s residents drink is treated at the Qodiriya plant in the Qibray district: it was commissioned in 1969, the water of the Bozsu canal is clarified with liquid alum, passed through 48 filters and chlorinated.
In 1960 the Aral Sea was a lake of about 68 000 km² with a salinity of around 10 g/L; after the water of the rivers was taken for irrigation, the salinity of its southern part rose above 100 g/L in the 2000s — nearly three times saltier than ocean water.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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